Quark Matter 2025

6 Apr 2025, 08:30 → 12 Apr 2025, 15:00 Europe/Berlin

Goethe University Frankfurt, Campus Westend, Theodor-W.-Adorno-Platz 1, 60629 Frankfurt am Main, Germany

Quark Matter 2025 is the XXXI International Conference on Ultra-relativistic Nucleus-Nucleus Collisions, which will be held in Frankfurt, Hesse, Germany. This conference brings together theoretical and experimental physicists from around the world to discuss new developments in high-energy heavy-ion physics. The focus of the discussions is on the fundamental understanding of strongly-interacting matter at extreme conditions, as formed in ultra-relativistic nucleus-nucleus collisions, as well as on emergent QCD phenomena in high-multiplicity proton-proton and proton-nucleus collisions.

The scientific program will cover the following topics:

• Chirality

• Collective dynamics & small systems

• Correlations & fluctuations

• Detectors & future experiments

• Electromagnetic probes

• Heavy flavor & quarkonia 

• Initial state of hadronic and electron-ion collisions & nuclear structure

• Jets

• Light and strange flavor physics & nuclei

• New theoretical developments

• Physics of ultraperipheral collisions

• QCD matter in astrophysics

• QCD phase diagram & critical point
  

 

The conference program is also available in the HEPCon app. It is available for free at the:

Apple App Store

Google Play Store

Please search for Quark Matter 2025 within the app, which is the most recent addition at the time of writing. Navigating talks/posters is most easily done by clicking on the Sessions tab.

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Sunday 6 April

08:30 → 13:00 Early Career Researcher Registration

09:00 → 17:30 Early Career Researcher Day

09:00 A: Theory for heavy ions and how to compare to data

Speaker: Dr Wilke Van Der Schee (CERN)

09:00 B: Detector concepts, road to particle detection

Speaker: Piotr Gasik (GSI - Helmholtzzentrum fur Schwerionenforschung GmbH (DE))

09:50 Short break

10:00 A: Question from the box/Discussion

10:00 B: Questions from the box/Discussion

10:30 Coffee break

11:00 Dynamical evolution/collective dynamics

Speaker: Yuuka Kanakubo (RIKEN iTHEMS / Lawrence Berkeley National Laboratory)

11:35 Polarization/Vorticity

Speaker: Diyu Shen

12:10 Question from the box/Discussion

12:30 Lunch break

14:00 Jets

Speaker: Carlota Andres (MIT)

14:35 Heavy flavor

Speaker: Ralf Averbeck (GSI - Helmholtzzentrum fur Schwerionenforschung GmbH (DE))

15:10 Questions from the box/Discussion

15:30 Coffee break

16:00 High density QCD/Phase diagram (Lecture hall H1)

Speaker: Dr Volodymyr Vovchenko (University of Houston)

16:35 Question from the box/Discussion

16:55 General discussion of remaining questions (Lecture hall H1)

Monday 7 April

08:00 → 09:00 Registration

09:00 → 10:00 Opening session

Convener: Dirk Rischke

10:00 → 10:30 Plenary session 1

Convener: Harald Appelshaeuser (Goethe University Frankfurt (DE))

10:00 Our Contribution to Cosmology

Speaker: Reinhard Stock (Johann-Wolfgang-Goethe Univ. (DE))

10:30 → 11:00 Coffee break

11:00 → 12:40 Plenary session 2

Convener: Tetyana Galatyuk

11:00 Experiment overview: ALICE

Speaker: Alexander Schmah (GSI - Helmholtzzentrum fur Schwerionenforschung GmbH (DE))

11:30 Experiment overview: ATLAS

Speaker: Riccardo Longo (Univ. Illinois at Urbana Champaign (US))

11:55 Experiment overview: CMS

Speaker: Jing Wang (CERN)

12:20 Experiment overview: HADES

Speaker: Hanna Zbroszczyk (Warsaw University of Technology (PL))

12:40 → 14:10 Lunch

14:10 → 16:05 Plenary session 3

Convener: Johanna Stachel (Heidelberg University (DE))

14:10 Experiment overview: LHCb

Speaker: Thomas Boettcher (Los Alamos National Laboratory (US))

14:35 Experiment overview: NA61

Speaker: Katarzyna Grebieszkow (Warsaw University of Technology (PL))

14:55 Experiment overview: PHENIX

Speaker: Dr Roli Esha (Center for Frontiers in Nuclear Science, Stony Brook University)

15:15 Experiment overview: sPHENIX

Speaker: Christof Roland (Massachusetts Inst. of Technology (US))

15:35 Experiment overview: STAR

Speaker: Sooraj Radhakrishnan (Kent State University/Lawrence Berkeley National Laboratory)

16:05 → 16:35 Coffee break

16:35 → 18:55 Parallel session 1: Chirality I

Convener: Prof. Qun Wang (University of Science and Technology of China)

16:35 Measurements of global and local spin polarization of hyperons in Au+Au collisions at RHIC-STAR

In non-central heavy ion collisions, substantial angular momentum is generated, resulting in significant vorticity and the spin polarization of particles with finite spin along the global angular momentum. Additionally, anisotropic flow can produce local vorticities in both in-plane (z) and out-of-plane (y) directions, leading to local polarization. The RHIC Beam Energy Scan II (BES-II) program, including the Fixed-Target (FXT) mode at STAR, provides a unique opportunity to investigate hyperon ($\Lambda$, $\overline{\Lambda}$, $\Xi^{\pm}$, $\Omega^{\pm}$) global polarization over a wide range of baryon chemical potential. This study probes the vortical characteristics of the medium and can serve as a tool for exploring the nuclear matter equation of state. Also, the second harmonic of local spin polarization ($P_{2y}$ and $P_{2z}$) for $\Lambda$ and $\overline{\Lambda}$ and their difference, are predicted to be a sensitive probe for the spin Hall effect driven by gradients in baryon chemical potential.

In this talk, we present measurements of the global polarization of $\Lambda$, $\overline{\Lambda}$, $\Xi^{\pm}$ and $\Omega^{\pm}$ at $\sqrt{s_{NN}} = 7.7$ - $27$\;GeV in BES-II Au+Au collisions, and the global $\Lambda$ polarization at $\sqrt{s_{NN}} = 3.0$ - $6.2$\;GeV in FXT Au+Au collisions. Additionally, we report the second harmonics of local $\Lambda$, $\overline{\Lambda}$ polarization and their difference at $\sqrt{s_{NN}} = 7.7$ - $27$\;GeV. These measurements provide valuable insights into the spin dynamics within the QCD medium produced in heavy ion collisions.

Speaker: Tan Lu (Institute of Modern Physics, Chinese Academy of Sciences)

16:55 Measurement of $\Lambda$ $\bar{\Lambda}$ spin correlation in proton-proton collisions at STAR

Approximately fifty years ago, the polarization of $\Lambda$ hyperons produced in unpolarized proton-beryllium collisions was discovered, though the origin of this phenomenon remains elusive. Many studies indicate that final-state effects, particularly from hadronization, play a significant role. Recently, it has been proposed that spin correlations of $\Lambda$ hyperons could provide insight into the underlying mechanisms of $\Lambda$ polarization. In this talk, we report the first experimental measurements of $\Lambda\bar{\Lambda}$, $\Lambda\Lambda$, and $\bar{\Lambda}\bar{\Lambda}$ spin-spin correlations in $p$+$p$ collisions at $\sqrt{s} = 200\,\textrm{GeV}$, as recorded by the STAR experiment in 2012. Both short-range ($|\Delta y| < 0.5$ and $|\Delta \phi| < \pi/3$) and long-range ($0.5 < |\Delta y| < 2.0$, or $\pi/3 < |\Delta \phi| < \pi$) $\Lambda$ hyperon pairs were measured. We will discuss the implications of the measured spin-spin correlations in relation to the longstanding puzzle of $\Lambda$ hyperon polarization, offering new insights into the hadronization of strange quarks.

Speaker: Jan Vanek (Brookhaven National Laboratory)

17:15 Investigating the Event-Shape Methods in Search for the Chiral Magnetic Effect in Relativistic Heavy Ion Collisions

Chiral Magnetic Effect (CME) is a phenomenon in which electric charge is separated by a strong magnetic field from local domains of chirality imbalance and parity violation in quantum chromodynamics. The CME-sensitive observable, charge-dependent azimuthal correlator $\Delta\gamma$, is contaminated by a major physics background proportional to the elliptic flow ($v_{2}$) of particles of interest (POI's). Event-shape engineering (ESE) [1] and event-shape selection (ESS) [2] are two methods to search for the CME by projecting $\Delta\gamma$ to the $v_2=0$ intercept. The former groups events according to the elliptic anisotropy vector $\vec{q}_2$ of particles in phase space different from that of POI's, thus to the dynamical fluctuations of the POI's $v_2$, whereas the latter groups events according to $\vec{q}_{2,{\rm pair}}$ of pairs of POI's themselves, thus to the statistical fluctuations of the POI's $v_2$. In this talk, we conduct a systematic study of these two methods using physics models as well as toy model simulations [3].
It is observed that the ESE method requires significantly more statistics than the ESS method to achieve the same statistical precision of the intercept, because of the limited dynamical fluctuation range of $v_2$ than that of statistical fluctuations.
It is found that the ESE method can faithfully probe the CME when the $\vec{q}_2$ phase space is distanced from the POI's, while nonflow effect cannot be neglected when the $\vec{q}_2$ and POI's phase spaces are close. The ESS method, on the other hand, is obscured because of the intertwining $\vec{q}_{2,{\rm pair}}$, $v_2$, and $\Delta\gamma$, all of which are computed from the same POI's. It is found that the intercept from the ESS method depends on the details of the event content, such as the relative mixtures of background-contributing sources, and thus is not a robust measure of the CME.

[1] J. Schukraft, A. Timmins, and S.A. Voloshin, Phys. Lett. B 719 (2013) 394.
[2] Z. Xu, B. Chan, G. Wang, A. Tang, and H.Z. Huang, Phys. Lett. B 848 (2024) 138367.
[3] H. Li, Y. Feng, and F. Wang, arXiv:2407.14489.

Speaker: Han-Sheng Li

17:35 Search for the quark flavor dependence of the Chiral Magnetic Effect through charge-dependent correlations with identified hadrons at ALICE

The interplay between the chiral anomaly and strong magnetic or vortical fields generated in off-central heavy-ion collisions is theorized to produce anomalous chiral effects in the quark–gluon plasma, such as the Chiral Magnetic Effect (CME) and the Chiral Vortical Effect (CVE). These effects have been actively studied by several experiments for over a decade as they could shed light on the topological structure of vacuum gauge fields and reveal potential local violations of P and/or CP symmetries in strong interactions. Although recent developments have shown that the signals are small, various underlying mechanisms remain unclear, including the quark flavor dependence and its contributions to both the signal and the background.
In this talk, we extend the ALICE measurements of charge-dependent two particle correlations from inclusive hadrons to identified hadrons (pions, kaons, and protons) in Pb–Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.36 TeV. Together with previous results of Λ–proton correlations aimed at searching for the CVE, we discuss several possible background mechanisms, including local charge conservation, coalescence, and flow. These studies offer new insights into the search for anomalous chiral effects and enhance our understanding of collectivity in heavy-ion collisions.

Speaker: Zhengqing Wang (Fudan University (CN))

17:55 Initial-state-driven spin polarization in heavy-ion collisions

In recent years, there has been a growing interest in the study of spin-polarization effects in heavy-ion collisions. Most approaches assume that final-state particles become polarized only if the thermal vorticity and shear are non-zero at freeze-out. In this sense, polarization is a final-state effect. Here, we propose a different polarization mechanism. For the first time, spin density is introduced in the initial state in a similar way as one introduces energy density. We argue that the collision of two nuclei, which occurs among participant nucleons carrying spin, deposits in the transverse plane a spin density that polarizes the produced QCD fireball along some random direction. If the subsequent evolution (e.g., kinetic or hydrodynamic) of the fireball does not alter this effect, the spins of the particles emitted at freeze-out will be polarized along the same direction. Using a simple model, we estimate that the polarization induced by initial-state fluctuations is on the order of 1% in central collisions, increasing to over 10% in noncentral collisions. At high energies, it is thus much larger than the polarization due to event-by-event fluctuations of the thermal vorticity. To measure this phenomenon, we introduce a two-particle angular correlation observable able to reveal net-spin fluctuations in the initial states, for both spin-1/2 and spin-1 hadrons. This new effect depends little on beam energy and could be promptly studied at both the LHC and RHIC. Finally, besides introducing new tools to probe spin effects in nuclear collisions, this work provides a framework to define spin initial conditions suitable for extensions of hydrodynamics with spin degrees of freedom.

Speaker: Enrico Speranza (CERN)

18:15 Hydrodynamic contributions to the spin polarization and spin alignment in heavy ion collisions

We investigate the hydrodynamic helicity polarization of $\Lambda$ hyperons and the spin alignment of $\phi$ mesons at RHIC-BES energies by utilizing the relativistic (3+1)D CLVisc hydrodynamics framework with SMASH and AMPT initial conditions.
As opposed to local spin polarization at high energy collisions, our hydrodynamic simulations demonstrate that the helicity polarization induced by the kinetic vorticity dominates over other contributions at intermediate and low collision energies. Our findings provide an opportunity to probe the fine structure of local kinetic vorticity as a function of azimuthal angle at intermediate and low collision energies by mapping our predictions to the future measurements in experiments.
When it comes to the spin alignment contributed by the hydrodynamic effects, our study indicates that when hydrodynamic effects are considered in conjunction with the fluctuations of effective $\phi$ meson field , the theoretical results exhibit improved agreement with experimental data on spin alignment as a function of $p_{T}$.

Speaker: Cong Yi

18:35 Initial conditions and bulk viscosity effects on $\Lambda$ polarization in high-energy heavy ion collisions

The $\Lambda$ polarization is a crucial probe of the gradients of velocity and temperature in the quark-gluon plasma generated in heavy-ion collisions. However, it is still not systematically used to tune hydrodynamic models. In this talk, we investigate the influence of different initial conditions and parametrization of the bulk viscosity on $\Lambda$ polarization, showing that they affect the local polarization significantly. These results highlight the impact that the use of local $\Lambda$ polarization can have on refining theoretical models. Finally, we compare our results, including feed-down corrections, with experimental data from high-energy heavy-ion collisions at STAR and ALICE, and demonstrate the crucial role of bulk viscosity in generating the correct sign of longitudinal polarization at LHC energies.

Based on Eur.Phys.J.C 84 (2024) 9, 920

Speaker: Andrea Palermo (Stony Brook University)

16:35 → 18:55 Parallel session 2: Detectors & future experiments

Convener: W.A. Zajc (Columbia University)

16:35 ALICE 3: a next-generation heavy-ion detector for LHC Run 5

The ALICE Collaboration has proposed a completely new apparatus, ALICE 3, for the LHC Run 5 (LoI, arXiv:2211.02491). The detector consists of a large pixel-based tracking system covering eight units of pseudorapidity, complemented by multiple systems for particle identification, including silicon time-of-flight layers, a ring-imaging Cherenkov detector, a muon identification system, and an electromagnetic calorimeter. Track pointing resolution of better than 10 micron for $p_{\rm T} > 200$ MeV/$c$ can be achieved by placing the vertex detector on a retractable structure inside the beam pipe. ALICE 3 will, on the one hand, enable novel studies of the quark-gluon plasma and, on the other hand, open up important physics opportunities in other areas of QCD and beyond. The main new studies in the QGP sector focus on low-$p_{\rm T}$ heavy-flavour production, including beauty hadrons, multi-charm baryons and charm-charm correlations, as as well as on precise multi-differential measurements of dielectron emission to probe the mechanism of chiral-symmetry restoration and the time-evolution of the QGP temperature. Besides QGP studies, ALICE 3 can uniquely contribute to hadronic physics, with femtoscopic studies of the interaction potentials between charm mesons and searches for nuclei with charm, and to fundamental physics, with tests of the Low theorem for ultra-soft photon emission. The presentation will cover the detector concept, the physics performance, and the status of detector R&D.

Speaker: Nicola Nicassio (Universita e INFN, Bari (IT))

16:55 Prospects for heavy-ion physics with the LHCb Upgrade II

Owing to its spectrometer acceptance, unique with respect to the other LHC experiments, and to its excellent tracking and particle identification, LHCb has been developing since the LHC Run2 a complete heavy-ion programme. In parallel, by exploiting the injection of gases in the LHC accelerator beam-pipe, LHCb is now simultaneously acquiring data in collider and fixed-target mode, with two independent interaction points. The sum of the
two configurations already gives unique inputs to theoretical models. With the foreseen LHCb Upgrade II, to be operated from Run5, even more possibilities will be opened. Detector granularity will be increased, and new instrumentation with timing capabilities will be added, opening a plethora of new possible analyses to be explored. In this contribution, a full overview of the heavy-ion opportunities with the LHCb Upgrade II, as discussed in a recent workshop with theoreticians, will be presented and discussed.

Speaker: Saverio Mariani (CERN)

17:15 CMS Phase-2 Upgrades for Heavy Ion Physics in the High-Luminosity LHC Era

The intriguing phenomena emerging in high-density quantum chromodynamics (QCD) matter are being extensively studied in the LHC’s heavy ion program, and will be explored in greater depth during the high-luminosity LHC (HL-LHC) era. To meet the challenges of HL-LHC, the CMS experiment is undergoing significant Phase-2 upgrades, including the MIP Timing Detector (MTD), which provides 30 ps timing resolution for minimum ionizing particles (MIPs). With its large pseudorapidity coverage up to $|\eta|<3$, the MTD enables particle identification (PID) via time-of-flight (TOF) measurements, offering new opportunities for probing QCD matter. Coupled with an upgraded wide-acceptance tracker ($|\eta|<4$) and high-granularity calorimetry ($|\eta|<5$), these upgrades will allow for more detailed investigations of the quark-gluon plasma (QGP), such as the (3+1)-dimensional evolution of heavy flavor quark dynamics and particle composition inside jets over a wide angular range. Additionally, the improved detector capabilities will significantly expand opportunities in ultraperipheral collisions (UPCs), enabling precise measurements of photon-induced processes, which are crucial for studying the gluonic structure of nuclei. This talk will discuss new physics opportunities enabled by the Phase-2 upgrades covering a wide range of topics. The latest status of the MTD project, including its R&D advancements and prototyping efforts, will also be presented.

Speaker: Bolek Wyslouch (Massachusetts Inst. of Technology (US))

17:35 The NA60+ experiment at the CERN SPS

A new fixed-target experiment, NA60+, is proposed at the CERN SPS to measure muon pairs in the centre-of-mass energy range from 6 to 17 GeV across various collision systems, from Pb--Pb to p--Be. Its physics scope spans topics from thermal radiation, to chiral symmetry restoration, strangeness, and charm production.

The experimental apparatus comprises a vertex spectrometer and a large-acceptance muon spectrometer., A vertex telescope with ultra-thin, large-area Monolithic Active Pixel Sensors (MAPS) is positioned close to the target and operates within a dipole magnetic field. Downstream, the set-up includes a muon spectrometer based on large-area gaseous detectors and a second dipole magnet. A high-intensity beam grants access to rare observables that have been scarcely explored.

The physics program includes searching for chiral symmetry restoration through $\rho$-$a_{1}$ mixing, studying the phase transition order at high baryochemical potential via a caloric curve, and detecting deconfinement onset through $J/\psi$ suppression. Additionally, NA60+ will measure medium transport properties using open charm states and study hadrochemistry through strange hadrons and hypernuclei production.

A technical proposal is being prepared for submission to the SPS committee in May 2025, with data taking targeted to begin in 2029/2030.

This talk will cover the technical aspects of the experimental apparatus, the R&D progress, and the physics program and its potential impact alongside other experiments.

Speaker: Giacomo Alocco (Universita e INFN, Torino (IT))

17:55 Exploring baryon-rich QCD matter with CBM at FAIR: status and prospects

The Compressed Baryonic Matter (CBM) experiment is under construction at the Facility for Antiproton and Ion Research (FAIR) in Darmstadt, Germany. It aims to explore the phase structure and microscopic properties of strong interaction (QCD) matter at large net-baryon densities and moderate temperatures using heavy-ion collisions in the energy range √sNN = 2.9 - 4.9 GeV.

CBM is a fixed-target experiment, equipped with fast and radiation hard detector systems and an advanced triggerless data acquisition scheme. It will collect data at unprecedented interaction rates by performing online reconstruction and event selection, thus allowing measurements of rare probes not accessible so far in this energy range. These include: multi-strange hadron production and their flow, higher-order cumulants, dileptons, as well as double-strange hypernuclei.

This contribution will provide an overview of the CBM physics performance and objectives, as well as the detector technologies being developed for the experiment. The status of preparations for CBM’s commissioning in 2028, including performance evaluations of CBM components at FAIR Phase-0 experiments, will also be presented.

Speaker: Petr Chaloupka

18:15 ALICE Forward Calorimeter upgrade (FoCal): physics program and expected performance

The FoCal is a high-granularity forward calorimeter to be installed as an ALICE upgrade during the LHC Long Shutdown 3 and take data in Run 4. It will cover a pseudorapidity interval of $3.2 < \eta < 5.8$, allowing to explore QCD at unprecedented low Bjorken-$x$ of down to $\approx 10^{-6}$ - a regime where non-linear QCD dynamics are expected to be sizable.
The FoCal consists of a compact silicon-tungsten sampling electromagnetic calorimeter with pad and pixel readout to achieve high spatial resolution for discriminating between isolated photons and decay photon pairs. Its hadronic component is constructed from copper capillary tubes with scintillator fibers. The detector design allows measuring a multitude of probes, including direct photons, jets, as well as photo-production of vector mesons in ultra-peripheral collisions and angular correlations of different probes.
After the recent completed of the Technical Design Report (https://cds.cern.ch/record/2696471), the FoCal project is entering the production phase in view of installation in 2028.
We will give an overview of the FoCal physics programme, of the detector design and of its expected performance using results from recent test beams of small-scale prototypes.

Speaker: Tommaso Isidori (The University of Kansas (US))

18:35 The ATLAS Transition Radiation Tracker and Zero Degree Calorimeter: the progress on triggering on ultraperipheral processe

A subset of heavy-ion collisions are ultra-peripheral collisions (UPC), where the ions collide with a large impact parameter, causing them to interact primarily through their intense electromagnetic fields. These interactions allow for the study of various photon-induced processes at high energies. The products of exclusive UPC interactions are typically characterized by very low transverse momenta products, which present several challenges to efficiently select with a trigger. In Run 3, the ATLAS Transition Radiation Tracker (TRT) FastOR trigger provided a novel tool to trigger on these processes at Level 1. The TRT trigger was, for the first time, adapted for use in heavy-ion collisions, allowing it to trigger on low-$p_\mathrm{T}$ particles ($p_\mathrm{T} > 200$ MeV), including muons from coherent J/Psi decays. Another feature of many UPC collisions is single-sided nuclear breakup or multiple photon exchange, which causes one or both of the colliding ions to emit forward neutrons.
The Zero-Degree Calorimeter (ZDC) detects these neutrons and provides a fully digital trigger. This tool allows for triggering on many distinct neutron topologies within the Level 1 system. In Run 4, the new joint ATLAS-CMS HL-ZDC will be installed, providing similar triggering capabilities in the more challenging environment of the HL-LHC. The combination of the ZDC and TRT along with other calorimeter systems allows for the efficient collection of data for many distinct UPC processes.

Speaker: Matthew Caleb Hoppesch (University of Illinois at Urbana-Champaign)

16:35 → 18:55 Parallel session 3: Heavy flavor & quarkonia I

Convener: Roberta Arnaldi (Universita e INFN Torino (IT))

16:35 Heavy Quark Diffusion Coefficient from Lattice QCD

We present new lattice results for the heavy quark diffusion coefficient in 2+1 flavor QCD in the temperature range from 163 MeV up to few GeV. Compared to previous lattice calculations with unphysical light quark masses, we consider near-physical values and a much wider temperature range. Our results for the spatial heavy diffusion coefficient near the crossover temperature are considerably smaller than the estimates obtained by comparing phenomenological models with experimental data and by T-matrix calculations, and are close to the AdS/CFT limit. At high temperatures, however, the spatial heavy diffusion coefficient increases and approaches the NLO weak coupling prediction within the estimated errors. We also find that the dependence of the spatial heavy quark diffusion coefficient on the heavy quark mass $M_Q$ on the lattice is weaker compared to the model calculations.

Speaker: Jorge Luis Dasilva Golan (Brookhaven National Laboratory)

16:55 Intrinsic Heavy Flavor Production of Tetraquarks

A number of new four-quark states containing from one to four $c$ or $\overline c$ quarks have been observed recently. Many of these new states have been discovered at the LHC. The production of these states via intrinsic charm in the proton is investigated. The tetraquark masses obtained in this approac agree well with the measured masses [1]. These calculations can provide some insight into the nature of the tetraquark candidates, whether as a bound meson pair or as a looser configuration of four individual partons which can influence their interactions in the nuclear medium, such as in heavy-ion collisions. The kinematic distributions of these states as a function of $y$ and $p_T$ are also studied. Previous investiagations [2-4] show that $J/\psi$ and $\overline D$ mesons produced from such states manifest themselves at forwrad rapidity and relatively high $p_T$. The extension to bottom tetraquark candidates is also considered.

[1] R. Vogt, Tetraquarks from Intrinsic Heavy Quarks, Phys. Rev. D in press, arXiv:2405.09018 [hep-ph].

[2] R. Vogt, Limits on Intrinsic Charm Production from the SeaQuest Experiment, Phys. Rev. C 103 (2021), 035204.

[3] R. Vogt, Energy dependence of intrinsic charm production: Determining the best energy for observation, Phys. Rev. C 106 (2022) 025201.

[4] Contribution from intrinsic charm production to fixed-target interactions with the SMOG Device at LHCb, Phys. Rev. C 108, 015201 (2023).

This work was performed under the auspices of the US DoE by LLNL under Contract DE-AC52-07NA27344 and supported by LDRD projects 21-LW-034 and 23-LW-036 and the HEFTY Collaboration.

Speaker: Ramona Vogt (LLNL and UC Davis)

17:15 Statistical Production of $B_c$ Mesons in Heavy-Ion Collisions at the LHC Energy

The recombination production of $B_c$ mesons in heavy-ion collisions at the LHC energy is facilitated by the abundant and highly thermalized charm ($c$) quarks transported in the deconfined medium created. We study the production of $B_c$ mesons via $c$ and bottom ($b$) quark recombination in a statistical fashion by placing $B_c$ in the position of a member of the family of open $b$ hadrons, which allows us to make quantitative predictions for the modifications of the production fraction ($f_c$) of $B_c$ mesons and its relative production to $B$ mesons in $\sqrt{s_{\rm NN}}=5.02$ TeV Pb-Pb collisions with respect to proton-proton ($pp$) collisions at the same energy [1]. The statistical production yield of $B_c$ mesons is converted into the transverse momentum ($p_T$) distribution with the shape computed from resonance recombination using the $c$- and $b$-quark phase space distributions that have been simulated via Langevin diffusion and constrained by open $c$- and $b$-hadron observables. Supplemented with the component fragmented from $b$-quark spectrum that dominates at high $p_T$, the total $p_T$ spectrum of $B_c$ mesons is obtained and converted into the $p_T$ dependent nuclear modification factor ($R_{\rm AA}$). Both $f_c$ and the integrated $R_{\rm AA}$ exhibit a $\sim5$-fold enhancement in central Pb-Pb collisions relative to the $pp$ reference. Comparison with data measured by the CMS experiment [2] shows decent agreement within theoretical and experimental uncertainties.

References:
[1] Shouxing Zhao and Min He, arXiv: 2407.05234
[2] CMS Collab., Phys. Rev. Lett. 128, 252301 (2022).

Speaker: Min He (Nanjing University of Science & Technology)

17:35 Investigating charm-quark dynamics in the QGP via the charm-hadron elliptic flow in Pb–Pb collisions with ALICE

Heavy quarks (charm and beauty) are useful probes for investigating the properties of the quark-gluon plasma (QGP) generated in ultra-relativistic heavy-ion collisions. Their participation in the collective motion of the medium can be assessed by measuring the charm-hadron elliptic-flow coefficient $v_2$, originating from the initial-state spatial asymmetry in non-central heavy-ion collisions. These measurements provide fundamental inputs to constrain theoretical models describing the charm-quark transport in the QGP, as well as its possible thermalization in the medium. In addition, the comparison between meson and baryon $v_2$ can provide further insights into medium-induced phenomena, such as the radial flow and the charm-quark hadronization via coalescence.

In this contribution, the first measurements of $\mathrm{D^0}$-, $\mathrm{D^+}$-, $\mathrm{ D_s^+}$-meson and $\Lambda_\mathrm{c}^+$-baryon $v_2$ in different centrality intervals of Pb–Pb collisions at $\sqrt{s_\mathrm{NN}}$ = 5.36 TeV collected by the ALICE experiment during the LHC Run 3 are shown. The measurements are compared to model predictions that incorporate various implementations of heavy-quark interaction and hadronization with the QGP constituents. Moreover, the measurement of the $\mathrm{D^0}$-meson elliptic flow in pp collisions is presented.

Speaker: Chuntai Wu (Universita e INFN, Padova (IT))

17:55 Studying charm and beauty quark production and hadronisation with charm hadron measurements with ALICE

Recent observations at the Large Hadron Collider (LHC) have revealed a significant enhancement in baryon production in hadronic collisions compared to e$^+$e$^-$ and ep collisions, challenging the long-held assumption of fragmentation universality. This phenomenon, initially observed in the charm sector, has also been observed in the beauty sector, indicating a broader implication for heavy-flavor hadronization processes. This observation has led to the development of several models attempting to explain these observations, by implementing modified hadronization approaches with respect to the in-vacuum fragmentation process. Proposed mechanisms include color reconnection beyond leading-color approximation, hadronisation via coalescence, and feed-down from unobserved resonant states in a statistical hadronization approach.

To further elucidate this phenomenon, recent studies have focused on baryon-to-meson ratios as a function of event multiplicity. Intriguingly, evidence of a multiplicity dependence, with increased ratios in high-multiplicity events, has been observed in both the charm and beauty sectors, suggesting a possible connection between baryon enhancement and the local parton density. These investigations have significant implications also for heavy-ion physics, potentially bridging the gap between phenomena observed in proton-proton and heavy-ion collisions. A comprehensive understanding of these effects is essential for refining our theoretical models of heavy-quark hadronisation in hadronic environments.

In this contribution, new ALICE measurements of prompt and non-prompt $\Lambda_{\rm c}^+$-baryon and ${\rm D}^0$-meson production cross sections, evaluated on the dataset of proton-proton collisions at $\sqrt{s} = 13.6$ TeV collected during the LHC Run 3, will be reported. The $\Lambda_{\rm c}^+/{\rm D}^0$ production-yield ratios in the same collision system, both multiplicity integrated and as a function of multiplicity, will also be discussed and compared to state-of-the-art model predictions implementing different hadronisation approaches.

Speaker: Luigi Dello Stritto (CERN)

18:15 Testing perturbative QCD calculations with beauty-meson production in proton–proton collisions with ALICE

Due to their large mass, beauty quarks are always produced in hard-scattering processes, and hence their production can be computed with perturbative quantum chromodynamics (pQCD) calculations. The production cross section of beauty hadrons can be theoretically described with the factorisation approach as a convolution of the parton distribution functions of the incoming projectiles, the perturbative partonic cross section, and the fragmentation functions describing the transition from quarks to hadrons. Measurements of the production cross section of beauty hadrons in proton–proton (pp) collisions are therefore excellent tests of pQCD calculations. Measurements down to low transverse momenta are also fundamental ingredients for the estimation of the $\mathrm{b\overline{b}}$ production cross section. Moreover, the relative abundances of different beauty-hadron species also gives insights about the beauty-quark hadronisation mechanisms.
Finally, measurements of beauty-hadron production cross sections in pp collisions provide a reference for Pb–Pb collisions, where modifications due to the creation of the quark–gluon plasma are expected.

In this contribution, the measurement of non-strange B-meson and $\mathrm{B_s}^0$-meson production in pp collisions at 13.6 TeV collected by the ALICE experiment during the LHC Run 3 are presented. The B mesons are fully reconstructed via their decay channels into a D meson and a charged pion. The measured production cross sections are compared with state-of-art pQCD calculations with next-to-leading order accuracy plus all-order resummation of next-to-leading logarithms.

Speaker: Fabrizio Chinu (Universita e INFN Torino (IT))

18:35 Charm-hadron polarization in hadronic collisions at the LHC with ALICE

Heavy quarks (charm and beauty) are mainly produced in hard-scattering processes among partons in ultrarelativistic hadronic collisions at the LHC. In heavy-ion collisions, where a quark-gluon plasma (QGP) state is produced, charm and beauty quarks are produced on a timescale shorter than the medium formation time, being sensitive to the initial angular momentum of the fireball and to the magnetic field produced perpendicularly to the reaction plane in non-central heavy-ion collisions. In the presence of a large angular momentum and initial magnetic field, the charm quark can be polarised, and the quark polarisation is expected to be transferred to the hadron during the hadronization process. Experimentally, heavy-flavour polarisation can be probed by measuring either the spin density matrix element $\rho_{00}$ of spin-1 bosons (as the D$^{*+}$ meson) or the longitudinal polarization $P_{\rm z}$ of spin ½ fermions (as the $\Lambda_{\rm c}^{+}$ baryon). Measurements in pp collisions provide a crucial baseline to disentangle the amount of charm-hadron polarization induced by the charm-quark polarization inherited from the initial stage of heavy-ion collisions.

In this contribution, the first measurement of the $\rho_{00}$ parameter of D$^{*+}$ meson in Pb-Pb collisions at $\sqrt{s_\mathrm{NN}}=5.02~$TeV, exploiting the data sample collected by the ALICE Collaboration during the LHC Run 2, is shown. Moreover, the $\rho_{00}$ parameter of D$^{*+}$ mesons and the longitudinal polarization $P_{\rm z}$ of $\Lambda_{\rm c}^{+}$ baryon measured in pp collisions at $\sqrt{s}=13.6~$TeV collected during the LHC Run 3 are presented. In this case, the measurements are performed also for charm hadrons originating from b-hadron decays, for which the helicity conservation in weak decays should imply a non-zero polarization of the produced charm hadrons.

Speaker: Mattia Faggin (CERN)

16:35 → 18:55 Parallel session 4: Initial state of hadronic and electron-ion collisions & nuclear structure I

Convener: Raju Venugopalan

16:35 Studies of nucleon structure at LHCb

The LHCb detector’s forward geometry provides unprecedented access to
the very low regions of Bjorken \textit{x} inside the nucleon. With full particle ID and a fast DAQ, LHCb is able to fully reconstruct plentiful charged particles and neutral mesons, as well as relatively rare probes such as heavy quarks, providing a unique set of constraints on nucleon structure functions. This contribution will discuss new LHCb measurements sensitive to the low-\textit{x} structure of nucleons, and discuss the impact of recent LHCb measurements that dramatically reduce nPDF uncertainties.

Speaker: Dr Imanol Corredoira Fernandez (Université Paris-Saclay (FR))

16:55 Experimental discovery of the nuclear shape phase transition and nuclear structure on ultrashort timescales at the LHC with ALICE

The second-order shape phase transition of the Xe isotope chain, predicted to lie around $^{128}\text{Xe}$ to $^{130}$Xe, is challenging to measure in low-energy nuclear experiments. Recently, heavy-ion collisions have demonstrated their potential as an imaging tool for nuclear structure by examining anisotropic flow and its correlations with the mean transverse momentum. Hints of a triaxial structure in $^{129}$Xe have been found experimentally and theoretically using these methods; however, these studies were limited to exploring the first moment of the $\gamma$ distribution and could not distinguish between a rigid rotor and $\gamma$-soft nuclei.

In this talk, a comprehensive study of the quadrupole deformation, nuclear diffuseness, and triaxial structure of $^{129}$Xe is presented through investigations of various anisotropic flow and mean transverse momentum observables. Additionally, the $\gamma$-soft structure of the $^{129}$Xe nuclei is investigated experimentally for the first time in heavy-ion collisions using a novel six-particle normalized cumulant, $ NC(v_2^4, \delta p_{\rm T}^2) $, which correlates the fourth moment of the elliptic flow, $ v_2^4$, with the fluctuations of the mean transverse momentum, $ \delta p_{\rm T}^2 $. This correlation is measured in Xe–Xe collisions at $ \sqrt{s_{\rm NN}} = 5.44$ TeV against the baseline of spherical Pb nuclei in Pb–Pb collisions at $ \sqrt{s_{\rm NN}}$ = 5.02 TeV using the ALICE experiment at the LHC. Comparisons with calculations from state-of-the-art models, both with and without fluctuating shape parameters, provide a high-precision probe of the nuclear structure of $^{129}$Xe and facilitate the discovery of the nuclear shape phase transition.

Speaker: Emil Gorm Nielsen (University of Copenhagen (DK))

17:15 Top-quark pair production in $p$+Pb collisions in the ATLAS experiment

Measurements of top-quark pairs in heavy-ion collisions are expected to provide novel probes of nuclear parton distribution functions at high Bjoerken-$x$ values, which are difficult to access experimentally using other available probes. We report the observation of top-quark pair production in proton-lead collisions at the centre-of-mass energy of 8.16 TeV in the ATLAS experiment at the LHC. Top-quark pair production is measured in the lepton+jets and the dilepton channels, with a significance well above 5 standard deviations in each channel separately. The nuclear modification factor $R_{p\mathrm{A}}$ is also measured for the first time for the top-quark pair process.

Speaker: Iwona Grabowska-Bold (AGH University of Krakow (PL))

17:35 Monte Carlo EKRT event generator for initializing 3+1 D fluid dynamics in high energy nuclear collisions

We present a novel Monte-Carlo implementation of the EKRT model, MC-EKRT, for computing partonic initial states in high-energy nuclear collisions [1]. Our new MC-EKRT event generator is based on collinearly factorized, dynamically fluctuating pQCD minijet production, supplemented with a saturation conjecture that controls the low-$p_T$ particle production. Previously, the EKRT model has been very successful in describing low-$p_T$ observables at mid-rapidity in heavy-ion collisions at the LHC and RHIC energies [2,3]. As novel features, our new MC implementation gives a full 3-dimensional initial state event-by-event, includes dynamical minijet-multiplicity fluctuations in the saturation and particle production, introduces a new type of spatially dependent fluctuating nuclear parton distribution functions, and accounts for the conservation of energy/momentum and valence-quark number. We show, by averaging a large set of event-by-event MC-EKRT initial conditions for 3+1 dimensional hydrodynamical evolution, that we obtain a good agreement with the rapidity-dependent multiplicity and elliptic flow data for LHC Pb+Pb and RHIC 200 GeV Au+Au collisions. This suggests that the same saturation mechanism that has successfully explained the mid-rapidity observables, works well also at larger rapidities. As a further application of MC-EKRT, we also study event-by-event fluctuations and decorrelations of initial eccentricities.

[1] M. Kuha, J. Auvinen, K. J. Eskola, H. Hirvonen, Y. Kanakubo, H. Niemi, arXiv:2406.17592 [hep-ph]
[2] H. Niemi, K. J. Eskola and R. Paatelainen, Phys. Rev. C 93, no.2, 024907 (2016)
[3] H. Hirvonen, K. J. Eskola and H. Niemi, Phys. Rev. C 106, no.4, 044913 (2022)

Speaker: Harri Niemi (University of Jyväskylä)

17:55 Pre-hydrodynamic jet momentum broadening beyond the jet quenching parameter

It has recently been widely acknowledged that the initial stages are important for correctly describing jet quenching in heavy-ion collisions. In particular, the initial stages influence the emission spectrum of hard partons traversing the plasma. Such calculations require the knowledge of the momentum-broadening kernel $C$, which describes the probability of a jet parton receiving a momentum kick. While it is often employed in a harmonic approximation using the jet quenching parameter $\hat q$, in this talk, I will present our results for the kernel $C$ from QCD kinetic theory beyond this approximation. We show that the kernel is consistent with previous extractions of $\hat q$ and find that it is highly anisotropic at early times and that small-momentum exchange processes are more likely than expected from a Landau-matched thermal system. Our results may lead to a better description of jet quenching during the initial stages, which can be particularly significant for the ongoing and upcoming light-ion collision programs.

Speaker: Florian Lindenbauer (TU Wien)

18:15 The (3+1)D structure of the dilute Glasma

I will discuss the (3+1)D structure of the Glasma [1] in the dilute approximation, a semi-analytic framework for the computation of rapidity-dependent early-time observables in relativistic heavy-ion collisions. Based on the Color Glass Condensate effective field theory, where the gluonic interactions between the large Bjorken-x, static sources localized in the colliding nuclei are described by classical Yang-Mills equations, I will present remarkably simple, analytic solutions for the Glasma field strength tensor by linearizing the Yang-Mills equations for weak sources. I will discuss our results for Pb+Pb collisions at LHC and Au+Au collisions at RHIC energies using a generalized three-dimensional McLerran-Venugopalan nuclear model with parametrized longitudinal correlations inside the nucleus. I will also discuss pA collisions using an advanced nuclear model with individual quark hot spots focusing on rapidity-dependent factorization breakdown and event plane fluctuations.

[1] Ipp, A., Leuthner, M., Müller, D. I., Schlichting, S., Schmidt, K., & Singh, P.
Energy-momentum tensor of the dilute (3+1)D Glasma. [Phys.Rev.D 109 (2024) 9, 094040]

Speaker: Kayran Schmidt

18:35 Perturbative high-energy evolution in the IP-Glasma initial state

We include the perturbative JIMWLK energy evolution into the IP-Glasma initial state description used to simulate the early-time dynamics in heavy ion collisions. By numerically solving the JIMWLK equation on an event-by-event basis, we obtain the energy (Bjorken-$x$) dependent structure of the colliding nuclei. This enables us to predict how the initial state evolves when moving from RHIC to LHC energies. [1]

Previously, the energy dependence in IP-Glasma was introduced through a Bjorken-$x$ dependent saturation scale extracted from fits to DIS structure data. In contrast, in our work we move beyond this parametrized approximation by providing a method to directly calculate the energy dependence. The energy-dependent initial state description is then coupled to relativistic hydrodynamics in the quark-gluon plasma phase and to the UrQMD hadronic afterburner, enabling us to simulate the full space-time evolution in heavy ion collisions. We demonstrate that incorporating the JIMWLK evolution significantly affects various flow observables.

The JIMWLK evolution applied in this work is constrained by the available $\mathrm{J}/\psi$ production data from HERA and from Ultra Peripheral Collisions at the LHC [2]. In addition to the JIMWLK-evolved initial state description, we determine an effective energy-dependent parametrization describing the energy-dependent nucleon structure, which can approximate the effect of JIMWLK evolution in IP-Glasma, enabling faster simulations.

References:
[1] H. Mäntysaari, B. Schenke, C. Shen, W. Zhao, in preparation.
[2] H. Mäntysaari, B. Schenke, F. Salazar, Phys.Rev.D 106 (2022) 7, 074019, arXiv:2207.03712 [hep-ph].

Speaker: Dr Heikki Mäntysaari (University of Jyväskylä)

16:35 → 18:55 Parallel session 5: Jets I

Convener: Yen-Jie Lee (Massachusetts Inst. of Technology (US))

16:35 Imaging the Jet-Induced Medium Response with Energy Correlators

In heavy-ion collisions, jets formed from hard-scattered partons experience an overall energy loss and have a modified internal structure compared to vacuum jets. These modifications are a result of the interactions between the energetic partons in a jet shower and the strongly coupled quark-gluon plasma (QGP). As the jet traverses the QGP, it loses momentum to the medium, which in turn responds to the presence of the jet. This “medium response” modifies the momentum distribution of the (soft) hadrons produced when the QGP freezes out. Since these hadrons carry the momentum lost by the parton shower, they and the modified shower both contribute to the energy flow in jets. The quantitative description of the medium response is an open question under active investigation. Recently, the projected N-point energy correlators (ENCs) have seen a resurgence of interest to probe vacuum QCD. For the first time [1], we will present a computation of the full three point energy-energy-energy correlation function in heavy-ion collisions and demonstrate its use for studying the shape of the energy flow originating from medium response. For this study, we utilize the Hybrid Model that implements a hydrodynamical medium response via the wake. We will show that measuring three-point correlation functions offer a promising experimental avenue for imaging the wake of the jet as when the three angles are well-separated the three-point correlator is dominated by the medium response.

[1] arXiv:2407.13818v2

Speaker: Arjun Srinivasan Kudinoor (Massachusetts Institute of Technology)

16:55 Tackling selection bias in heavy-ion jets with energy correlators

The first measurement of the two-point energy correlator (EEC) in Pb-Pb collisions has revealed its sizeable modifications with respect to the p-p baseline. Nevertheless, challenges arise in comparing these measurements due to selection bias caused by energy loss, which leads to a shift in the Pb-Pb EEC spectrum toward smaller angles as compared to the p-p measurement. In this talk, we will show that jets retain a memory of their original distribution before losing energy, allowing us to define a new EEC-based observable that corrects for the leading-order effects of this selection bias while keeping sensitivity to other medium effects. This represents the first-ever jet substructure observable whose medium modifications cannot be explained solely by selection bias and/or changes in quark/gluon fractions. Finally, we will extend this procedure to charged energy correlators, which correlate the energy flux carried by hadrons of different quantum numbers

Speaker: Jack Holguin

17:15 Initial stage jet momentum broadening in a light-front Hamiltonian approach

We study the momentum broadening of a high energy quark jet in the high-density gluon medium created right after the collision of two ultrarrelativistic heavy nuclei, the Glasma. Previous Glasma studies consider the jet as a classical probe particle, for which position and momentum are simultaneously determined. In this talk, we use the light-front QCD Hamiltonian formalism to treat the jet as a fully quantum state and compute its real-time evolution while propagating through the Glasma classical background fields, that appear as an interaction potential in the quantum evolution of the jet. We present results for the momentum broadening and jet quenching parameter $\hat{q}$ experimented by a jet at approximately mid-rapidity. We then pay special attention to the anisotropies in the momentum broadening between the longitudinal and transverse directions with respect to the collision axis. In opposition to the classical jet analyses, we find that, as the momentum modes of the boost-invariant Glasma are distributed mainly along the transverse direction, the momentum broadening is larger in the transverse than in the longitudinal direction.

Speaker: Carlos Lamas (IGFAE-USC)

17:35 Measuring energy-energy correlators in p-Pb collisions at ALICE

Jet substructure is a powerful tool for testing QCD in elementary particle collisions. The two-point energy-energy correlator (EEC), defined as the energy-weighted cross section of particle pairs inside jets, is a novel jet substructure observable probing the correlation of energy flow within jets. In pp collisions, the angular dependence of the EEC cross section shows a distinct separation of the perturbative and non-perturbative regimes, revealing the partonic dynamics of jet formation and the confinement of partons into hadrons. There have been an increasing number of theoretical studies predicting how the presence of a medium modifies the EECs with respect to the vacuum case. In this talk, we present measurements of the EECs for inclusive jets in p-Pb collisions at 5.02 TeV. By comparing our results to a measured pp baseline, we study the modifications of jet evolution caused by interactions with a cold nuclear medium and the presence of a high multiplicity environment. Understanding modifications due to cold nuclear matter effects is vital in interpreting EEC results in Pb$-$Pb collisions and separating hot and cold nuclear effects.

Speaker: Anjali Nambrath (University of California Berkeley (US))

17:55 Probing jet modification in the QGP using N-Point Energy Correlators in Pb-Pb collisions with ALICE

In heavy-ion collisions, jets propagating through the quark-gluon plasma undergo interactions with the medium. These interactions modify the internal structure of jets, making jet substructure an invaluable tool for probing the microscopic properties of the QGP. N-point Energy Correlators, defined as energy-weighted N-particle correlations inside jets, have emerged as a novel substructure observable in proton-proton collisions, where they show a clear separation of perturbative and non-perturbative effects. Recent measurements in Pb-Pb and p-Pb collisions show striking modifications of the Energy-Energy Correlators (EECs) compared to vacuum QCD baselines.

In this talk we present a measurement of the EECs in Pb-Pb collisions, presenting results at low jet transverse momentum for the first time. Such a measurement is essential in mapping jet modification across different kinematic regimes in the QGP. In addition we present the first measurements of the 3-point Energy Correlator in Pb-Pb collisions, along with their ratios to the EECs. This higher order correlator is predicted to have an enhanced sensitivity to medium effects such as the medium response and offers unique insights into the interactions of the jet and the plasma.

Speaker: Ananya Rai (Yale University (US))

18:15 Measurement of flavor dependence of jet quenching in heavy ion collisions with ATLAS

High-energy partons are known to lose energy when passing through the hot and dense medium produced in heavy ion collisions. This energy loss is expected to depend on the mass and flavor of the fragmenting parton. For light partons, energy loss via gluon bremsstrahlung is expected to dominate, while for heavy quarks, collisional energy loss may play a more important role. Comparisons between the suppression observed in the inclusive sample of jets with the suppression of b-tagged jets and jets produced at forward rapidity (quark enhances sample of jets) are needed to understand the impact of parton mass and parton flavor on energy loss. Tagging of the $b$-jets is achieved using neural networks trained on Monte Carlo simulation samples and validated in a data-driven way with an independent approach involving the detection of muons from semileptonic decays of B-hadrons. A detailed quantification of the differences between the $b$-jet $R_\mathrm{AA}$ and the inclusive jet $R_\mathrm{AA}$ is also provided, together with comparisons to theory. The measurements are performed in $pp$ and Pb+Pb collisions at $\sqrt{s_\mathrm{NN}}$ = 5.02 TeV using the ATLAS detector at the LHC.

Speaker: Dominik Karol Derendarz (Polish Academy of Sciences (PL))

18:35 Search for jet-induced diffusion wake in heavy ion collisions using photon-jet and di-jet events with the ATLAS detector

In heavy-ion collisions, hard-scattered partons lose energy as they traverse the medium, which is simultaneously modified by this interaction. One expected medium modification, known as the "diffusion wake," is a particle enhancement in the direction of the parton and a depletion in the opposite direction. Although theoretically predicted, clear experimental evidence of the diffusion wake remains elusive. This talk presents jet-track correlations in photon-jet and di-jet events measured in 5.02 TeV Pb+Pb collisions with the ATLAS detector to search for diffusion wake signals. While photon-jet events do not show a significant signal within current uncertainties, upper limits on the probability are provided, and the results are consistent with CoLBT theory predictions at the 68% confidence level. New measurements from di-jet events are also presented, offering higher statistical precision, though complicated by potential contamination from the convolution of wakes of the jet pair.

Speaker: Soumya Mohapatra (Columbia University (US))

19:00 → 21:00 Welcome reception

Tuesday 8 April

08:40 → 10:20 Parallel session 11: Chirality II

Convener: Gergely Endrodi

08:40 Search for the Chiral Magnetic Wave at STAR with Isobar and Au+Au collisions

The chiral magnetic wave (CMW) in heavy-ion collisions probes the topological sector of quantum chromodynamics, where parity and charge-parity symmetries are locally violated. Unlike the Chiral Magnetic Effect (CME), where the induced dipole moment fluctuates randomly from event to event, the CMW generates an electric quadrupole moment with a same sign for a given net charge state of the system, thus CMW can be observed even when CME is not detected. The CMW leads to charge-dependent elliptic flow ($v_2$) which correlates with charge asymmetry ($A_{ch}$) of the system.
Experimental searches for the CMW, however, face challenges from background contributions related to elliptic flow and nonflow.
The correlation between triangular flow $v_3$ and $A_{ch}$ will be discussed as a background indicator.
Similar to the CME search, we use RHIC isobar collisions ($^{96}_{44}\rm Ru+^{96}_{44}Ru$ and $^{96}_{40}\rm Zr+^{96}_{40}Zr$) at STAR for the CMW search, anticipating a stronger signal in $^{96}_{44}\rm Ru+^{96}_{44}\rm Ru$ due to its larger magnetic field.
In addition, we compare with Au+Au to study the system dependence at the top RHIC energy, examining the magnetic field effects from isobar to Au+Au.
The prerequisite of the CMW is expected to change with collision energy.
We present the measurement of slope $ r_2$ of $v_2$ vs. $A_{ch}$ correlation in isobar collisions and Au+Au collisions with BES-II data at $\sqrt{s_{NN}} =$ 7.7, 19, 27 and 200 GeV.
We use the spectator planes from the Zero Degree Calorimeter (ZDC) at $\sqrt{s_{NN}} =$ 200 GeV and from the Event Plane Detector (EPD) at lower beam energies to determine the magnetic field direction. The large pseudorapidity gap between the particles of interest and the EPD or ZDC helps minimize nonflow contributions.
We will also explore the feasibility of the event-shape selection scheme to subtract the flow-related background in CMW search.

Speaker: Ankita Nain

09:00 Global quark spin correlations in relativistic heavy ion collisions

The observation of the vector meson's global spin alignment by the STAR Collaboration reveals that strong spin correlations may exist for quarks and antiquarks in relativistic heavy-ion collisions in the normal direction of the reaction plane. We propose a systematic method to describe such correlations in the quark matter. The correlations can be classified as local and long range types. We show in particular that the effective quark spin correlations contain the genuine spin correlations originated directly from the dynamical process as well as those induced by averaging over other degrees of freedom. We also show that such correlations can be studied by measuring the vector meson's spin density matrix and hyperon-hyperon and hyperon-anti-hyperon spin correlations. We present the relationships between these measurable quantities and spin correlations of quarks and antiquarks.

Speaker: Prof. Qun Wang (University of Science and Technology of China)

09:20 Vortex Rings in Event-by-Event Relativistic Heavy-Ion Collisions

The appearance of vortical structures originating from gradients is a ubiquitous signature of collective fluid dynamic motion. We present event-by-event simulations for central asymmetric light+heavy and Au+Au collisions to investigate the formation and evolution of vortex-ring structures in the longitudinal flow velocity profile [1]. The production-plane polarization of $\Lambda$ hyperons, defined with respect to the $\Lambda$'s momentum and the beam, can track the "vortex ring" feature in the event, a characteristic vortical structure generated by longitudinal flow gradients. We make comprehensive model predictions for the rapidity-dependent vortex-ring observables for different collision system sizes at $\sqrt{s_\mathrm{NN}} =$ 200 and 72 GeV. Our predictions at the latter energy can be explored in the future LHCb fixed-target experiment (SMOG2) at the Large Hadron Collider. We further make hydrodynamic model comparisons with the preliminary CMS hyperon polarization measurements in p+Pb collisions at 8.16 TeV.

[1] D. Dobrigkeit Chinellato, M. A. Lisa, W. Matioli Serenone, C. Shen, J. Takahashi and G. Torrieri, "Vortex Rings in Event-by-Event Relativistic Heavy-Ion Collisions," arXiv:2407.02212 [nucl-th]

Speaker: David Dobrigkeit Chinellato (Austrian Academy of Sciences (AT))

09:40 Baryon Electric Charge Correlation as a Magnetometer of QCD

Although strong magnetic fields are expected in off-central heavy ion collisions, directly measuring their imprints, such as chiral magnetic effects, remains challenging. In this talk, we propose baryon electric charge correlation, $\chi^{\rm BQ}_{11}$, as well as the chemical potential ratio, $\mu_{\rm Q}/\mu_{\rm B}$, as useful probes for the detection of magnetic fields in heavy ion collisions [1]. This is based on (2+1)-flavor lattice QCD simulations at the physical pion mass. Along the transition line, $\chi^{\rm BQ}_{11}$ and $\mu_{\rm Q}/\mu_{\rm B}$ for Pb+Pb collisions increase by factors of 2.1 and 2.4, respectively, at a magnetic field strength of $eB\simeq 8m_{\pi}^2$. The latter also varies across different collision systems, increasing by a factor of 1.5 from Zr+Zr to Ru+Ru isobar collisions. We will also discuss possible experimental analyses which could be carried out across various centrality classes or in different collision systems exhibiting distinct $eB$ values.

[1] Heng-Tong Ding, Jin-Biao Gu, Arpith Kumar, Sheng-Tai Li and Jun-Hong Liu, Phys. Rev. Lett. 132, 201903(2024)

Speaker: Arpith Kumar (Central China Normal University)

10:00 Dissipative contributions to spin polarization

The observation of global and local spin polarization in heavy-ion collisions is explained as being induced by thermodynamics gradients of the QGP at local thermal equilibrium, such as the thermal vorticity and the thermal shear. However, since not all the features of the measurements can be well reproduced by the models, the assumption that the spin degrees of freedom of the QGP are equilibrated can be questioned. In this talk, I will derive the dissipative, out-of-equilibrium contributions to spin polarization at first order in gradients using the Zubarev method for spin hydrodynamics. I will prove that the thermal shear can not give rise to dissipative corrections to spin polarization. I will derive momentum dependent kubo formulas that gives the intensity of each contributions and that can be evaluated for any underlying microscopic theory. At the end I will discuss consequences on the interpretation of the measurements.

Speaker: Matteo Buzzegoli (West University of Timisoara)

08:40 → 10:20 Parallel session 12: Collective dynamics & small systems I

Convener: Ulrich Heinz

08:40 Measurement of directed flow of $K^{*0}$ and $\phi$ Resonances in Au+Au collisions at RHIC BES energies

It is known from earlier studies that the hadronic interaction affects the measured yield of short lived resonance e.g. $K^{*0}$. Recent studies [1] show that the directed flow ($v_{1}$) of $K^{*0}$ is strongly affected during the hadronic stage due to asymmetric loss in different sides of the $p_{x}$ axis in momentum space caused by the tilted fireball and density dependent rescattering. Therefore the $v_{1}$ of $K^{*0}$ can be a good probe to study the late-stage hadronic interaction and provide more information (e.g rescattering affected by variations in matter density across phase-space) compared to the yield measurement integrated over full azimuth and rapidity.

We present the first measurement of directed flow of $K^{*0}$ resonances in Au+Au collisions at $\sqrt{s_{NN}}$ = 14.6, 19.6 and 27 GeV. Centrality-dependent difference in directed flow between charged kaons and $K^{*0}$ resonances will be presented. The difference in directed flow between charged kaons and $\phi$ mesons will be also shown. The results will be compared with a hydrodynamic model that incorporates a hadronic afterburner [1]. The measured $v_{1}$ of $K^{*0}$ resonances can provide more insights into hadronic rescattering and regeneration processes in heavy-ion collisions and can constrain transport-based models of QCD matter.

[1] Phys. Rev. C 109, 044905 (2024)

Speaker: Md. Nasim (University of California, Los Angeles)

09:00 Recent flow and correlation measurements from ATLAS in $pp$, $p$+Pb and Pb+Pb collisions

This talk presents recent ATLAS measurements investigating correlations between the 'ridge' - azimuthal correlations between particles in the underlying event that extend over all rapidities - and hard scattering processes in $pp$ and $p$+Pb collisions. To study this soft-hard correlation, measurements of two-particle correlations in with two different particle-pair selections are studied. In the first case, charged particles associated with jets are excluded from the correlation analysis. In the second, correlations are measured between particles within jets and charged particles from the underlying event. In $pp$ collisions, excluding jet-associated particles does not affect the ridge, and no significant azimuthal correlations are found between jet-associated particles and the underlying event, ruling out hard-process contributions to the $pp$ ridge. New measurements extending this analysis to $p$+Pb collisions are also presented. Prior $p$+Pb flow measurements have indicated a small but significant high-$p_{\mathrm{T}}$ $v_2$ up to a $p_{\mathrm{T}}$ of 50 GeV. This is in contrast to suppression measurements which indicate no clear suppression of high-$p_{\mathrm{T}}$ hadrons. These jet-tagged measurements can thus provide insight into the origin of the high-$p_{\mathrm{T}}$ $v_2$ observed in $p$+Pb collisions.

Speaker: Tomasz Bold (AGH University of Krakow (PL))

09:20 Long-range near-side correlation in e+e- collisions at 183-209 GeV with ALEPH archived data

The first measurement of two-particle angular correlations for charged particles with LEP-II data is presented. The study is performed using archived hadronic $e^+e^-$ data collected by ALEPH at center-of-mass energies up to 209 GeV, above the $WW$ production threshold, which provide access to unprecedented charged-particle multiplicities and more complex color-string configurations if compared to previous measurements at LEP-I energies. An intriguing long-range near-side excess is observed in the correlation function measured with respect to the thrust axis in the highest multiplicity interval ($\rm N_{\mathrm{trk}}\geq 50$). Such a structure is not predicted by the Monte-Carlo simulation. The harmonic anisotropy coefficients $v_n$, which result from the Fourier expansion of the two-particle correlation functions, were also measured for the first time in $e^+e^-$ data, and compared to PYTHIA6 predictions and to the results obtained in proton-proton collisions. The results presented in this talk provide novel experimental constraints on the formation of collective phenomena in point-like $e^+e^-$ collisions.

Speaker: Michael Joseph Peters (Massachusetts Inst. of Technology (US))

09:40 What happens in hydrodynamic simulations of heavy-ion collisions when causality is violated?

It was recently discovered that Israel-Stewart theory is not as causal as we thought [1]. When a fluid expands too fast, as in heavy ion simulations at early times, the large viscous stresses modify the propagation speeds, leading to superluminal signaling [2]. Since we would not trust hydrodynamic information speeds anyway, this may not sound like such disconcerting news, were it not for some recent theorems that link causality violations to instabilities due to time-reversed dissipation [3]. In this talk, I will discuss the physical implications of early-time causality violations in heavy-ion simulations, and how current state-of-the-art hydrodynamic solvers such as MUSIC deal with the expected early-time pathologies [4]. Particular attention will be devoted to the following pressing question: How can hydrodynamic simulations of heavy-ion collisions be seemingly stable when theory indicates they should not be?

[1] Bemfica, Disconzi, Hoang, Noronha, Radosz; Phys. Rev. Lett. 126 (2021) 22, 222301
[2] Plumberg, Almaalol, Dore, Noronha, Noronha-Hostler; Phys. Rev. C 105 (2022) L061901
[3] Gavassino; Phys. Rev. X 12 (2022) 4, 041001
[4] Gavassino, Soares-Rocha, Singh, Paquet; to appear.

Speaker: Lorenzo Gavassino

10:00 EPJ Featured Talk: Transport Coefficients from pQCD to the Hadron Resonance Gas at finite BSQ densities

We present calculations of the shear viscosity in two limits using perturbative QCD and an excluded-volume hadron resonance gas at finite BSQ densities. We then develop a framework that interpolates these two limits to calculate shear viscosity across a wide range of finite BSQ densities. The pQCD and hadron resonance gas calculations have different BSQ densities dependence, leading to a non-trivial shear viscosity at finite densities. In addition, we extend the next-to-leading order weak-coupling shear viscosity of QCD to finite baryon chemical potential μ, and we show that the convergence is better at μ > T than at μ = 0. Finally, we present leading order calculations of flavor diffusion for weak-coupling QCD at finite baryon chemical potential μ for three flavors.

Speaker: Ms Isabella Danhoni

08:40 → 10:20 Parallel session 13: Heavy flavor & quarkonia II

Convener: Pol-Bernard GOSSIAUX

08:40 Heavy flavor correlations and Quarkonia production in high energy proton-proton collisions in the EPOS4HQ framework

A heavy quark-antiquark ($Q\bar Q$) pair can be produced in several pQCD processes, which impose different correlations between the $Q$ and $\bar Q$. Employing the recently advanced EPOS4HQ event generator, which contains these processes, we show that they explain the measured $D$$\bar D$ and $DD$ correlations and how they influence the $p_T$ distributions of open heavy flavor mesons in pp and AA collisions. It turns out that this influence is different for D and B mesons.

These correlations have also an influence on hidden heavy flavour mesons produced in pp collisions. To explore them we present a model of quarkonia formation based on the convolution between the Wigner densities, associated to the quarkonia eigenstates (excited states up to 3S states), and the Wigner density of the two-body $Q \bar Q$ pairs produced those collisions. This model allows to predict quantitatively the rapidity and $p_T$ distribution of ground states and excited states of charmonia as well as of bottomonia in pp collisions. We compare the results with experiments and identify the influence of the correlations. Finally, we extend the density matrix approach to study the production of $B_c$ mesons and compare our results with experimental data.

Speaker: Dr Jiaxing Zhao (Subatech)

09:00 A comprehensive study of open heavy flavor transport and hadronization in heavy-ion collisions

Heavy quarks serve as a clean probe of the quark-gluon plasma (QGP) produced in high-energy nuclear collisions. They are primarily generated from early-stage hard scatterings and retain information about the entire evolution of the QGP fireball. We have developed a state-of-the-art model to provide a comprehensive description of the heavy quark evolution in a realistic QGP medium from the heavy quark's production in the initial stages of the collision all the way to hadronic freeze-out.

The transport of heavy quarks in the QGP medium is described by a Langevin-based transport model that has been augmented to incorporate medium-induced radiation. This transport model is coupled to a bulk evolution model based on 2+1D relativistic viscous fluid dynamics. The heavy quark transport coefficients are derived from non-perturbative T-matrix calculations, which account for resonant correlations near the QGP transition temperature. Hadronization of the heavy quark is described by a fragmentation plus coalescence model. We utilize the resonance recombination model that satisfies energy conservation and provides an equilibrium mapping between quark and meson distributions. The recombination probabilities are derived from resonant heavy-quark scattering rates.

We report on key observables in open heavy flavor physics, including the nuclear modification factor, elliptic flow, baryon-to-meson ratio, as well as D-Dbar angular correlations and compare our results to experimental data from the ALICE collaboration in Pb-Pb collisions at 5 TeV.

Speaker: Steffen Bass (Duke University)

09:20 Top-quark pair production in Pb+Pb collisions in the ATLAS experiment

In relativistic heavy-ion collisions, top quarks are expected to be attractive candidates for probing the quark-gluon plasma as well as to bring unique information about the time evolution of strongly interacting matter. We report the first study of top-quark pair production in lead-lead collisions at the centre-of-mass energy of 5.02 TeV with the ATLAS experiment at the LHC. The dataset was recorded in 2015 and 2018, amounting to an integrated luminosity of $1.9~\mathrm{nb}^{-1}$. Top-quark pair production cross section is studied in the eµ channel. The result is compared to theory predictions based on different nuclear PDF sets.

Speaker: Patrycja Anna Potepa (AGH University of Krakow (PL))

09:40 Prompt/Non-prompt J/ψ production in pp collisions at forward and midrapidity with ALICE

Quarkonium production in high-energy hadronic collisions is sensitive to both perturbative and non-perturbative aspects of quantum chromodynamics (QCD) calculations. Charmonium cross section can be split into prompt and non-prompt components, the first corresponding to directly produced charm-anticharm pairs, the second originating from the decay of beauty hadrons. Both components are relevant for the investigation of the properties of the quark–gluon plasma (QGP), with the latter allowing a study the mass dependence of heavy-quarks in-medium energy-loss mechanism. In this contribution the recent measurement of prompt and non-prompt $\rm{J}/\psi$ carried out by the ALICE Collaboration in pp and Pb–Pb collisions at midrapidity (|y| < 0.8) will be shown, including the newest results from LHC Run 3. Moreover, thanks to the installation of the new muon forward tracker (MFT), prompt/non-prompt charmonium separation is now possible in LHC Run 3 at forward rapidity (2.5 < y < 3.6). Using pp collisions at $\sqrt{s} = 13.6$ TeV, performances for the prompt and non-prompt $\rm{J}/\psi$ separation at forward rapidity will be discussed. For the first time, preliminary results on the prompt/non-prompt $\rm{J}/\psi$ fraction will be presented.

Speaker: Shreyasi Acharya (Universita e INFN, Bari (IT))

10:00 Studying charm-quark hadronization via charm-baryon production measurements in pp at the LHC with ALICE

Charm-baryon production measurements in proton-proton (pp) collisions at the LHC are fundamental to investigate the charm-quark hadronization, and to test perturbative QCD-based calculations. Recent measurements in pp collisions show baryon-to-meson ratios significantly higher than those in $\mathrm{e^+e^-}$ collisions, challenging the validity of theoretical calculations based on the factorisation approach and assuming universal fragmentation functions across collision systems. Several QCD-inspired effective models (e.g. Catania, POWLANG, QCM) and Monte Carlo generators (e.g. PYTHIA 8, EPOS 4) take different approaches to describe the charm-quark hadronization, and to explain the observed baryon production at the LHC. However, most of them do not manage to describe simultaneously the production of strange and non-strange charm baryons. Precise measurements of strange and non-strange charm-baryon production are crucial to put constraints on model calculations, and to understand the mechanisms governing the charm-quark hadronization in pp collisions at the LHC.

In this contribution, the first measurement of strange ($\Xi_\mathrm{c}^{0,+}$) and non-strange ($\Lambda_\mathrm{c}^+$, $\Sigma_\mathrm{c}^{0,++}(2455)$, $\Sigma_\mathrm{c}^{0,++}(2520)$) charm-baryon production utilising the large data sample of pp collisions at $\sqrt{s} = 13.6$ TeV harvested from the start of LHC Run 3 are presented, and the comparison with model predictions are discussed.

Speaker: Himanshu Sharma (Universita e INFN, Padova (IT))

08:40 → 10:20 Parallel session 14: Jets II

Convener: Urs Wiedemann (CERN)

08:40 sPHENIX measurement of dijet correlations in p+p collisions over a large kinematic range

The sPHENIX experiment is a next-generation collider detector at RHIC designed for rare jet and heavy flavor probes of the Quark-Gluon Plasma. The experiment includes large-acceptance, hermetic electromagnetic (EMCal) and hadronic (HCal) calorimeter systems, along with a very high-rate data acquisition plus trigger system. In RHIC Run-24, sPHENIX sampled 107/pb of p+p collision data at 200 GeV using an efficient high-p$_T$ jet trigger. This dataset represents a major increase in the luminosity times acceptance compared to previous measurements for this collision energy, along with the first HCal at mid-rapidity at RHIC. This talk presents a measurement of dijet p$_T$ asymmetries and azimuthal acoplanarities, over a wide range of leading jet p$_T$, performed with the full sPHENIX calorimeter system. These measurements provide the pQCD baseline for measurements of dijet p$_T$ asymmetries and angular decorrelation in future sPHENIX Au+Au data-taking at RHIC. The tighter initial kinematic correlations at RHIC compared to the LHC are expected to greatly increase the sensitivity to physics effects.

Speaker: Virginia Bailey (Georgia State University)

09:00 3D structure of jet-induced diffusion wake in heavy-ion collisions

The diffusion wake accompanying the jet-induced Mach cone provides a unique probe of the properties of quark-gluon plasma in high-energy heavy-ion collisions. It can be characterized by a depletion of soft hadrons in the opposite direction of the propagating jet. We explore the 3D structure of the diffusion wake induced by γ-triggered jets in Pb+Pb collisions at the LHC energy within the coupled linear Boltzmann transport and hydro model. We identify a valley structure caused by the diffusion wake on top of a ridge from the initial multiple parton interaction (MPI) in jet-hadron correlation as a function of rapidity and azimuthal angle. This leads to a double-peak structure in the rapidity distribution of soft hadrons in the opposite direction of the jets as an unambiguous signal of the diffusion wake. The diffusion wake valley is found to deepen with the jet energy loss as characterized by the γ-jet asymmetry. Its sensitivity to the equation of state and shear viscosity is also studied.

Speaker: Dr Xin-Nian Wang (Lawrence Berkeley National Lab. (US))

09:20 Color Coherence on Recoils

In the last years, it has become clearer that recoiling particles originated due to elastic scatterings between the energetic partons within a jet and the medium constituents within the QGP are of great importance for jet quenching phenomenology. Current implementations in state-of-the-art models consider that the elastic scattering process takes place between an on-shell energetic parton and the medium constituent. Crucially, however, highly energetic partons produced during jet evolution cannot be considered in general as on-shell states coming from infinity, and their correct description is in terms of a collection of color correlated dipoles. In this talk we present the first computation of the properties of the soft recoils produced as a result of perturbative elastic scatterings between a color coherent dipole and a quark medium constituent.

Our findings [1] indicate that the angular phase space of these soft recoiling quarks is tightly constrained by the opening angle of the color coherent dipole, owing to quantum interference effects. In this long wavelength limit, one can effectively consider that interactions take place with each of the legs of the dipole separately, provided that the angular constraints dictated by the corresponding color flow topologies are respected. This is in complete analogy with the case of soft gluon emission in vacuum, where the recoil quark plays the role of the emitted gluon. As a direct phenomenological application we provide the first estimate of the collisional energy loss rate of a color antenna. Importantly, these results indicate the way in which color coherence effects can be implemented in jet quenching models that account for the recoils from elastic scatterings, improving in this way our description of medium response physics.

[1] arXiv:2406.08550v2

Speaker: Daniel Pablos (IGFAE, USC)

09:40 Probing the evolution of the charm-quark dead cone and splitting function with ALICE

The flavour dependence of parton showers presents a unique opportunity to explore fundamental properties of QCD interactions. In particular, the substructure of low-energy jets containing heavy-flavour hadrons provides experimental access to both mass effects as well as the impact of Casimir colour factors. Using novel tracing methods, splittings of charm quarks are isolated to measure fundamental properties such as the dead-cone effect and the charm-quark splitting function. By using the new high statistics data samples of Run 3, we present new measurements of the dead-cone effect and splitting function of charm quarks, differentially in the energy of the emitter, through measurements of jets tagged with a reconstructed $\rm{D}^{0}$ meson. At the low energies uniquely accessible to the ALICE experiment, comparisons of these measurements to an inclusive-jet baseline probe the evolution of the dead-cone of charm quarks with unprecedented accuracy. With increasing energy of the emitting charm quark, the dead-cone angle is predicted to get smaller, allowing for the isolation of the impact of Casimir colour factors. Such differential measurements in the charm sector not only illuminate the role of mass and Casimir colour factors when compared to inclusive jets, but also provide a lower mass heavy quark-enriched sample which is crucial for comparisons to measurements in the beauty sector, in order to isolate the mass dependence of the dead-cone effect.

Speaker: Nima Zardoshti (CERN)

10:00 Measurement of flavor effects in charm-induced jet showers at the perturbative/non-perturbative boundary with ALICE

Jet substructure provides precise tests of Quantum Chromodynamics (QCD) and offers more differential ways to study hadronization mechanisms, compared to measurements of single hadrons. QCD predicts that jet radiation patterns depend on the mass and color charge of the initiating parton. Parton showers, in particular, are sensitive to the different Casimir factors of quarks and gluons, as well as to the mass of partons through the dead-cone effect.

In this talk we present three key charm-tagged jet measurements, using jets tagged with a reconstructed $\rm{D^{0}}$-meson. The first are measurements of the differences between three different axis definitions inside the jets. These different definitions vary in their sensitivity to wide-angle soft radiation and systematically probe the radiation pattern off of charm quarks. Next are the jet angularities, which characterise the weighted angular distribution of the jet constituents. These powerful observables probe the parton shower, with sensitivity to mass and Casimir effects that can be tuned by adjusting the importance of wide-angle radiation. Finally, we present the first measurement of the energy-energy correlator for $\rm{D^{0}}$-meson tagged jets. This observable is defined as the energy-weighted cross section of particle pairs within jets and reveals the detailed structure of QCD radiation from heavy quarks, allowing for a distinction between perturbative and non-perturbative effects in charm showers. Comparisons of all these results to measurements of dominantly gluon-initiated jets, as well as to various event generators, are sensitive to flavor-dependent effects in both the parton shower and hadronization mechanisms. Moreover, comparisons with next-to-leading order calculations emphasize the need for improved theoretical modelling of heavy-quark jets and provide critical insights into the parton-to-hadron transition in QCD.

Speaker: Preeti Dhankher (University of California Berkeley (US))

08:40 → 10:20 Parallel session 15: Physics of ultraperipheral collisions I

Convener: Andre Govinda Stahl (CERN)

08:40 New probes of nuclear gluon dynamics through photoproduction of charm in inelastic ultra-peripheral Pb–Pb collisions with ALICE

In an ultra-peripheral collision, where the impact parameter is larger than the sum of the nuclear radii, the nuclei mainly interact electromagnetically. These reactions are mediated by virtual photons. A photon can interact with a gluon in the target nucleus and produce a pair of charm quarks. These charm quarks then fragment and are observed as open charm hadrons ($\rm D^{0}$, $\rm D^{∗} (2010)^{\pm}$ etc.) or vector mesons (J/ψ and ψ(2S)). This process has been used in e-p collisions to set stringent limits on the proton gluon distribution at low-$x$. The current measurements can provide similar constraints on the much less known nuclear gluon distributions. The cross sections and transverse momentum distributions for D mesons and J/ψ measured in ALICE will be presented. The results will be compared with model calculations.

Speaker: Sigurd Nese (University of Oslo (NO))

09:00 EPJ Featured Talk: Investigating subnucleonic structures via new measurements of incoherent J/ψ photoproduction in ultra-peripheral Pb–Pb collisions with ALICE

According to quantum chromodynamics, at high energy, hadrons exhibit a dynamic equilibrium between gluon splitting and recombination, known as saturation. Diffractive photonuclear production of J/ψ vector mesons provides unique insights into the gluon distribution of hadrons. The Mandelstam-t variable, representing the momentum transfer, probes hadron structure within the impact-parameter plane, with different t ranges sensitive to gluon field dynamics at various spatial scales. In this talk, we will present new ALICE measurements on the energy dependence of incoherent photonuclear production of J/ψ mesons off lead ions across Mandelstam-$t$ intervals, testing gluon saturation models in an unprecedented way.

Speaker: Vendulka Filova (Czech Technical University in Prague (CZ))

09:20 Measurements of $J/\psi$ and impact of coincident photon induced processes in ultra-peripheral Pb+Pb collisions at $\sqrt{s_\mathrm{NN}}=5.36$ TeV with the ATLAS detector

The fully stripped ions used in heavy ion collisions at the LHC are an excellent source of high-energy quasi-real photons. These can interact with photons emitted by the oncoming nucleus, or with the nucleus itself, either directly in inelastic processes or diffractively via pomeron exchange. Diffractive photonuclear processes can produce exclusive vector mesons that are uniquely sensitive to the spatial and momentum structure of the nuclear parton distribution functions, as well as spatial fluctuations (hotspots). In Run 3, the ATLAS experiment utilized a low-multiplicity track trigger in heavy ions for the first time, allowing the collection of a large sample of events with a few tracks. A substantial fraction of these are sensitive to dilepton decays of vector mesons, including the $J/\psi$. First results on $J/\psi$ yields, in association with various topologies of forward neutron emission will be presented using the widest continuous rapidity range available at the LHC. Discussion of important backgrounds from dissociative and multiple-UPC processes will also be provided.

Speaker: Mateusz Dyndal (AGH University of Krakow)

09:40 Measurement of dijet production, strange particle production, and bulk properties in photonuclear collisions with ATLAS

In ultra-relativistic heavy ion collisions, the charged ions produce an intense flux of equivalent photons. Photon-induced processes are the dominant interaction mechanism when the colliding nuclei have an impact parameter larger than the nuclear diameter. In these ultra-peripheral collisions (UPCs), the photon provides a clean, energetic probe of the partonic structure of the nucleus, analogous to deep inelastic scattering. This talk presents a measurement of jet production in UPCs performed with the ATLAS detector using high-statistics 2018 Pb+Pb data. Events are selected using requirements on jet production, rapidity gaps, and forward neutron emission to identify inclusive photo-nuclear hard-scattering processes. These measurements also include detailed studies of rapidity gap distributions and nuclear break-up effects, allowing for precise comparisons between data and theory for inclusive photo-nuclear processes. The measured cross-sections are compared to theoretical models in phase-space regions where significant nuclear PDF modifications are expected but not well constrained by world data, demonstrating the potential of these data to provide a strong new constraint on nPDF effects.

Speaker: Benjamin Jacob Gilbert (Lawrence Livermore Nat. Laboratory (US))

10:00 Neutron emission from the photon-induced reactions in UPC of heavy ions - impact of new ALICE data

The ultraperipheral collisions are a source of various interesting phenomena based on photon-induced reactions. We calculate cross sections for single and any number of n, p, $\alpha$, $\gamma$ in ultraperipheral heavy-ion collision for LHC energies emitted in forward/backward directions. We analyze the production of a given number of neutrons relevant for a recent ALICE experiment [1] using zero degree calorimeters, for $\sqrt{s_{NN}}$ = 5.02 TeV. In our approach, we include both single and multiple photon exchanges as well as the fact that not all photon energies are used in the process of equilibration of the residual nucleus. We propose a simple two-component model in which $E_{exc} \neq E_{\gamma}$ and compare its results with the results of HIPSE and EMPIRE codes. We discuss the role of preequilibrium and equilibrium processes. In addition we analyze the role of scattering of high-energy photons for small neutron multiplicities. Emission of the small number of neutrons at high (virtual) photon energies ($E_{\gamma} >$ 1 GeV) seems to be crucial to understand the new ALICE data. All effects work in the desired direction, but the description of the cross section for four- and five-neutron emission is rather difficult. Several open issues will be discussed.

[1] S. Acharya et al. (ALICE colaboration),
Phys. Rev. C107, 064902 (2023).

Speaker: Antoni Szczurek

10:20 → 10:50 Coffee break

10:50 → 12:30 Parallel session 16: Collective dynamics & small systems II

Convener: Charles Gale

10:50 Quarkonia collectivity in large collision systems with ALICE

Quarkonium production has long been identified as one of the golden probes to study the quark-gluon plasma (QGP). Among many observables, the measurement of azimuthal anisotropies in quarkonium production has a special role to shed light on the collective behavior of particles in a strongly interacting medium. In particular, the magnitude of the $\rm{J}/\psi$ elliptic flow measured at the LHC in Pb-Pb collisions is interpreted as a signature of the charm-quark thermalization in the QGP, supporting the scenario of charmonium (re)generation at low $p_{\rm T}$. Interestingly, the observation of collective-like effects in high-multiplicity pp and p-Pb collisions raises questions about the minimal conditions needed for QGP formation. In this contribution, measurements of quarkonium flow coefficients from small (pp) to large (Pb–Pb) collision systems carried out by the ALICE collaboration will be presented. New results on $\rm{J}/\psi$ $v_{2}$ in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.36 TeV using event-plane and scalar product methods as a function of $p_{\rm T}$ and rapidity will be discussed. In addition, multi-particle cumulant $\rm{J}/\psi$ $v_{2}${4} will be shown, allowing further insight into charm quark thermalization into the QGP and, for the first time, access to flow fluctuations.

Speaker: Chi Zhang (Université Paris-Saclay (FR))

11:10 Latest Anisotropic Flow Measurements in Pb–Pb Collisions with ALICE: New Insights into QGP Properties

Characterizing the quark--gluon plasma (QGP) created in ultrarelativistic heavy-ion collisions requires identifying observables sensitive to different stages of the collision. Recent studies show that measuring collective anisotropic flow and the complex interplay between its Fourier harmonics ($v_n$) and $(\Psi_n$) can effectively disentangle contributions from various collision phases, providing insights into QGP transport properties and the initial state of the system.

In this talk, the latest flow measurements from ALICE for Pb–Pb collisions at the highest center-of-mass collision energy, $\sqrt{s_{\rm NN}}$ = 5.36 TeV, ever achieved in a laboratory are presented. This includes the flow coefficient $v_n$, and mixed harmonic cumulants. Added to this are the first measurements of multiharmonic correlations using symmetric cumulants for hexagonal flow harmonics and new symmetry plane correlators, totaling 25 correlators—16 of which are newly introduced with enhanced estimators. Differential results for selected observables are also provided as a function of kinematic variables, alongside standard centrality dependence. In particular, $p_{\rm T}$-differential flow is further extended to extremely high-$p_{\rm T}$ regions, providing new inputs for hard processes. These results are compared with state-of-the-art hydrodynamic calculations, offering unique insights for future model developments and further constraining the properties of QCD matter in heavy-ion collisions. Furthermore, multi-particle elliptic flow measurements in ultra-central Pb–Pb collisions will be discussed, evaluating higher-order flow coefficients and the role of octupole deformation in shaping the initial state of the system.

Speaker: Zhiyong Lu (China Institute of Atomic Energy (CN))

11:30 Exploring the origin of collective phenomena in small collision systems with ALICE at the LHC

The observation of collective-like behaviors in pp and p–Pb collisions at LHC energies has sparked debate about the similarities between the dynamics of small systems and heavy-ion collisions. In this talk, our latest published results are presented, showing baryon-meson $v_{\rm 2}$ grouping (within 1$\sigma$) and significant splitting (approximately 5$\sigma$) at intermediate $p_{\rm T}$ in high-multiplicity p–Pb and pp collisions, similar to those observed in heavy-ion collisions. The Hydro-Coal-Frag model, which incorporates partonic flow and quark coalescence, best describes the data, while alternative models fail to reproduce the pattern, providing strong evidence of a collectively flowing partonic medium in high-multiplicity pp and p–Pb collisions. The key question of how far down in system size the dynamics of small systems and heavy-ion physics remain similar is also investigated. The observed baryon-meson $v_{\rm 2}$ grouping and splitting, down to lower multiplicities $N_{\rm{ch}} > 25 $, suggest that partonic collectivity may persist even in smaller systems. Additionally, our newly published results on ultra-long-range two-particle correlations, $|\Delta \eta| >$ 5.0(6.5), extending down to or below minimum bias multiplicity in pp and p–Pb collisions, explore the limits of collective medium formation in small systems.

The unique sensitivities of flow vector fluctuations to initial geometry and event-by-event fluctuations offer new insights into the initial conditions of both heavy-ion and small collision systems. Newly published results on two-particle correlation observables are presented, revealing $p_{\rm T}$- and $ \eta$-dependent flow vector fluctuations in p–Pb collisions at the LHC, similar to those observed in heavy-ion collisions. These measurements, together with comparisons to state-of-the-art theoretical model calculations, allow us to constrain the initial conditions with unprecedented detail in both transverse and longitudinal directions, advancing our understanding of the origin of anisotropic flow phenomena observed in small collision systems at the LHC.

Speaker: Dr Debojit Sarkar (Niels Bohr Institute, University of Copenhagen, Denmark)

11:50 Initial momentum and conserved charges to final flow

The evolution of a relativistic heavy-ion collision is typically understood as a process that transmutes the initial geometry of the system into the final momentum distribution of observed hadrons, which can be described via a cumulant expansion of the initial distribution of energy density and is represented at leading order as the well-known eccentricity scaling of anisotropic flow. We extend this framework to include the contribution from initial momentum-space properties as encoded in other components of the energy-momentum tensor, as well initial conserved charge densities. We confirm the validity of the framework in state-of-the-art hydrodynamic simulations
of large and small systems.
With this new framework, it is possible to separate the effects of early-time dynamics from those of final-state evolution, even in the case when the distribution of energy does not fully determine subsequent evolution, as for example, in small systems. Specifically, we answer the question of when and how azimuthal correlations from the initial state survive to the final state.

In very small systems such as $p$-$p$, for example, initial momentum degrees of freedom dominate over energy. Thus, even if the system forms a quark-gluon plasma that is well described by hydrodynamics, the usual hydrodynamic picture of the transmutation of initial geometry to final momentum anisotropy is broken. Nevertheless, we show that the hydrodynamic response to the full energy-momentum tensor can be well understood in a similar manner as larger systems. Additionally, this framework elucidates the generic features of the system's evolution that are responsible for the impressive success of hydrodynamic simulations, but which may still hold even in cases where hydrodynamics is not applicable.

Based on: arXiv:2405.13600 and work in progress

Speaker: Prof. Matthew William Luzum (University of São Paulo)

12:10 BHAC-QGP: three-dimensional MHD simulations of relativistic heavy-ion collisions

We present BHAC-QGP, an advanced numerical code designed to simulate the evolution of quark-gluon plasma (QGP) in heavy-ion collisions with enhanced accuracy in the presence of magnetic field. Building on the foundation of the Black Hole Accretion Code (BHAC), which solves general-relativistic magnetohydrodynamic (GRMHD) equations for astrophysical phenomena, BHAC-QGP adapts these capabilities to the unique challenges of high-energy nuclear collisions. BHAC-QGP leverages Adaptive Mesh Refinement (AMR) to dynamically enhance resolution in regions requiring high precision, enabling detailed tracking of QGP properties over a wide range of length scales. We present applications of BHAC-QGP to gold-gold (Au-Au) collisions at RHIC energies, demonstrating its superior accuracy over existing models and confirming its consistency with established results. These advancements underscore BHAC-QGP's potential as a robust tool for precision studies in relativistic heavy-ion physics.

Speaker: Ashutosh Dash

10:50 → 12:30 Parallel session 17: Correlations & fluctuations I

Convener: Jana N. Guenther (University of Wuppertal)

10:50 New constraints to Chiral Effective Field Theories via Σ$^+$ -p femtoscopy with ALICE

Experimental information on the strong interaction between $\Sigma$ hyperons and protons is a crucial input for both the description of the equation of state of neutron stars with hyperon content and theoretical predictions regarding potential $\Sigma$-hypernuclei. Data on this interaction are scarce, solely based on scattering experiments. Since data points are only available at rather high relative momenta and their uncertainties are sizeable, theory calculations are not well constrained. Particularly the characteristics of the interaction in the triplet channel spin configuration are highly uncertain, and it is not yet clear if the interaction in this channel is attractive or repulsive. In this regard, two-particle intensity interferometry (femtoscopy) of $\Sigma$ baryons and nucleons can provide valuable information.
The reconstruction of the weak decay of $\Sigma^+$ baryons is very challenging. Its decay into a proton and a neutral pion via the weak interaction has a branching ratio of around 50%, with the neutral pion decaying electromagnetically almost exclusively into two photons. In this talk, a novel reconstruction method will be shown which makes use of sophisticated reconstruction algorithms and machine learning techniques to improve the reconstruction efficiency and purity of the $\Sigma^+$ baryons, enabling the measurement of their correlation function with protons for the first time. The obtained correlation function will be discussed and related to latest theoretical calculations using Chiral Effective Field Theories, providing new constraints on the $\Sigma$–nucleon interaction.

Speaker: Benedict Heybeck (Goethe University Frankfurt (DE))

11:10 Search for the Strange Dibaryons with Baryon Correlations in Isobar Collisions at STAR

Dibaryons, exotic states composed of six quarks, have long been a subject of interest in understanding the strong interaction beyond conventional hadrons. Among these, strange dibaryons, which contain strange quarks, offer an important role of studying the hyperon-nucleon ($YN$) and hyperon-hyperon ($YY$) interactions . Of particular interest are the spin-0 $H$ ($S$ = -2) and the spin-2 $N\Omega$ ($S$ = -3) dibaryon state, which are considered promising candidates for the strange dibaryon bound state. In heavy-ion collisions, two-particle femtoscopy is a powerful and unique method for extracting information about the spatio-temporal properties of the source, characterising the final state interactions (FSI), and searching for the possible bound states.

In this talk, we will present the measurements of baryon-baryon correlation functions, including $p$-$\Xi^{-}$, $\Lambda$-$\Lambda$ and $p$-$\Omega^{-}$ pairs, in Isobar collisions (Ru+Ru, Zr+Zr) at $\sqrt{s_\mathrm{NN}}$ = 200 GeV. The correlation functions are analyzed within the Lednicky-Lyuboshitz formalism. The extracted scattering length and effective range will be presented. Those measured parameters will be compared with recent Lattice QCD and effective theory model calculations. Most importantly, the physics implications for the formation of strange dibaryon ($S=-2$ and $S=-3$) bound state will be discussed.

Speaker: Kehao Zhang

11:30 NA61/SHINE results on multiplicity and net-charge fluctuations at CERN SPS energies

The NA61/SHINE strong interaction program is dedicated to exploring the phase diagram of strongly interacting matter. Its primary goals are to study the onset of deconfinement and to search for the critical point within this type of matter. To accomplish these objectives, the program conducts scans over a range of beam momenta (from 13A to 150A/158A GeV/c) and system sizes, involving collisions such as p+p, p+Pb, Be+Be, Ar+Sc, Xe+La, and Pb+Pb. Achieving these goals necessitates a diverse array of measurements.
In this contribution, we present final results on how multiplicity and net-charge fluctuations depend on system size and energy, analyzed using higher-order moments by comparing p+p and Ar+Sc most central interactions. The results take into account detector bias and centrality selection in the case of Ar+Sc. The results are compared across different energies, as well as against model predictions.

Speaker: Maja Mackowiak-Pawlowska (Warsaw University of Technology (PL))

11:50 Microscopic study of non-Gaussian particle number fluctuations near the critical point

We calculate non-Gaussian cumulants of particle number near the 3D-Ising critical point by means of GPU-accelerated molecular dynamics simulations. We perform ensemble averaging with large statistics, and study the equilibration of cumulants near the critical point. We find that scaled variance, skewness, and kurtosis reflect the expected critical behavior of fluctuations and equilibrate on comparable time scales. We also incorporate Bjorken-like collective flow model to study the behavior of non-Gaussian fluctuations in the rapidity space. The results are put in the context of measurements of cumulants and factorial cumulants of protons in heavy-ion collisions at RHIC.

Speaker: Volodymyr Kuznietsov

12:10 EPJ Featured Talk: The collectivity of transverse momentum fluctuations

Event-by-event fluctuations of the transverse momentum per particle $[p_T]$ have been measured in heavy-ion collisions at RHIC and LHC, but this topic has so far received less attention than anisotropic flow. Yet the physical mechanism is identical for both phenomena: A long-range correlation in the initial stages, followed by collective expansion. While the origin of anisotropic flow is a fluctuation in the shape of the fireball, that of $[p_T]$ fluctuations is a fluctuation in the initial temperature. It generates a fluctuation in the fluid velocity, which is carried over to the $p_T$ spectrum of outgoing particles. The same fluctuation is present at all rapidities and generates a long-range correlation.

We make hydrodynamic predictions for the fluctuation of the spectrum, which is a new observable $v_0(p_T)$ introduced by Schenke, Shen and Teaney [https://arxiv.org/abs/2004.00690] which has not yet been measured, and for which first experimental results will hopefully be shown at this conference. Like anisotropic flow $v_n(p_T)$, $v_0(p_T)$ is predicted to have a strong mass ordering in the $p_T$ range where hydrodymamics applies. We show that our hydrodynamic calculation reproduces precisely the observed dependence of $[p_T]$ fluctuations on $p_T$ cuts which has previously been observed by PHENIX and ATLAS. We argue that the mechanism of quark coalescence should give rise to a splitting between the $v_0(p_T)$ of baryons and mesons at higher $p_T$, similar to that observed for anisotropic flow.

Based on Parida, Samanta, Ollitrault, Phys.Lett.B 857 (2024) 138985 [2407.17313].

Speaker: Mr Tribhuban Parida (IISER Berhampur)

10:50 → 12:30 Parallel session 18: Jets III

Convener: John William Harris (Yale University (US))

10:50 Studies of jet quenching in O+O collisions at $\sqrt{s_{NN}}$ = 200 GeV by STAR

Jet quenching — modifications to the energy and substructure of high-energy parton showers in the Quark Gluon Plasma (QGP) — serves as a key experimental tool to probe properties of QGP in relativistic heavy-ion collisions. On one hand, the quenching effect, often quantified through the nuclear modification factor (e.g., $R_{AA}$), is well established in large collision systems, such as Au+Au and Pb+Pb collisions. On the other hand, it is absent in smaller p+A collisions despite the observation of collectivity in these collisions, either because the medium is not formed, or its temperature or lifetime is too low to cause jets to lose much energy. This highlights the need for studying the system size dependence of the jet quenching phenomenon. And O+O collisions provide a great opportunity for such studies as they bridge the gap between these small and large systems.

In this talk, we present the first measurements of the following three observables related to the jet quenching: inclusive charged hadron $R_{AA}$ at high transverse momentum ($p_{T}$), inclusive jet $R_{cp}$, and semi-inclusive hadron+jet $I_{cp}$, using O+O collisions data collected with the STAR detector in 2021 at $\sqrt{s_{NN}} = 200$ GeV. For jet measurements, combinatorial background is subtracted based on the event mixing technique, while background fluctuations and detector effects are corrected via unfolding. These measurements provide a valuable opportunity to explore possible jet quenching effects in O+O collisions. The $p_{T}$ and centrality differential results will be presented and compared to similar measurements in collision systems of various sizes. Such comparisons will also help disentangle the effects of system size and collision geometry.

Speaker: Sijie Zhang (Shandong University)

11:10 Measurements of Baryon-to-Meson Ratios Inside Jets in Au+Au and $p$+$p$ Collisions at $\sqrt{s_{NN}} = 200$ GeV at STAR

Measurements at RHIC and the LHC show strongly enhanced baryon-to-meson yield ratios at intermediate transverse momenta ($p_{\rm{T}}$) in inclusive measurements from high-energy nuclear collisions compared to $p$+$p$ baseline. This enhancement is attributed to strong hydrodynamic flow and parton recombination in the Quark-Gluon Plasma (QGP).
Jet probes have been used extensively to gain insights into QGP properties, with substantial modifications to jet yields and internal structures seen across multiple measurements. Despite apparent medium-induced changes to jet fragmentation patterns, modification of in-jet hadro-chemistry has not yet been found. To search for such effects with the STAR detector at RHIC, we couple the jet-hadron correlation technique with particle identification to measure in-cone baryon-to-meson yield ratios associated with fully reconstructed jets from Au+Au and $p$+$p$ collisions at $\sqrt{s_{\mathrm{NN}}} = 200$ GeV. These in-jet ratios are studied with jet selections of jet radius, R $= 0.2, 0.3, 0.4$, and jet constituent $p_{\rm{T}}$, $p^{\rm{cons}}_{\rm{T}} > 2.0$, $3.0$ GeV/$c$, to probe jets with different levels of QGP interaction. We present in-jet $p/\pi$ ratios as a function of $p_{T}$ as well as $\Delta R$, alongside jet shapes for identified hadrons, and compare Au+Au and $p$+$p$ measurements to examine QGP effects on hadronization.

Speaker: Gabe Dale-Gau (University of Illinois at Chicago)

11:30 Probing jet hadrochemistry modification with measurements of π, K, and p in jets and the underlying event in pp and Pb–Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV with ALICE

Measurements of jet substructure observables in heavy-ion collisions provide powerful constraints on the microscopic mechanisms of interactions between energetic partons and the QGP. Though there has been remarkable progress in measuring inclusive charged-particle jet substructure observables, a complete understanding of the identified particle production inside jets (jet hadrochemistry) and its modification in heavy-ion collisions remains elusive. Jet quenching models predict modifications to jet hadrochemical composition in heavy-ion collisions, arising from both jet-medium interactions and altered particle production in the jet wake. Measurements of identified particles in jets can help discriminate between different proposed jet-medium interaction mechanisms.
Enabled by the excellent PID capabilities of ALICE, we present the first measurements of π, K, and p ratios within jets and in the underlying event as a function of particle transverse momentum in pp and Pb–Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV. This study aims to advance our understanding of soft particle production mechanisms and distinguish modified jet fragmentation from bulk medium effects.

Speaker: Sierra Cantway (Yale University (US))

11:50 Modification of the Jet Energy-Energy Correlator in Cold Nuclear Matter

We compute medium corrections to the energy-energy correlator (EEC) for jets in electron-nucleus collisions at leading order in the QCD coupling and the interaction of the jet with the medium. We derive an analytical expression for the modification of the EEC as a function of the opening angle and show that the modification is strongest at large angles within the jet cone. We obtain explicit results for the dependence of the modification on the jet energy, the scattering power of cold nuclear
matter, and the path length within the medium. We extend our calculations to gluon jets in proton-nucleus collisions and compare our results with recent preliminary data for proton-lead collisions at the LHC. We also discuss the role of comovers on the EEC in p+Pb collisions.
Reference: arXiv:2411.04866

Speakers: Dr Chathuranga Sirimanna (Duke University), Yu Fu (Duke university)

12:10 Comprehensive study of jet substructure using a multi-stage jet evolution framework

We present a comprehensive study of jet substructure modifications in heavy-ion collisions using the JETSCAPE framework. Our approach utilizes the multi-stage jet energy loss description, which includes suppression of jet-medium interaction at high virtuality, reflecting the virtuality dependence of the process. The parameters used in our simulations are tuned to reproduce hadron RAA and inclusive jet RAA, particularly in the high-pT region at 5.02 TeV. Using these simulations, we analyze key observables such as Soft Drop observables, jet mass, and fragmentation functions. Our results show good agreement with current experimental data, highlighting the model’s reliability. This study provides a solid baseline for further detailed investigations of jet structures in heavy-ion collisions, offering insights into jet-medium interactions and their effects on jet evolution.

Speaker: Chathuranga Sirimanna

10:50 → 12:30 Parallel session 19: New theoretical developments I

Convener: Berndt Mueller (Duke University)

10:50 Understanding quantum corrections to momentum broadening in a weakly coupled QGP

In heavy-ion collisions, the transverse momentum broadening coefficient, $\hat{q}$ plays an integral role in characterising how a high-energy parton loses energy to the quark-gluon plasma (QGP) through which it passes. Over the last few years, it has been shown [1,2] that $\hat{q}$ starts to receive radiative corrections at $\mathcal{O}(g^2)$, featuring potentially large logarithms. Indeed, better understanding the nature of these logarithmic corrections is extremely relevant for the goal of obtaining a precise theoretical description of parton energy loss.
During this talk, I will begin by presenting our results from [3], where we investigate how the inclusion of thermal effects impacts these radiative corrections. I will then report on our progress in understanding how the logarithmic phase space is deformed, once one relaxes the so-called harmonic oscillator approximation. In this direction, we make use of the improved opacity expansion [4], which has recently found success in analytically describing, at leading order, the interface of the single and multiple-scattering regimes.

References:
[1] T.Liou, A.Mueller, B.Wu, arxiv:1304.7677
[2] J.P.Blaizot, F.Dominguez, E.Iancu, Y.Mehtar-Tani, arxiv: 1311.5823
[3] J.Ghiglieri, E.Weitz, arxiv: 2207.08842
[4]J.Barata, Y.Mehtar-Tani, A.Soto-Ontoso, K.Tywoniuk, arxiv: 2106.07402

Speaker: Eamonn Weitz (Universität Bielefeld)

11:10 An end-to-end generative model for heavy-ion collisions

The multistage approach based on hydrodynamics has achieved remarkably success in modeling heavy-ion physics, providing an accurate description of experimental particle spectra as well as various signatures of collective flow, flow correlations, and fluctuations. However, traditional numerical simulations of hydrodynamics are challenged by recent high-precision measurements: to probe the finer details in the system via statistics-demanding observables, 10^9-10^10 collisions events are commonly required. As heavy-ion collision physics enters a high-precision era, theoretical modeling needs to evolve to meet growing computational demands.

In this work, we introduce DiffHIC, a generative diffusion model designed to bridge this gap. The model simulates ultra-relativistic heavy-ion collisions from end to end, taking initial entropy density profiles as input and producding two-dimensional final particle spectra. By comparing observables derived from particle spectra generated by both traditional numerical simulations and our trained generative model, we demonstrate that DiffHIC not only accurately replicates integrated and differential observables but also effectively captures higher-order fluctuations and correlations. These results indicate that DiffHIC successfully learns the intricate mapping from initial entropy density profiles to final particle spectra, governed by a set of nonlinear hydrodynamic and Boltzmann transport equations. While preserving the fine details of the underlying physical processes, DiffHIC significantly accelerates end-to-end heavy-ion collision simulations. For example, DiffHIC can simulate a single central collision event in just 0.1 seconds on a GeForce GTX 4090 GPU.

The code and trained model are available at https://huggingface.co/Jing-An/DiffHIC/tree/main.

[1]arxiv:2410.13069

Speaker: Jing-an Sun (Fudan University)

11:30 Exploring the impact of strong-force fields on $\phi$- meson spin alignment within a quantum relativistic transport approach

The unexpected pattern of global spin alignment of vector mesons observed in relativistic heavy-ion collisions for different particle species have posed a strong challenge to theoretical interpretation, as it cannot be explained solely through conventional polarization sources such as vorticity and electromagnetic fields. We argue that fluctuations of strong-force fields with short correlation length may be the key to solve the puzzle. We derive a relativistic spin Boltzmann equation for vector mesons from Kadanoff-Baym equations, incorporating an effective quark-meson model for interaction and the quark coalescence process for hadronization. Our calculations of the spin density matrix element $\rho_{00}$ for unflavored vector mesons reveal that all field contributions appear in squared terms. This finding indicates that anisotropies in local field correlations, relative to the spin quantization axis, lead to the spin alignment of unflavored vector mesons. We propose that the large spin alignment observed for $\phi$ mesons at lower energies arises from effective $\phi$ fields that polarize the strange quarks and antiquarks, which in turn form polarized $\phi$ mesons via coalescence during the hadronization stage. The strength of the $\phi$ field can be inferred from the experimental data on $\phi$-meson spin alignment as functions of collision energy reported by STAR Collaboration. Our findings show a transverse momentum dependence of $\rho_{00}$ in fair agreement with STAR data, and we also predict the azimuthal angle dependence of $\rho_{00}$. The proposed polarization mechanism driven by fluctuating strong-force fields may open a new window into a hitherto unknown behaviour of QCD interaction.
[X.-L. Sheng, L. Oliva, Z.-T. Liang, Q. Wang and X.-N. Wang, Phys. Rev. Lett. 131, 042304 (2023)]
[X.-L. Sheng, L. Oliva, Z.-T. Liang, Q. Wang and X.-N. Wang, Phys. Rev. D 109, 036004 (2024)]

Speaker: Lucia Oliva (Università di Catania, INFN Catania)

11:50 Nonlinear causality and strong hyperbolicity of baryon-rich Israel-Stewart hydrodynamics

We present the first set of fully-nonlinear, necessary and sufficient conditions guaranteeing causal evolution of the initial data for the Israel-Stewart equations with shear and bulk viscosity from kinetic theory coupled to a nonzero baryon current. These nonlinear constraints not only provide causality: they also (a) guarantee the existence of a locally well-posed evolution of the initial data (they enforce strong hyperbolicity) when excluding the endpoints of the bounds, (b) arise from purely algebraic constraints that make no underlying symmetry assumptions on the degrees of freedom and (c) propagate the relevant symmetries of the degrees of freedom over the entire evolution of the problem. Our work enforces a mathematically rigorous foundation for future studies of viscous relativistic hydrodynamics with baryon-rich matter including neutron star mergers and heavy-ion collisions.

Speaker: Mr Ian Cordeiro (University of Illinois at Urbana Champaign)

12:10 Resummed spin hydrodynamics from quantum kinetic theory

In the past years, different observables related to the spin polarization of hadrons such as $\Lambda$-Baryons have attracted much attention and continue to find new applications.
While some measurements (such as the global polarization) are straightforwardly reproduced by assuming the spins of the particles to be in equilibrium, the explanation of more differential observables (such as the local polarization) remains a subject of active discussion.

A crucial ingredient in a more thorough description of spin dynamics is a formulation of dissipative relativistic spin hydrodynamics that accounts for the finite (and possibly long [1]) timescale of spin relaxation, i.e., of the approach of the spin potential $\Omega_0^{\mu\nu}$ to the thermal vorticity $\varpi^{\mu\nu}$. In such a theory, the spin potential remains a dynamical quantity that follows certain evolution equations, akin to the Bloch equations for the magnetization in the (nonrelativistic) description of spin dynamics.

The main result to be presented are the equations of motion for such a theory, derived from first principles based on quantum kinetic theory [2]. Employing the inverse-Reynolds dominance (IReD) approach [3], a resummation scheme based on a power counting in Knudsen and inverse Reynolds numbers is constructed, leading to hydrodynamic equations that are accurate to second order.
It is found that the spin dynamics can be characterized by eleven equations: six of them describe the evolution of the components of the spin potential $\Omega_0^{\mu\nu}$, while the remaining five provide the equation of motion of a dissipative irreducible rank-two tensor $\mathfrak{t}^{\mu\nu}$. For a simple truncation, all first- and second-order transport coefficients are computed explicitly.

[1] DW, M. Shokri, D. H. Rischke, arXiv: 2405.00533
[2] DW, arXiv: 2409.07143
[3] DW, A. Palermo, V. E. Ambruș, Phys.Rev.D 106 (2022) 1, 016013

Speaker: David Wagner (Florence University)

Talk_QM25_Wagner.pdf

10:50 → 12:30 Parallel session 20: QCD phase diagram & critical point I

Convener: Nu Xu (Lawrence Berkeley National Lab)

10:50 Covariant formulation of spinodal decomposition for simulating first-order QCD phase transition

We expect to probe the QCD phase diagram in the region beyond the critical point at the beam energy scan program at Relativistic Heavy-Ion Collider (RHIC) and at the upcoming Facility for Antiproton and Ion Research (FAIR). In this region, the heavy-ion collision systems will undergo a first-order phase transition. This phase transition is expected to proceed via spinodal decomposition as the system explores the metastable and unstable areas of the equation of state. To conclusively claim the discovery of the first-order phase transition curve, it is crucial to include spinodal effects in phenomenological simulations of these collisions and identify their experimental signatures. For the first time, we present a covariant formulation of relativistic hydrodynamics including spinodal decomposition [1]. These equations account for surface effects between the two phases and can be used in hydrodynamic models of heavy-ion collisions. We solve them for a nearly boost invariant flow and discuss the effects of phase separation on system evolution.

[1] J. I. Kapusta, M. Singh and T. Welle; arXiv:2407.16963

Speaker: Mayank Singh (Vanderbilt University)

11:10 Simulating stochastic diffusion in critical and near-critical fluids in (3+1) dimensions

We describe numerical simulations of stochastic fluid dynamics
with a conserved charge coupled to the momentum density of the
fluid. This theory is known as model H, and it is expected to
describe universal dynamics in the vicinity of a possible
critical endpoint in the QCD phase diagram. We verify dynamical
scaling, and compute the scaling exponent and the renormalized
viscosity. We also describe exploratory simulations that include
a relativistic background fluid.

Speaker: Thomas Schaefer

11:30 Exploring QCD matter in extreme conditions with Machine Learning

In recent years, machine learning has emerged as a powerful computational tool and novel problem-solving perspective for physics, offering new avenues for studying strongly interacting QCD matter properties under extreme conditions. In this talk I will aim to provide an overview of the current state of this intersection of fields, focusing on the application of machine learning to theoretical studies in high energy nuclear physics. It covers diverse aspects, including heavy ion collisions, lattice field theory, and neutron stars, and discuss how machine learning can be used to explore and facilitate the physics goals of understanding QCD matter. The talk will also provide a commonality overview from a methodology perspective, from data-driven perspective to physics-driven perspective. The talk may also discuss the challenges and future prospects of machine learning applications in high energy nuclear physics, also underscoring the importance of incorporating physics priors into the purely data-driven learning toolbox.

Ref:
[1]K. Zhou, L. Wang, L. Pang, S. Shi, Prog. Part. Nucl. Phys. 135 (2024) 104084
[2]L. Wang, G. Aarts, K. Zhou, JHEP 05 (2024) 060
[3]M. O.K, J. Steinheimer, K. Zhou, H. Stoecker, Phys.Rev.Lett. 131 (2023) 20, 202303
[4]W. He, Y. Ma, L. Pang, H. Song, K. Zhou, Nucl.Sci.Tech. 34 (2023) 6, 88

Speaker: Prof. Kai Zhou (CUHK-Shenzhen)

11:50 Probing sound propagation in the QGP via relativistic ultra-central collisions with ALICE

Relativistic heavy-ion collisions create a hot, dense state of QCD matter called Quark–Gluon Plasma (QGP). In ultra-central collisions, the QGP volume saturates and remains constant; instead, entropy fluctuations cause temperature variations in the system. This property can be probed by measuring the correlation between the average transverse momentum (⟨$p_{\rm T}$⟩) and the multiplicity of charged hadrons. This contribution shows the latest ALICE measurements
of charged-hadron ⟨$p_{\rm T}$⟩ and its higher-order cumulants in ultra-central Pb–Pb collisions. Results are in close agreement with state-of-the-art hydrodynamic models. Furthermore, by fitting the relative increase of ⟨$p_{\rm T}$⟩ to the relative change in the average charged-particle density at midrapidity, it is possible to extract the speed of sound ($c_s$) in the QGP, which indicates how fast compression waves propagate through the QGP medium. The extracted cs shows a strong dependence on the choice of the centrality estimators used to select ultra-central collisions, highlighting the sensitivity of this measurement to experimental biases. This observation suggests a need for careful reassessment of methods for determining cs from heavy-ion data. The measurements are compared with predictions from state-of-the-art models in ultra-central events.

Speaker: Omar Vazquez Rueda (University of Houston (US))

12:10 Higher-Order Fluctuations: Unveiling the Final Frontier of QCD at the LHC with ALICE

Lattice QCD calculations predict that chiral symmetry is restored in a smooth crossover transition between a quark--gluon plasma and a hadron resonance gas (HRG) at vanishing net-baryon density, a condition realized in heavy-ion collisions at the LHC. In this regime, the net-baryon number cumulants computed using the HRG and lattice QCD partition functions are in good agreement up to third order. However, starting with the fourth-order cumulants, the LQCD results are significantly lower than the corresponding HRG results. This offers a unique opportunity to experimentally verify the full QCD partition function by measuring the fourth-order cumulants of the net-proton number distributions.

In this talk, ALICE data for net-proton number cumulants up to sixth order in proton-proton collisions at top LHC energy are presented to search for effects of a possible chiral phase transition in this small system. An extension of the net-proton measurements in Pb-Pb collisions to fourth order is also presented. In addition to providing experimental access to the full QCD partition function, these measurements will, for the first time, allow to distinguish between different mechanisms of baryon production.

Speaker: Ilya Fokin (Heidelberg University (DE))

12:30 → 14:00 Lunch

14:00 → 15:40 Poster session 1

15:40 → 16:10 Coffee break

16:10 → 18:30 Parallel session 21: Correlations & fluctuations II

Convener: Bedangadas Mohanty (National Institute of Science Education and Research (NISER) (IN))

16:10 Evidence for the collective nature of radial flow in Pb+Pb collisions with the ATLAS detector

Radial flow and anisotropic flow are key observables used to study the expansion dynamics of the quark-gluon plasma (QGP). While anisotropic flow has been extensively explored, the collective nature of radial flow has remained less understood. This talk presents the first measurement of transverse momentum ($p_{\mathrm{T}}$) dependence of radial flow fluctuations, $v_0(p_{\mathrm{T}})$, over $0.5 < p_{\mathrm{T}} < 10$ GeV in Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02$ TeV.

These measurements reveal three key features that establish the collective nature of radial flow: (i) long-range correlations in pseudorapidity, (ii) factorization in $p_{\mathrm{T}}$, and (iii) a centrality-independent shape in $p_{\mathrm{T}}$. The observed patterns provide new experimental constraints on the origin and fluctuations of radial flow. A comparison with hydrodynamic models demonstrates the sensitivity of $v_0(p_{\mathrm{T}})$ to bulk viscosity, offering a novel probe of QGP transport properties. These results introduce a new and powerful tool for investigating the collective dynamics of heavy-ion collisions, providing fresh insights into the fundamental properties of the QGP.

Speaker: Somadutta Bhatta (Stony Brook University (US))

16:30 Strong forces in three-baryon systems with ALICE Run 3 data

The interactions between Λ hyperons and nucleons are of crucial importance for understanding the composition of the inner core of neutron stars. The equation of state for these dense objects requires accurate knowledge of two- and three-body interactions at short distances, which remain poorly constrained by existing experimental data. ALICE has introduced a novel approach for probing these interactions by measuring femtoscopic correlation functions of particles emitted at distances of 1–2 fm in high-energy pp collisions. This technique enables the study of various hadron–nucleus pairs and provides, for the first time, direct access to the 3 → 3 free scattering process.
In this talk, ALICE measurements of p–d, p–p–p, and p–p–Λ correlation functions in proton-proton
collisions at $\sqrt{s}$ = 13.6 TeV, obtained using three-body software triggers, will be presented. These results offer a fifteenfold increase in the statistical sample compared to Run 2. Additionally, the corresponding first-ever measurement of Λ–d pairs in proton-proton collisions will be showcased. The measured correlation functions will be compared with theoretical predictions, highlighting their sensitivity to the dynamics of three-body systems.

Speaker: Laura Šerkšnytė (CERN)

16:50 Probing the strong interaction with femtoscopy measurements between charm hadrons and charged particles with ALICE

Studies of strong interactions between hadrons provide a valuable opportunity to test Quantum Chromodynamics at nucleon-scale distances. The femtoscopy technique has proven to be an effective tool for studying interactions between unstable hadrons by measuring the correlation function of hadron pairs in momentum space. This approach offers insights into the interaction between two hadrons, based on the spatial extension of the emitting source. While several measurements of residual strong interactions between light and strange hadrons have been conducted using this technique, studies of charm hadrons have been limited. These studies can shed light on the formation of exotic charm states or, in case of baryons, nuclei with charm content.

In this talk, we will present measurements of residual strong interaction between charm hadrons and light-flavor hadrons using the femtoscopy technique. The final results on the correlation functions and residual strong interactions between D mesons and light mesons measured in pp collisions at $\sqrt{s} = 13$ TeV will be discussed. Additionally, new studies of correlations between $\Lambda_{\rm c}^+$ and protons in pp collisions at $\sqrt{s} = 13.6$ TeV, utilizing the new data samples collected from LHC Run 3, will be presented.

Speaker: Biao Zhang (Heidelberg university(DE))

17:10 Cumulants of the multiplicity distributions of identified particles measured in heavy-ion collisions by HADES

We report on the analysis of event-by-event (E-by-E) fluctuations of protons and light nuclei, in heavy-ion collisions (Au+Au at $\sqrt{s_{NN}}$ = 2.4 GeV and Ag+Ag at $\sqrt{s_{NN}}$ = 2.55 GeV) measured with the HADES (High-Acceptance Di-Electron Spectrometer) apparatus. The results of various analysis techniques, including cut-based [1], fuzzy logic [2], and machine learning [3], will be presented and compared. Different experimental artifacts, such as contributions from volume/participant fluctuations, will be addressed as well [4, 5]. Finally, efficiency-corrected cumulants of multiplicity distributions will be compared with expectations from non-critical phenomena, i.e., with baselines without critical fluctuations associated with phase transitions between hadronic matter and quark-gluon plasma.

[1] HADES Collaboration, Phys.Rev.C 102 (2020) 2, 024914
[2] A. Rustamov, 2409.09814 [nucl-th]
[3] M. Backes, A. Butter, M. Dunford and B. Malaescu, SciPost Phys.Core 7 (2024) 1, 007
[4] A. Rustamov, R. Holzmann, J. Stroth, Nucl.Phys. A 1034 (2023) 122641
[5] R. Holzmann, V. Koch, A. Rustamov, J. Stroth, Nucl.Phys.A 1050 (2024) 122924

Speaker: Marvin Nabroth

17:30 EPJ Featured Talk: First direct measurement of radial flow in heavy-ion collisions with ALICE

The observation of long-range correlations is a crucial indicator of collectivity in heavy-ion collisions and has been instrumental in establishing the formation of quark-gluon plasma (QGP) at RHIC and LHC. In this study, the first direct measurement of radial flow is presented using a new observable, $ v_{\rm 0}(p_{\rm T}) $, formulated to capture the long-range correlations in the transverse momentum $ p_{\rm T} $ spectrum. Unlike anisotropic flow coefficients $ v_{\rm n}(p_{\rm T}) $, which are influenced by both shear and bulk viscosity, $v_{\rm 0}(p_{\rm T}) $ is sensitive solely to the bulk properties of the QGP medium, making it a valuable probe for studying bulk viscosity and the hydrodynamic evolution of the system formed in heavy-ion collisions.

The measurement of $ v_{\rm 0}(p_{\rm T})$ is reported across various centrality classes for inclusive charged particles and identified particles, such as pions, kaons, and protons, in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV with the ALICE detector. To minimize short-range non-flow correlations, a rapidity gap technique similar to that used in anisotropic flow measurements is employed. At low $p_{\rm T}$, characteristic mass ordering, typically associated with collective flow in $v_{\rm n}(p_{\rm T})$, is observed. At higher $ p_{\rm T} $, where hydrodynamic descriptions fail, a baryon-meson splitting indicative of quark coalescence is observed. Measurements are compared with the hydrodynamic model MUSIC under varied initial conditions, as well as non-hydrodynamic models like HIJING and EPOS.

Speaker: Swati Saha (National Institute of Science Education and Research (NISER) (IN))

17:50 Hadron resonance gas is bad model for hadronic matter in strong magnetic field

We study the effect of magnetic field on particle ratios and charge fluctuations in hadron resonance gas. We argue that the big change in the pion to proton ratio is due to ill-defined description of higher-spin states, and that because of detailed balance, neutral resonances must be affected by the field too. The calculated fluctuations of conserved charges are likewise suspicious and must be treated with care.

Reference: arXiv:2405.15745

Speaker: Pasi Huovinen (University of Wroclaw)

18:10 Charge fluctuations at LHC with local charge conservation as a signature of QGP

The D-measure of the event-by-event net charge fluctuations was introduced over 20 years ago as a potential signal of quark-gluon plasma (QGP) in heavy-ion collisions, based on the fractional charges of quarks leading to suppression of fluctuations relative to hadron gas baseline. Measurements have been performed at RHIC and LHC, but in the absence of quantitative calculations for both scenarios, the conclusion has been elusive.

Here, we employ a recently developed formalism of density correlations to incorporate the effects of resonance decays, local charge conservation, and experimental kinematical cuts to perform quantitative calculations of charge fluctuations in central Pb-Pb collisions at LHC energies. We find that the hadron gas scenario is in fair agreement with the experimental data of the ALICE Collaboration only when a very short rapidity range of local charge conservation is enforced. On the other hand, the QGP scenario predictions show small sensitivity to the range of local charge conservation and yield excellent agreement with experimental data. We present predictions for LHC Run 2 as a precision test of the two scenarios.

Speaker: Mr Jonathan Parra (University of Houston)

16:10 → 18:30 Parallel session 22: Initial state of hadronic and electron-ion collisions & nuclear structure II

Convener: Prof. Kari J. Eskola (University of Jyväskylä (FI))

16:10 New constraints on nPDFs using dijets in pPb collisions at 8.16 TeV with CMS

Measurements of dijet production in heavy ion collisions can be used to probe the nuclear matter. In proton-lead collisions, the normalized average dijet pseudorapidity distributions can be used as a sensitive tool for constraining the nuclear modifications of parton distribution functions (nPDF) at different $Q^2$ scales and Bjorken-$x$. In such studies, it is possible to investigate, with a highest precision, the shadowing, anti-shadowing and EMC effects. In this talk, the first measurement of dijet average pseudorapidity measurements in pPb collisions at 8.16 TeV in the widest dijet transverse momentum ranges will be presented with the data samples collected with CMS at the LHC. The measured distributions are compared to pQCD calculations with state-of-art proton and nuclear PDFs.

Speaker: Austin Alan Baty (University of Illinois Chicago)

16:30 Multiparticle Cumulants up to 8th Order to Constrain the Initial State in Xe-Xe and Pb-Pb Collisions in the CMS Experiment

Xenon (Xe) nuclei are deformed and have a non-zero quadrupole moment, whereas lead (Pb) nuclei are considered spherical in shape. The study of Xe-Xe collisions at a center-of-mass energy per nucleon pair of $\sqrt{s_{_{\mathrm{NN}}}} = 5.44$ TeV opens up a window to study nuclear deformation at LHC. When compared to Pb-Pb collisions at $\sqrt{s_{_{\mathrm{NN}}}} = 5.36$ TeV, one can explore the dependence of the Fourier flow harmonics ($v_{n}$) on the size and initial-state geometry of the colliding systems. For the first time, correlations between higher-order moments ($\langle v_{n}^{k}v_{m}^{l}\rangle$, where $n,m = 2,3,4$ and $k,l = 2,4,6$) between two ($v_{2}$ and $v_{3}$ or $v_{2}$ and $v_{4}$) or even three flow harmonics ($v_{2}$, $v_{3}$ and $v_{4}$) are measured and compared between Xe-Xe and Pb-Pb collisions as a function of collision centrality. These new measurements have been calculated with multiparticle mixed harmonic cumulants (upto 8th order) using charged particles in the wide pseudorapidity region of the CMS detector ($|\mathrm{\eta}| < 2.4$) and in the transverse momentum range of $0.5 < p_\mathrm{T} < 3.0$ GeV/c. The results have also been compared to several theoretical model predictions. The observables measured in this analysis have been used to closely probe the non-linearities between $v_{2}$, $v_{3}$ and $v_{4}$ and their corresponding eccentricities $\epsilon_{2}$, $\epsilon_{3}$ and $\epsilon_{4}$. This not only helps us to constrain the deformation parameters of Xe nuclei, but can also significantly constrain initial-state model parameters and give us a better understanding of the evolution of the quark-gluon plasma created in heavy-ion collisions at LHC.

Speaker: Aryaa Dattamunsi (Indian Institute of Technology Madras (IN))

16:50 Imaging shapes of atomic nuclei in high-energy nuclear collisions at STAR experiment

The shape and orientation of colliding nuclei play a crucial role in determining the initial conditions of the QGP formed in central collisions, which influence key observables such as anisotropic and radial flow. For instance, central collisions of near-spherical Au nuclei create a QGP with a fixed, circular geometry, whereas prolate-shaped uranium nuclei can collide in a variety of orientations, producing QGP droplets of diverse shapes and sizes. Hence, by comparing systems with similar mass numbers, such as $^{238}$U and $^{197}$Au, we can map out their shape differences and gain deeper insights into the initial conditions of heavy ion collisions. In this talk, we present measurements of $v_2$, $p_T$ fluctuations, and
$v_2 - p_T$ correlations in $^{238}$U + $^{238}$U and $^{197}$Au + $^{197}$Au collisions. Our results reveal large differences in these observables between the two systems, particularly in central events. A comparison with hydrodynamic model calculations indicates a large deformation in uranium nuclei, consistent with previous low-energy experiments. However, data also imply a small deviation from axial symmetry in the ground states of the colliding $^{238}$U nuclei [1]. Our work introduces a novel approach for imaging nuclear shapes, enhances the modeling of QGP initial conditions, and sheds light on nuclear structure evolution across different energy scales. The potential applications of this method for other nuclear species are explored.

[1] STAR Collaboration, Nature 635, 67-72 (2024), https://doi.org/10.1038/s41586-024-08097-2

Speaker: Chunjian Zhang (Fudan University)

17:10 A global analysis of the shapes of Xe-129 and Pb-208 at the Large Hadron Collider

The six-hour Xe-129 run at the LHC has produced a remarkable wealth of physics, among which several studies of its nuclear shape. Previous works indicate that the elliptic flow ratio with Pb-208 collisions determines the quadrupole deformation ($\beta_2$) of Xe, while the $\rho_2$ elliptic flow-mean $p_T$ correlator informs us about the so-called triaxiality ($\gamma$). In this work, we go one step further and perform a global Bayesian analysis of Xe-Xe and Pb-Pb data at the LHC varying simultaneously the shapes of Xe, Pb and relevant Trajectum model parameters. This allows us to systematically assess the sensitivity of observables or ratio of observables to the shape of both Pb (in particular its neutron skin) and Xe (in particular $\beta_2$, its octupole $\beta_3$, $\gamma$ and its skin). Our Bayesian extractions lead to a picture for the ground states of these nuclei that is in remarkable agreement with complementary results using methods in low-energy nuclear structure physics. We comment on implications of the analysis of the shapes of other nuclei via future LHC experiments, with particular focus on isotopes relevant for neutrinoless double beta decay searches.

Speaker: Dr Wilke Van Der Schee (CERN)

17:30 Effects of sub-nucleonic fluctuations on the longitudinal structure of heavy-ion collisions

Subnuclear fluctuations in the initial state of heavy-ion collisions impact not only transverse long-range correlations of small systems, but also the creation of longitudinal structures, measured in longitudinal decorrelation observables [1]. In this work, we study the emergence of long-range rapidity correlations in nuclear collisions due to the inclusion of event-by-event subnuclear fluctuations in the initial state, by using the 3D resolved McDIPPER for the initial state of ultra-relativistic heavy-ion collisions [2], and for the first time, we connect it to 3D phenomenology by evolving through a full hybrid evolution, including a pre-equilibrium stage, using the novel KøMPøST with charges [3], 3+1D viscous hydrodynamical evolution [4], and hadronic rescattering [5]. We present new results, focusing on the effect of sub-nuclear hotspots and (sub) nuclear thickness fluctuations on the longitudinal structure of observables, such as the flow decorrelations and directed flow. We include phenomenological results for large systems (Pb-Pb at $\sqrt{s_{\rm NN}}=5.02$ TeV) as well as predictions for the upcoming O+O and Ne+Ne runs ($\sqrt{s_{\rm NN}}=6.8$ TeV) at the LHC.

References
[1] U. Heinz and R. Snellings. Ann.
Rev. Nucl. Part. Sci., pp. 123–151, 2013, 1301.2826;ATLAS collaboration, Eur. Phys. J. C 78, 142 (2018)
[2] O. Garcia-Montero, H. Elfner and S. Schlichting, Phys.Rev.C 109 (2024) 4, 044916; O. Garcia-Montero, S. Schlichting and J. Zhu, in preparation
[3] T. Dore, X. Du, S. Schlichting, EPJ Web Conf. 296 (2024) 10003
[4] X.-Y. Wu, G.-Y. Qin, L.-G. Pang, and X.-N. Wang,
Phys. Rev. C 105, 034909 (2022)
[5] Weil et al, Phys.Rev.C 94 (2016) 5, 054905

Speaker: Oscar Jesús Garcia Montero

17:50 A new perspective on nuclear SIDIS and Drell-Yan data from dynamical TMD effects analysis

The semi-inclusive deep inelastic scattering (SIDIS) and Drell-Yan (DY) processes are primary channels for studying the parton distributions of the proton and the nucleus, and for deducing the properties of cold nuclear matter. However, for nuclear targets, the DY and SIDIS differential cross-sections are modified by multiple interactions between the active parton and spectator nucleons, which we refer to as dynamical nuclear effects. Only recently have theoretical developments allowed us to understand these processes in the nuclear environment from first principles. We present the complete calculation of next-to-leading order (NLO) dynamical nuclear corrections to the longitudinal and transverse-momentum dependent (TMD) cross-sections in both reactions using the Soft Collinear Effective Theory with Glauber gluons. The main result that this talk focuses on is the first TMD analysis of the nuclear SIDIS and DY data that incorporates these dynamical effects. We show for the first time medium-induced renormalization group (RG) equations improved to second order in opacity to estimate the truncation uncertainty and a rapidity RG (a Blaitsky-Fadin-Kuraev-Lipatov evolution) applied in the initial state for DY and the final state for SIDIS. We discuss the implications of the new theoretical analysis for the interpretation of existing data, and for the physics program of the future EIC.

Speaker: Prof. Weiyao Ke (Central China Normal University)

18:10 Imaging event-by-event nuclear shapes in relativistic nuclear collisions

The event-by-event shapes of colliding nuclei are imprinted on flow patterns measured in relativistic heavy-ion collisions because of the strong hydrodynamic response to collision geometry. In this work, we perform high-statistics simulations to study the impacts of nuclear structure on the ratios of anisotropic flow observables between different collision systems. We study $^{208}$Pb+$^{208}$Pb and $^{129}$Xe+$^{129}$Xe collisions at the Large Hadron Collider, and find the ratios of anisotropic flow in the same centrality class between the two collision systems to be strongly affected by the nuclear structure inputs in the initial state, providing a novel opportunity to probe the deformed nuclear geometry at very high energies [1]. The ratios of $v_2\{4\}/v_2\{2\}$ in these collisions are sensitive to the nuclear skin thickness of the colliding nuclei, providing indirect constraints on the nuclei’s neutron skin. We will also highlight the unique opportunities at the upcoming fixed-target SMOG2 experiments at the LHCb. With input from ab initio calculations of the structure of $^{16}$O and $^{20}$Ne, we compute 3+1D hydrodynamic predictions for the anisotropic flow of Pb+Ne and Pb+O collisions to be tested with upcoming LHCb data. Elliptic flow in Pb+Ne collisions is significantly enhanced compared to the Pb+O baseline due to the shape of $^{20}$Ne, which is deformed in a bowling pin geometry [2]. These studies open a new and emerging avenue for synergy between the nuclear structure and heavy-ion communities, advancing the precision era of relativistic nuclear collisions.

[1] H. Mäntysaari, B. Schenke, C. Shen and W. Zhao, “Probing Nuclear Structure of Heavy Ions at the Large Hadron Collider,'' arXiv:2409.19064 [nucl-th]

[2] G. Giacalone, W. Zhao, et al. “The unexpected uses of a bowling pin: anisotropic flow in fixed-target $^{208}$Pb+$^{20}$Ne collisions as a probe of quark-gluon plasma,'' arXiv:2405.20210 [nucl-th]

Speaker: Chun Shen (Wayne State University)

16:10 → 18:30 Parallel session 23: Light and strange flavor physics & nuclei I

Convener: Boris Hippolyte (IPHC / Université de Strasbourg (FR))

16:10 Hadron Production in Au+Au Collisions from STAR Fixed-Target Experiment

Hadrons have been suggested as sensitive probes for the medium properties of nuclear matter created in heavy-ion collisions. A dense baryon-rich medium is formed during collisions at center-of-mass energies of a few-GeV. Since (anti-)strange hadrons and anti-protons are produced near or below the threshold, their phase space distribution and yield ratios may provide strong constraints on the equation of state (EoS) of high baryon density matter.

In this presentation, the recent results on $\pi^{\pm}$, $\rm{K}^{\pm}$($\rm{K}^0_S$), $\phi$, $p$($\overline{p}$), $\Lambda (\overline{\Lambda})$ and $\Xi^-(\overline{\Xi}^{+})$ production at $\sqrt{s_{\rm{NN}}}$ = 3.0 - 7.7 GeV in Au + Au collisions with fixed-target mode from the STAR experiment will be presented. The transverse momentum spectra ($p_{\rm T}$), rapidity density distributions (dN/dy) and their yield ratios as a function of centrality and collision energy are able to provide unique information about QCD phase structure. The $p_{\rm T}$ spectra and dN/dy ratios of baryon to anti-baryon will provide important input to differentiate between hadron production via quark coalescence and hadronic scattering. Enhancement of baryon to meson ratio hints at an onset of deconfinement in this energy range. We will also explore the centrality dependence of hadron yields, and the evolution of the chemical and kinetic freeze-out temperature, baryon chemical potential and average radial expansion flow in the reported energy range, which can give insights on the EoS of the created medium. These results will be compared with those from higher collision energies and the physics implications will be studied by comparing to the model calculations.

Speaker: Hongcan Li

16:30 Study of Hypernuclei Production Mechanisms and Formation Regions with HADES

In this report, we present measurements of $^3_\Lambda H$ and $^4_\Lambda H$ in Ag+Ag collisions at $\sqrt{s_{NN}}$ = 2.55 GeV, conducted with the HADES experiment at GSI. While at RHIC and LHC energies hypernuclei production is typically discussed with a focus on two scenarios, statistical production in full equilibrium versus production via coalescence, at SIS energies, the spatial region of hypernuclei formation is also considered, distinguishing between production in the participant zone and formation in the spectator matter fragmentation region.
We address both of these aspects by performing a multi-differential analysis of the kinematic distributions of hypernuclei, hyperons, and light nuclei. These studies provide crucial insights into the dynamics of nuclear matter and the interactions between hyperons and nucleons in a dense, strongly interacting medium, enhancing our understanding of hypernuclei production mechanisms in low-energy heavy-ion collisions.

Speaker: Dr Simon Spies (Johann-Wolfgang-Goethe Univ. (DE))

16:50 ALICE charming take on strangeness enhancement in pp collisions

Understanding strangeness enhancement in proton-proton (pp) collisions at the LHC remains a challenge for hadronization models. Recent observations of the $\Omega_\mathrm{c}^0$ baryon indicate that a substantial fraction of the detected Ω baryons may originate from the decay of charm hadrons, e.g. from $\Omega_\mathrm{c}^0 \rightarrow \Omega^- + \pi^+$. However, the unknown absolute branching ratios prevent an exact estimation of the contribution of $\Omega$ baryons coming from charm-hadron decays.

In this talk, we present the first measurement of the fraction of Ω baryons originating from charm-hadron decays in pp collisions at $\sqrt{s} = 13.6$ TeV, performed by the ALICE collaboration. This result is enabled by ALICE's new silicon Inner Tracking System, which allows the direct tracking of the Ω baryon prior to its decay. By studying the evolution of the fraction of Ω coming from charm hadrons as a function of charged-particle multiplicity, we will explore the role of charm-hadron production as a driver of strangeness enhancement.

Speaker: Andrea Sofia Triolo (University of Messina (IT), INFN Catania (IT))

17:10 Deuterons emergence through coalescence with ALICE

One of the unresolved issues in hadron–hadron collisions is the microscopic understanding of how light (anti)(hyper)nuclei are created. Several differential measurements of (anti)(hyper)nuclei yields, momentum distributions, and fluctuations have been carried out in pp, p–A, and A–A collisions at ultra-relativistic energies. Comparisons to models based on statistical hadronization or nucleon coalescence generally provide a good fit to the data. However, these models are grounded in different underlying scenarios, leading to a highly model dependent interpretation of the results.
In this work, new results based on femtoscopy correlations of deuteron proton and deuteron–pion pairs measured in pp and Pb–Pb collisions by ALICE at collision energies of $\sqrt{s}$ = 13 TeV and $\sqrt{s_{\rm NN}}$ = 5.02 TeV, respectively, are presented. Deuteron–proton correlations allow for studying the particle-emitting source size of deuterons compared to other hadrons. The deuteron source aligns with the proton source across all colliding systems. In pp collisions, the deuteron–pion correlation reveals the residual interaction between pions and nucleons from short lived Δ decays, followed by nucleon coalescence into deuterons. This effect is not observed in Pb–Pb collisions due to the large pion multiplicities.
These results demonstrate, in a model-independent manner, that coalescence is a viable mechanism for (anti)deuteron production in ultra-relativistic hadron–hadron collisions.

Speaker: Maximilian Mahlein (Technische Universitaet Muenchen (DE))

17:30 EPJ Featured Talk: News on strangeness production from the NA61/SHINE experiment

Strangeness production in high-energy hadronic and nuclear collisions continues to be one of the central topics in the study of strongly interacting matter. The data collected by the NA61/SHINE experiment at the CERN SPS North Area allows for a comprehensive scan of the strangeness production across various collision energies and system sizes.

This presentation will focus on the new results of the strangeness production in central collisions of medium-sized nuclei, such as Ar+Sc, at the SPS energy range. In particular, the results for Lambda hyperons and charged and neutral K mesons will be shown. The energy and system size dependencies of Lambda-to-pion and strangeness-to-pion ratios are also explored. Moreover, an unexpected excess of charged over neutral meson production in Ar+Sc and pi-+C interactions will be presented. The obtained results will be compared with predictions from selected particle production models, as well as with existing world data from proton-proton and nucleus-nucleus collisions.

Speaker: Yuliia Balkova (University of Silesia (PL))

17:50 Probing the structure of light exotic hadron $f_0(980)$ with elliptic flow in p-Pb collisions at the LHC

The study of exotic hadrons has long been a topic of great interest for the understanding of Quantum Chromodynamics (QCD). As one of the light exotic hadrons, the structure and constituent quark content of $f_0(980)$ have been debated for decades, with theories suggesting it could be a tetraquark state ($s\bar sq\bar q$) or a hadronic molecule ($K\bar K$). Recently, the CMS experiment has measured the elliptic flow anisotropy $v_2$ of $f_0(980)$ in p-Pb collisions and concluded that $f_0(980)$ is an ordinary $s\bar s$ meson, based on the number-of-constituent-quarks (NCQ) scaling of elliptic flow [1]. Assuming that loosely bound light exotic hadrons can only survive at the kinetic freeze-out of the expanding hadronic matter, we have implemented the first $K\bar K$ coalescence model to a hybrid viscous hydrodynamic and hadronic transport model to calculate the $p_T$-spectra and elliptic flow of $f_0(980)$ in p-Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV. Using the well tuned phase-space distributions of kaons in this collision, our coalescence model results agree well with the CMS flow measurements, providing thus a strong evidence for the $K\bar K$ molecular nature of $f_0(980)$. Our study also indicates that the CMS Collaboration may have unjustifiably ruled out the $K\bar K$ molecular state of $f_0(980)$ by not considering the different $v_2$ scalings in the coalescence of hadrons and the coalescence of quarks[2].

References
[1] A. Hayrapetyan et al. (CMS), arXiv:2312.17092, 2023.
[2] Y. Wang, W. Zhao, C. M. Ko, F. Guo, J. Xie, H. Song, in preparation.

Speaker: Yili Wang (Peking University)

18:10 Production of nuclei and hypernuclei in relativistic ion collisions

Production of nuclei and hypernuclei in relativistic ion collisions.

A. Botvina and M. Bleicher.

Institute for Theoretical Physics, J. W. Goethe University,
D-60438 Frankfurt am Main, Germany.

We investigate the formation of light nuclei and hypernuclei in the
rapidly expanding nuclear matter after relativistic nucleus-nucleus
collisions. The primary hadronization phase including the strangeness
production is described with UrQMD model. It is demonstrated that the
nucleation process can be explained within the statistical approach by
applying the local equilibrium concept. We subdivide the expanding
nuclear system into several parts (clusters) consisting of nucleons and
hyperons which are close in the phase space. The production of nuclei
takes place inside these clusters, and it can be described as their
statistical decay in the coexistence region of the nuclear phase
transition. The local chemical equilibrium concept explain consistently
many experimental data which was not possible with the statistical
models under assumption of global chemical equilibrium [1,2]. This model
was also successfully applied to describe experimental data on both
normal nuclei and strange hypernuclei production in the GSI and RHIC-BES
energy range [3,4]. We predict the production of double hypernuclei up
to $^4_{\Lambda \Lambda}$H and further intermediate mass nuclei up to
$^8$Be for Au+Au central collisions at $\sqrt{s_{NN}}$=3 GeV, recently
explored by the STAR experiments [4]. The production of charmed nuclei
is discussed too. These new nuclei can be identified by the measurement
of the correlated particles.

[1] A. S. Botvina, N. Buyukcizmeci and M. Bleicher,
Phys. Rev. C {\bf 103}, 064602 (2021).

[2] A. S. Botvina, N. Buyukcizmeci and M. Bleicher,
Phys. Rev. C {\bf 106}, 014607 (2022).

[3] N. Buyukcizmeci, T. Reichert, A.S. Botvina, and M. Bleicher,
Phys. Rev. C {\bf 108}, 054904 (2023).

[4] N. Buyukcizmeci, T. Reichert, A.S. Botvina, and M. Bleicher,
arXiv: 2410.17449 (2024).

Speaker: Alexander Botvina (ITP, University of Frankfurt am Main)

16:10 → 18:30 Parallel session 24: New theoretical developments II

Convener: Owe Philipsen (Goethe University Frankfurt)

16:10 Limit cycle of hydrodynamic attractor in ultracold quantum gases

Hydrodynamic attractors are a universal phenomenon of strongly interacting systems that describe the hydrodynamic-like evolution far from local equilibrium. The rapid hydrodynamization of the Quark-Gluon Plasma is behind the remarkable success of hydrodynamic models of high-energy nuclear collisions. Until now, hydrodynamic attractors have been studied only in monotonically expanding systems, e.g., Bjorken expansion. This work shows that a system undergoing an oscillating isotropic expansion exhibits a novel attractor behavior that resembles a limit cycle in dynamical systems. This phenomenon can be studied in ultracold quantum gases with externally modulated scattering length, which opens the way to the experimental discovery of hydrodynamic attractor phenomenon.

Refs.:
1. Fuji, Enss, Hydrodynamic Attractor in Ultracold Atoms, Phys.Rev.Lett. 133, 2404.12921
2. Enss, Mazeliauskas, in preparation.

Speaker: Dr Aleksas Mazeliauskas (Heidelberg University (DE))

16:30 Finite density lattice QCD without extrapolations

Finite density lattice QCD usually relies on extrapolations in baryon
chemical potential ($\mu_B$), be it Taylor expansion, T' expansion or
analytical continuation. However, their range of validity is difficult to
control. In the canonical formulation, the baryon density is the
parameter of the system, not $\mu_B$.
Here we demonstrate that we can access finite density QCD in the canonical formulation
with physical quark masses along the strangeness neutral line.
We present first results with both the strangeness ($n_S$) and baryon
($n_B)$ densities as parameters. Specifically, we compute the QCD pressure and
chemical potentials as functions of $n_B$ and $n_S$ and construct
an equation of state. Our computations rely on
high-statistics simulations with 2+1 4HEX-staggered fermions.

Speaker: Ludovica Pirelli (Bergische Universität Wuppertal)

16:50 A new effective theory for stochastic relativistic hydrodynamics

Thermal fluctuations are a fundamental feature of dissipative systems that are essential for understanding physics near the expected critical point of QCD and in small systems. Such fluctuations can be modeled by including a stochastic source in the standard hydrodynamics equations. However, when this is done naively strange features can appear such as negative self-correlation functions [1–3]. We construct an effective theory for nonlinear stochastic relativistic hydrodynamics that ensure a well-posed mathematical formulation. Using Crooks fluctuation theorem [4], we derive a symmetry of the effective action that incorporates fluctuations through a suitable free energy functional [5]. For divergence type theories [6], the action can then be fully specified using a single vector generating current. This generating current is related to the free energy, which allows for a systematic understanding the stability properties of the system. The equations of motion obtained using this procedure are guaranteed to be flux conservative and symmetric hyperbolic. This ensures that these equations are well-posed (for suitable initial data) and are in a form that can easily be simulated, including with Metropolis techniques [7].

[1] N. Mullins, M. Hippert, and J. Noronha, “Stochastic fluctuations in relativistic fluids: Causality, stability, and the information current,” Phys. Rev. D, vol. 108, no. 7, p. 076013, 2023.
[2] N. Mullins, M. Hippert, L. Gavassino, and J. Noronha, “Relativistic hydrodynamic fluctuations from an effective action: Causality, stability, and the information current,” Phys. Rev. D, vol. 108, no. 11, p. 116019, 2023.
[3] L. Gavassino, N. Mullins, and M. Hippert, “Consistent inclusion of fluctuations in first-order causal and stable relativistic hydrodynamics,” Phys. Rev. D, vol. 109, no. 12, p. 125002, 2024.
[4] G. E. Crooks, “Entropy production fluctuation theorem and the nonequilibrium work relation for free energy differences,” Physical Review E, vol. 60, p. 2721–2726, Sept. 1999.
[5] L. Gavassino, M. Antonelli, and B. Haskell, “Thermodynamic Stability Implies Causality,” Phys. Rev. Lett., vol. 128, no. 1, p. 010606, 2022.
[6] R. P. Geroch and L. Lindblom, “Dissipative relativistic fluid theories of divergence type,” Phys. Rev. D, vol. 41, p. 1855, 1990.
[7] G. Basar, J. Bhambure, R. Singh, and D. Teaney, “Stochastic relativistic advection diffusion equation from the Metropolis algorithm,” Phys. Rev. C, vol. 110, no. 4, p. 044903, 2024.

Speaker: Nicki Mullins

17:10 Nonlinear dynamics of jet quenching

We develop a comprehensive analytic framework for jet quenching in QCD media, based on a medium-induced parton cascade sourced by collinear virtual splittings. We show that the energy flow out of the jet cone, driven by turbulent gluon cascades, is governed by a non-linear rate equation that resums gluon splittings at arbitrary angles and is enhanced by the medium length, L. The solution of this equation sets the initial condition for a non-linear DGLAP-like evolution equation, which describes the collinear early vacuum cascade resolved by the medium at angles exceeding the medium resolution angle, $\theta_c$. For asymptotic jet energies, the medium-induced cascade displays an exponential behavior that generalizes the Poisson-like distribution of parton energy loss. This formulation enables the resummation of leading contributions in $\ln(1/R)$, and $\ln(R/\theta_c)$, and powers of $L$. We briefly explore the limit of strong quenching, where analytic treatments are feasible, offering insights into the impact of parton cascades on jet quenching. These results provide guidance for future numerical simulations and analytical investigations.

Speaker: Dr Yacine Mehtar-Tani (Brookhaven National Laboratory)

17:30 Machine learning approach to QCD kinetic theory

The effective kinetic theory of QCD provides a possible picture of various non-equilibrium processes in heavy- and light-ion collisions. While there have been substantial advances in simulating the EKT in simple systems with enhanced symmetry, eventually, event-by-event simulations will be required to test this physical picture. As of now, these simulations are prohibitively expensive due to the numerical complexity of the Monte Carlo evaluation of the collision kernels. In this talk, we show how the evaluation of the collision kernels can be performed using neural networks paving the way to full event-by-event simulations.

Speaker: Sergio Barrera Cabodevila (Instituto Galego de Física de Altas Enerxías - Universidade de Santiago de Compostela)

17:50 Stochastic Hydrodynamics and the Density Frame

We describe a unique formulation of relativistic viscous fluid dynamics based on the density frame. The equations of motion are strictly first order in time and have no non-hydrodynamic modes. The only fields are the energy and momentum densities $T^{0\mu}$ and the only parameters are the equilibrium equation state and the shear and bulk viscosities. We compare numerical results to QCD kinetic theory simulations for a set of one dimensional tests -- we find excellent agreement within the regime of applicability of hydrodynamics, and the simulation is well behaved outside of this regime. The results are compared to a second-order formulation developed by Bemfica, Disconzi, Noronha, and Kovtun (BDNK) as well as to second-order hydrodynamics as implemented in MUSIC.

In the density frame we propose a strikingly simple algorithm to simulate stochastic relativistic fluid dynamics based on the Metropolis updates. Each step of the algorithm begins with an update based on ideal hydrodynamics. Then random (three) momentum transfers between fluid cells are proposed. These proposals are then accepted or rejected using the change in entropy as the Boltzmann weight. On average the algorithm reproduces viscous hydrodynamics in the density-frame. The algorithm can be generalized to general coordinates and space-time foliations by parallel transporting the proposed momentum transfers.

References:

G. Basar, J. Bhambure, R. Singh and D. Teaney, ``Stochastic relativistic advection diffusion equation from the Metropolis algorithm,'' Phys. Rev. C (2024) [arXiv:2403.04185].

J. Bhambure, R. Singh and D. Teaney, ``Stochastic relativistic viscous hydrodynamics from the Metropolis algorithm,'' to appear.

J. Bhambure, A. Mazeliauskas, J-F Paquet, M. Singh, R. Singh, D. Teaney, F. Zhou. ``Relativistic Viscous Hydrodynamics in the Density Frame: Numerical Tests and Comparisons,'' to appear.

Speaker: Derek Teaney

18:10 Universal critical dynamics near the chiral phase transition and the QCD critical point

We use a novel real-time formulation of the functional renormalization group (FRG), as a valuable tool complementary to classical-statistical simulation, for dynamical systems with reversible mode couplings to study Model H, the conjectured dynamic universality class of the QCD critical point. We emphasize the structural similarities with Model G, conjectured to be the dynamic universality class of the chiral phase transition in the limit of two massless quark flavors. Importantly, our formulation of the real-time FRG preserves all relevant symmetries throughout the FRG flow which, e.g., guarantees the non-renormalization of the reversible mode couplings, as but one exact result. We derive non-perturbative RG flow equations for the kinetic coefficients of both, Model G and H, in parallel, and discuss commonalities and differences in the resulting fixed-point structure and dynamic critical exponents, such as weak scaling relations which hold in either case versus the characteristic strong scaling of Model G which is absent in Model H. For Model G, we also extract a novel dynamic scaling function that describes the universal momentum and temperature dependence of the diffusion coefficient of iso-vector and iso-axial charge densities in the symmetric phase.

Speaker: Yunxin Ye

16:10 → 18:30 Parallel session 25: QCD matter in astrophysics

Convener: Eero Aleksi Kurkela

16:10 Listening to the long ringdown: a novel way to pinpoint the equation of state in neutron star cores

Multimessenger signals from binary neutron star (BNS) mergers are promising tools to infer the largely unknown properties of nuclear matter at densities that are presently inaccessible to laboratory experiments.
The gravitational waves (GWs) emitted by BNS merger remnants, in particular, have the potential of setting tight constraints on the neutron-star equation of state (EOS).
In this talk I will present a novel and tight correlation between the ratio of the energy and angular momentum losses in the late-time portion of the post-merger signal, i.e., the “long ringdown”, and the properties of the EOS at the highest pressures and densities in neutron-star cores.
By applying this correlation to post-merger GW signals, I will show a significant reduction of the EOS uncertainty at densities several times the nuclear saturation density, where no direct constraints are currently available.

Speaker: Christian Ecker (Goethe)

16:30 Quark matter at four loops

Completing the perturbative four-loop pressure of cold quark matter (QM) has been shown to significantly constrain the neutron-star-matter equation of state. Building on the milestone results in [1, 2], where all screened gluonic contributions to the QM pressure at this order were computed, the remaining challenge lies in evaluating the missing hard contributions arising from four-loop QCD vacuum diagrams at finite density. In this talk, we will present the latest progress in calculating these four-loop contributions [3], with a particular focus on the new algorithmic technique based on Loop Tree Duality (LTD), originally developed for perturbative computations in high-energy collider physics, which we recently generalized to finite density [4]. This novel approach tackles multiloop computations via direct numerical integration of entire Feynman diagrams, enabling the treatment of complex four-loop diagrams at finite density that were previously considered impossible to evaluate.

[1] Gorda, Kurkela, Paatelainen, Säppi, Vuorinen, Phys.Rev.Lett. 127 (2021)
[2] Gorda, Paatelainen, Säppi, Seppänen, Phys.Rev.Lett. 131 (2023)
[3] Kärkkäinen, Navarrete, Nurmela, Paatelainen, Seppänen, Vuorinen, In preparation
[4] Navarrete, Paatelainen, Seppänen, arxiv:2403.02180

Speaker: Risto Paatelainen

16:50 EPJ Featured Talk: Dissipative Magnetohydrodynamics in accretion disks

We generalize the relativistic magnetohydrodynamic (MHD) theory recently derived in Ref. [1] to describe the dissipative dynamics occurring in accretion disks in presence of large magnetic fields. For this purpose we consider a relativistic dilute gas composed of electrons and ions described by the Boltzmann-Vlasov equation and simplify it using the 14-moment approximation. This procedure leads to an extended version of the MHD theory traditionally employed in astrophysics simulations [2,3] which typically considers only the parallel component of the shear-stress tensor aligned with magnetic field, B. In our framework, we consider all three distinct, non-degenerate components of the shear-stress tensor relative to B, each governed by unique dynamical equations. We demonstrate that this novel framework exhibits oscillatory behavior in strong magnetic fields that cannot be captured by the conventional MHD formulation, similar to what was observed in Ref. [1]. We finally compare both frameworks in the linear and nonlinear regimes. In particular, we analyze the equations in conditions that lead to firehose instability [4], which is usually observed in simulation-based frameworks.

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References:
[1] K. Kushwah and G. S. Denicol, “Relativistic dissipative magnetohydrodynamics from the Boltzmann equation for a two-component gas,” Phys. Rev. D 109, 096021 (2024).
[2] F. Foucart, M. Chandra, C. F. Gammie, and E. Quataert, “Evolution of Accretion Discs around a Kerr Black Hole using Extended Magnetohydrodynamics,” Mon. Not. Roy. Astron. Soc. 456, 1332 (2016), arXiv:1511.04445 [astro-ph.HE].
[3] F. Foucart, M. Chandra, C. F. Gammie, E. Quataert, and A. Tchekhovskoy, “How important is non-ideal physics in simulations of sub-Eddington accretion onto spinning black holes?” Mon. Not. Roy. Astron. Soc. 470, 2240 (2017), arXiv:1706.01533 [astro-ph.HE].
[4] I. Cordeiro, E. Speranza, K. Ingles, F. S. Bemfica, and J. Noronha, “Causality Bounds on Dissipative General-Relativistic Magnetohydrodynamics,” Phys. Rev. Lett. 133, 091401 (2024).

Speaker: Khwahish Kushwah (Universidade Federal Fluminense (UFF), Rio de Janeiro, Brazil)

17:10 Astrophysical Equation-of-State Constraints on the Color-Superconducting Gap

We demonstrate that astrophysical constraints on the dense-matter equation of state place an upper bound on the color-superconducting gap in dense matter above the transition from nuclear matter to quark matter. Pairing effects in the color-flavor locked (CFL) quark matter phase increase the pressure at high density, and if this effect is sufficiently large then the requirements of causality and mechanical stability make it impossible to reach such a pressure in a way that is consistent with what is known at lower densities. The intermediate-density equation of state is inferred by considering extensions of chiral effective field theory (CEFT) to neutron star densities, and conditioning these using current astrophysical observations of neutron star radius, maximum mass, and tidal deformability (PSR J0348+0432, PSR J1624-2230, PSR J0740+6620, GW170817). At baryon number chemical potential $\mu = 2.6~\text{GeV}$ we find a 95% upper limit on the CFL pairing gap $\Delta$ of $457~\text{MeV}$ using overly conservative assumptions and
$216~\text{MeV}$ with more reasonable assumptions. This constraint may be strengthened by future astrophysical measurements as well as by future advances in high density QCD calculations.

Speaker: Rachel Steinhorst (Massachusetts Institute of Technology)

17:30 Lambda potential in dense matter from chiral EFT: Bridging heavy-ion collisions, hypernuclei, and neutron stars

We investigate the validity of the $\Lambda$ single-particle potential ($\Lambda$ potential) in dense matter that is based on chiral effective field theory (chiral EFT) that is sufficiently repulsive to solve the hyperon puzzle of neutron stars. We discuss that the model calculations with the $\Lambda$ potential are consistent with the $\Lambda$ hypernuclear spectroscopy [1] and the $\Lambda$ directed flow $v_1$ [2].

A unified approach integrating heavy-ion collisions, other nuclear experiments, and neutron stars with the modern nuclear force from chiral EFT has provided valuable insights into the properties of the equation of state (EOS) of dense matter (e.g. Ref. [3]). This approach contributes to a more comprehensive understanding of both nuclear experiments and astrophysical observations. For a microscopic description of dense matter, such an approach must be performed also for the hyperon sector. A precise understanding of the hyperon potentials in nuclear matter is essential for addressing the long-standing hyperon puzzle of neutron star physics.

In this talk, we consider the $\Lambda$ directed flow $v_1$ [2] at RHIC-BES energies to verify the $\Lambda$ potential based on chiral EFT [4]. This potential is sufficiently repulsive at high densities to solve the hyperon puzzle and has proven by us to be consistent with the $\Lambda$ hypernuclear spectroscopy [1]. We employ a relativistic quantum molecular dynamics (RQMD) implemented into the Monte-Carlo event generator JAM2. We show that the model with the $\Lambda$ potential from chiral EFT can reproduce the experimental data [2], while a more attractive $\Lambda$ potential also reproduces the data. Thus, additional constraints on the Λ potential are demanded.

We will also report the analysis of $v_1$ for $\Lambda$ with various updates including the covariant collision term [5] and the hyperon-nucleon scattering cross section from up-to-date chiral EFT forces [6]. A consistent treatment of the $\Lambda$ and $\Sigma$ potentials based on chiral EFT will also be discussed.

[1] A. Jinno, K. Murase, Y. Nara, and A. Ohnishi, Phys. Rev. C 108 (2023) 065803.
[2] Y. Nara, A. Jinno, K. Murase, and A. Ohnishi, Phys. Rev. C 106 (2022) 044902.
[3] S. Huth, et al., Nature 606 (2022) 276.
[4] D. Gerstung, N. Kaiser, and W. Weise, Eur. Phys.J. A 56 (2020) 175.
[5] Y. Nara, A. Jinno, K. Murase, and A. Ohnishi, Phys. Rev. C 108 (2023) 024910.
[6] J. Haidenbauer, U.-G. Meißner, A. Nogga, and H. Le, Eur. Phys. J. A 59 (2023) 63.

Speaker: Asanosuke Jinno

17:50 Finite-temperature expansion of the dense-matter equation of state

Intermediate to low energy heavy-ion collisions are sensitive to the finite temperature ($T$), high baryon number density ($n_B$) QCD equation of state (EoS) at a ratio between charged and total number of baryons that is fixed by the colliding species. For typical nuclei used in collisions, like gold and lead, this ratio, also known as the charge fraction ($Y^{\rm QCD}_Q$), is around 0.4. On the other hand, studies of isolated neutron stars and the dynamics of their birth and mergers require $Y^{\rm QCD}_Q$ as low as 0.01. We propose a new finite-temperature expansion of the dense-matter EoS [1] that can be used to describe neutron star mergers and core-collapse supernova explosions using as a starting point observations of isolated neutron stars and nuclear experiments. We suggest new thermodynamic quantities of interest that can be calculated from theoretical models or directly inferred by experimental data from low to intermediate energy heavy-ion collisions. With our new method, we can separately quantify the uncertainty in the $Y^{\rm QCD}_Q$ and finite $T$ components of the EoS without making assumptions about the underlying degrees of freedom. We check the accuracy of our framework using a microscopic equation of state up to T = 100 MeV for baryon chemical potential $\mu_B \geq$ 1100 MeV (∼ 1 − 2 $n_{\rm sat}$) and find that the error introduced is less than 5%, with even better results for larger $\mu_B$ and/or lower $T$.

[1] D. Mroczek, N. Yao, et. al, 2404.01658.

Speaker: Débora Mroczek (University of Illinois at Urbana-Champaign)

18:10 Dependence of the bulk viscosity of neutron star matter on the nuclear symmetry energy

We investigate the weak-interaction-driven bulk-viscous transport properties of npe matter in the neutrino transparent regime. This work complements our recent study that shows that the dynamics of neutron star mergers away from beta equilibrium can be precisely described by an Israel-Stewart formulation with far-from-equilibrium bulk and relaxation time transport coefficients, which are strongly affected by the nuclear symmetry energy [1]. In this work, we designed a simple parametric model for the equation of state and the bulk viscosity that satisfies all the thermodynamics (causality and stability) and observational constraints from LIGO/VIRGO and NICER. This parametric model explicitly depends on the nuclear symmetry energy parameters and is ideal for studying their effects on equilibrium and out-of-equilibrium properties. With this new model, we systematically show how the transport coefficients vary with the nuclear symmetry energy. We also discuss how the nuclear symmetry energy affects gravitational-wave observables, such as the dissipative tidal deformability [2] present in the inspiral phase of neutron star mergers.

[1] Y. Yang, M. Hippert, E. Speranza, and J. Noronha, ``Far-from-equilibrium bulk-viscous transport coefficients
in neutron star mergers’’, Phys. Rev. C 109, 015805 (2024).

[2] J. L. Ripley, A. Hegade K. R., R. S. Chandramouli, and N. Yunes,``First constraint on the dissipative tidal deformability of neutron stars," Nat. Astron. 8, 1277 (2024)

Speaker: Yumu Yang

18:30 → 20:10 Poster session 2

Wednesday 9 April

08:40 → 10:40 Parallel session 26: Electromagnetic probes I

Convener: Ralf Rapp (TAMU)

08:40 Low $p_T$ direct photon results with PHENIX and their bearing on the direct photon puzzle

Despite decades of theoretical and experimental achievements, a coherent picture of the evolution in colliding heavy ion systems is still missing, particularly in the non-perturbative regime. Since photons are penetrating probes, the combined information of direct photon yields at low transverse momenta ($p_T$) and their azimuthally asymmetric emission (flow) provides important constraints on theories. Simultaneous observation of high yields and significant (almost hadron-like) flow of low $p_T$ direct photons in Au+Au collisions by PHENIX, dubbed the "direct photon puzzle" still eludes a coherent explanation. A closely related issue is the apparent universality of photon production, that depends only on charged particle multiplicity, but not on the size of the colliding ions, except for the most asymmetric systems. In this talk, we will present new results on direct photon flow in Au+Au and yields in Au+Au, Cu+Au and $^3$He+Au at low $p_T$. Also, we will discuss their impact on our current understanding of the various phases of heavy-ion collisions.

Speaker: Gabor David

09:00 Direct virtual photon measurements in Au+Au collisions with STAR BES-II data

In a heavy-ion collision, photons, which do not interact strongly, have the advantage of escaping unimpeded from their emission source. Consequently, photons can carry valuable information about the properties and dynamics of the hot QCD medium created in heavy-ion collisions. Particularly, the transverse momentum distribution of direct virtual photons emitted from the hot QCD medium exhibits sensitivity to the system temperature. The yield of direct virtual photons depend strongly on fireball volume, emission time, total baryon chemical potential, and temperature.
The STAR experiment has recorded large datasets of Au+Au collisions in the Beam Energy Scan Phase-II (BES-II) program, spanning center-of-mass energies $\sqrt{s_{\text{NN}}}$ = 3 - 54.4 GeV.

In this talk, new results on the direct virtual photon measurements in Au+Au collisions at $\sqrt{s_{\text{NN}}}$ = 14.6 and 19.6 GeV will be presented, including $p_{T}$ spectra and invariant yields. Additionally, prompt photons from initial hard scattering are estimated using model calculations and the effective temperature is extracted from non-prompt direct photons at $\sqrt{s_{\text{NN}}}$ = 14.6 - 54.4 GeV. The physics implications of direct virtual photon yields and effective temperature will be discussed.

Speaker: xianwen bao

09:20 Studying properties of baryon-dominated matter with dileptons

Electromagnetic probes ($\gamma$, $\gamma*$) are an excellent probe to investigate strong-interaction matter under extreme conditions. Their penetrating nature not only enables e.g. temperature measurements unbiased by the collective expansion of the fireball, but also an insight to the microscopic structure of the matter under investigation. The HADES experiment has measured virtual photons in their di-electron decay channel in heavy-ion collisions as well as in proton- and pion-induced reactions in the energy range of a few GeV.
In this contribution, we emphasize multi-differential spectra and collective behavior of dilepton sources in Au+Au collisions at $\sqrt{s_{NN}}$ = 2.42 GeV and Ag+Ag collisions at $\sqrt{s_{NN}}$ = 2.55 and 2.42 GeV and contrast them with hadronic probes at the freeze-out stage.
The exclusive $\pi^-+p \to n_\mathrm{missing}+e^++e^-$ reactions at the center-of-mass energy in the second resonance region revealed the role of the intermediary $\rho$ meson and was the first measurement of a baryon electromagnetic transition form factor in the time-like region.
It is now accompanied by the measurement in p+p reactions at $\sqrt{s}$ = 3.7 GeV and 2.65 GeV that also serves as a baseline for dilepton studies in heavy-ion collisions at facilities under construction such as FAIR.
We also give an outlook to the study of microscopic properties of the matter with low-invariant-mass and low-momentum dileptons and the sensitivity to the phase structure of baryon-dominated matter with the new HADES programme at even lower beam energies collected at the nominal and reduced values of the magnetic field.

Speaker: Niklas Schild

09:40 Dielectron production in pp and Pb-Pb collisions with ALICE in Run 3

The measurement of dielectron production is a fundamental piece of the puzzle in the understanding of the hot and dense matter produced in ultra-relativistic heavy-ion collisions. The dielectron spectrum provides information that penetrates the veil of final-state hadronic interactions and provides direct access to the early phases of the collision. However, the interpretation of the measured spectra relies on a precise understanding of all the contributing sources.

In this talk, we present the measurement of dielectron production in the high-precision proton-proton data collected with the upgraded ALICE detector at $\sqrt{s} = 13.6$ TeV. This establishes a crucial baseline for all measurements of dielectron production in larger collision systems. Utilizing the new inner tracking system, dielectron pairs from semi-leptonic decays of heavy-flavor hadrons are identified and subtracted using a data-driven approach.
The possibilities of using the extracted spectrum of prompt dielectrons over a wide mass range are discussed in the context of a possible onset of thermal radiation or the production of dielectron pairs via the Drell--Yan process in a regime where no reliable calculations based on perturbative QCD are available. We will in addition show the status of the analysis of the Pb-Pb data collected during LHC Run 3.

Speaker: Florian Eisenhut (Goethe University Frankfurt (DE))

10:00 Imprints of dynamic fluidization on dilepton production

We present a newly developed hybrid hadronic transport + hydrodynamics geared towards heavy ion collisions (HICs) at low to intermediate beam energies, and report on the resulting excitation function of dileptons. In this range of energies, covered by the STAR Beam Energy Scan program and the future CBM experiment at FAIR, it is unclear how to best decribe the medium evolution. At which beam energy is hydrodynamics necessary? How should it be initialized? The electromagnetic radiation in a HIC is heavily dependent on the cumulative emission throughout the expansion, so these questions can be resolved by comparing dilepton observables in different model calculations.

In this work, we propose a dynamic approach to fluidizing hadrons in state-of-the-art evolutions of intermediate beam energy HICs, based on the local energy density. This provides a natural core-corona separation, in contrast to commonly employed initalizations based on a constant proper time. In the presence of a first-order phase transition, the dilepton yield is greatly enhanced, which can be seen in the dilepton excitation function. Moreover, we highlight the need for event-by-event initial conditions compared to smooth averaged profiles, since hotspots account for a significant part of the dilepton spectra.

Speaker: Renan Góes-Hirayama (Frankfurt Institute for Advanced Studies)

10:20 The electric conductivity of nuclear matter along the QCD phase transition line

Transport coefficients play an important role in characterizing hot and dense nuclear matter, such as that created in ultra-relativistic heavy-ion collisions (URHIC). The electric conductivity can be accessed via the electromagnetic (EM) spectral function's low-energy transport peak, which can be measured via thermal dilepton emission. Several facilities including the Schwer-Ionen Synchrotron (SIS), the Relativistic Heavy-Ion Collider (RHIC), and the Large Hadron Collider (LHC) have potential to probe low energy dilepton signals in ongoing and future experiments.
In this contribution, we present our recently published study on the electric conductivity of hot and dense nuclear matter. We implement the vector dominance model (VDM), in which the photon couples to hadronic currents predominantly through the $\rho$ meson. Therefore, hadronic many-body theory is utilized to calculate the $\rho$-meson's self-energy in hot and dense hadronic matter, by dressing its pion cloud with $\pi$-$\rho$, $\pi$-$\sigma$, $\pi$-$K$, N-hole, and $\Delta$-hole loops. Vertex corrections are then introduced to maintain gauge invariance. In particular, we examine the transport peak and electric conductivity along a proposed phase transition line, and under conditions comparable to those expected in future heavy-ion experiments. We compare the transport properties for hadronic matter and a pion gas, in order to examine the individual contributions of various particles to the electric conductivity. In addition, we calculate the transport peak along a proposed phase transition line, and find that the electric conductivity shows a decreasing tendency from high to low collision energies.

Speaker: Joseph Atchison (Abilene Christian University)

08:40 → 10:40 Parallel session 27: Heavy flavor & quarkonia III

Convener: Dr Hendrik van Hees

08:40 Unscreened potential and quarkonium width in QGP

Screening of the heavy quark anti-quark potential was suggested as the cause of quarkonium melting in quark gluon plasma (QGP). Perturbative QCD calculations within potential non-relativistic QCD (pNRQCD) on the other hand do not lead to a screened potential in general and the thermal correction to the potential also has an imaginary part. We determine the complex potential in QGP using lattice calculations on lattices with large temporal extent $N_{\tau}=16-36$. The complex potential is extracted from the Wilson line correlators and physically motivated forms of the corresponding spectral functions [1,2]. We find that the real part of the complex potential is not screened in QGP independently of the detailed form of the spectral functions. We also estimate the imaginary part of the potential using prior information from various theoretical calculations [2].

Lattice calculations also find that the wave function of bottomonium states does not change much across the QCD transition [3]. This is consistent with the observation of the unscreened potential. Therefore, we use the vacuum wave function of different quarkonium states and the imaginary part of the potential obtained by us on the lattice to estimate the size of in-medium widths of $J/\psi$, $\psi(2S)$, $\chi_c(1P)$, $\Upsilon(1S)$, $\Upsilon(1S)$, $\Upsilon(1S)$ and $\chi_b(1P)$ and $\chi_b(2P)$ states
in QGP [2] and compare to lattice NRQCD results [3].

[1] Alexei Bazavov, Daniel Hoying, Rasmus N. Larsen, Swagato Mukherjee, Peter Petreczky, Alexander Rothkopf, Johannes Heinrich Weber, Phys. Rev. D 109 (2024) 074504

[2] Rasmus N. Larsen, Swagato Mukherjee, Peter Petreczky, Johannes Heinrich Weber, work in progress

[3] Rasmus N. Larsen, Stefan Meinel, Swagato Mukherjee, Peter Petreczky, Phys. Rev. D 102 (2020) 114508

Speaker: Rasmus Larsen

09:00 Determination of space diffusion coefficient by heavy flavour $R_{AA}$ and $v_n$ in an event-by-event approach

The strong interaction in the non-perturbative regime typical of the system created in Heavy-Ion collision has been studied in the past years through an effective Quasi-Particle Model ($QPM$): we present an extension to a more realistic model, named $QPMp$, incorporating momentum-dependent parton masses as entailed by QCD asymptotic free dynamics. This model naturally improves the description of quark susceptibilities previously underestimated in the simple $QPM$. Coupling the $QPMp$ modeling to an event-by-event transport approach based on Langevin equations, we evaluate the heavy-flavour $R_{AA}(p_T)$ and $v_{n}(p_T)$ for $D$, $\Lambda_c$, $B$, $\Lambda_b$ hadrons in $PbPb$ as well as for $OO$ at $5$ $ATeV$. In $PbPb$ we find a quite good prediction for both $D$ meson and leptons from $B$ decays at least within current experimental uncertainties; we also discuss the additional information that can be carried by collisions of light $OO$ system.
A main general result is that for charm quark the extracted space diffusion Ds(T) appears to be significantly larger (about $30-40\%$) wrt the recent lQCD/NREFT results at momentum $p=0$. We propose a novel analysis that point out the crucial role of the momentum dependence of the interaction to draw solid conclusions about the agreement between the phenomenological approach and lQCD calculation as well as to determine the thermalization time of HQ and in particular its mass dependence from charm to bottom quark. We will point out that $\Lambda_c$ production and the $R_{AA}(p_T)$ of D meson at very low momenta ($p_T<1.5$ $GeV$) will allow for a novel and more solid determination of the space diffusion coefficient.

M.L.Sambataro, V. Minissale, S. Plumari and V. Greco, Phys.Lett.B 849 (2024) 138480.

M. L. Sambataro, V. Greco, G. Parisi and S. Plumari, Eur.Phys.J.C 84 (2024) 9, 881.

M.L.Sambataro et al., in preparation.

Speaker: Maria Lucia Sambataro (Università degli Studi di Catania)

09:20 Efficiently simulating quarkonium's evolution beyond the dipole approximation

The open quantum system framework allows one to compute quarkonium's evolution in a medium, keeping track of the needed quantum features. However, computing this evolution is a computationally demanding task. QTRAJ is an efficient code that allows one to simulate the behavior of quarkonium in a medium in the case in which the medium sees quarkonium as a small color dipole $rT\ll 1$. While this limit is accurate for $\Upsilon(1S)$, its applicability to other quarkonium states is unclear. In this talk, wwe present a generalization of this code that incorporates the regime where $rT \sim 1$ in the one-gluon exchange approximation. In its new version, QTRAJ implements new jump operators connecting different states, which are then expanded in plane waves, giving rise to a variation of the algorithm present in QTRAJ 1.0 where jumps with $\Delta \ell > 1$ are allowed. We will show a review of this approach comparing the $rT\ll 1$ and $rT\sim 1$ cases, and we present preliminary phenomenological results.

Speaker: Jorge Manuel Martínez Vera (Università di Torino and Universitat de Barcelona)

09:40 Open charm production from small to large systems at LHCb

Open charm production is a sensitive probe of both hot and cold nuclear matter effects. Charm meson production provides strong constraints on nuclear parton distributions, while charm baryon and strange charm hadron production can be used to probe strangeness- and baryon-enhancing hot QCD effects, respectively. The LHCb detector is designed to study heavy flavor hadrons at the LHC, providing unique opportunities to study open charm production in heavy-ion collisions. In this contribution, recent LHCb results on open charm production will be discussed, as well as progress on studies of strangeness and baryon enhancement in the heavy flavor sector with 2024 PbPb data.

Speaker: John Matthew Durham (Los Alamos National Laboratory)

10:00 Charmonium production measurements from small to large systems at LHCb

Modifications of quarkonia production in hadronic collisions provide an important experimental observable to probe the heavy quark interaction with the nuclear medium. The excited ψ(2S) state, with a relatively low binding energy, is especially sensitive to these effects. In thiscontribution, we will present a new LHCb result on ψ(2s)/J/ψ production in PbPb collisions, along with progress on new measurements with Run 3 data.

Speaker: Camilla De Angelis (Universita e INFN, Cagliari (IT))

10:20 Quarkonium production in heavy-ion collisions with ALICE

Charmonium production has long been identified as one of the golden probes to study the quark­­–gluon plasma (QGP). In fact, the early production of heavy quarks ($c\bar{c}$ and $b\bar{b}$) make charmonia an ideal tool to investigate the evolution of the hot and dense medium produced in ultra-relativistic heavy-ion collisions. Moreover, at LHC energies the recombination of uncorrelated charm quarks pairs, namely (re)generation, was found to significantly affect charmonium observables, in contraposition to the well known suppression mechanism. This aspect makes the production measurements of ground and excited states more and more relevant to discriminate among the scenarios foreseen by the different theoretical models. In addition, the possibility to isolate the non-prompt charmonia, i.e. originating from beauty hadron decays, provides an insight into the energy loss experienced by the ancestor beauty quarks inside the QGP. In this contribution, recently published results by the ALICE collaboration of inclusive charmonia production obtained at central and forward rapidity in Pb--Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV will be presented. At midrapidity, the measurements of prompt and non-prompt J/$\psi$ production will be shown, while at forward rapidity the measurement will be extended to the excited state as the $\psi$(2S). Finally, the first preliminary measurement of the $\psi$(2S)-to-J/$\psi$ ratio obtained with the new data sample collected in Pb--Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.36 TeV will be presented.

Speaker: Ingrid Mckibben Lofnes (University of Bergen (NO))

08:40 → 10:40 Parallel session 28: Jets IV

Convener: James Lawrence Nagle (University of Colorado Boulder)

08:40 How loss of colour coherence affects intra-jet observables

When a hard parton fragments in vacuum, subsequent emissions are angular ordered due to colour coherence. In contrast to this, in a dense coloured medium interactions with the background change the parton's colour and disrupt colour coherence. The phase space for the next emission is then not constrained by angular ordering leading to potentially measurable differences in the distribution of fragments. The effect is not due only to a broader angular distribution of partons, because angular ordering also affects the energy sharing. In this talk I will present the improved treatment of angular ordering in the upcoming release of JEWEL and discuss the effect of loss of angular ordering on different intra-jet observables. I will also quantify related theoretical uncertainties.

Speaker: Korinna Zapp

09:00 Gluon emission by a quark-antiquark antenna with realistic parton-medium interactions

The spectrum of coherent gluon radiation from a quark-antiquark pair experiencing multiple scatterings within a coloured medium is central for understanding in-medium parton cascades. Despite its foundational importance, current results are limited by reliance on simplified scattering rates, such as the harmonic oscillator approximation, valid only in restricted phase-space regions. Using the formalism introduced in [1], we express the gluon emission spectrum as a set of differential equations that can be solved numerically, circumventing conventional approximations. We present the transverse momentum and energy distributions of emitted gluons for realistic interaction models, illustrating the breakdown of color coherence across the entire accessible phase-space, and consequently enabling a higher-precision description of jet observables.

[1] Andrés, Apolinário, Dominguez, JHEP 07 (2020) 114

Speaker: André Cordeiro (LIP/IST)

09:20 Jet Elliptic Anisotropy and Suppression in PbPb, AuAu and OO

The elliptic anisotropy of energetic particles produced in heavy-ion collisions is understood as an effect of a geometrical selection bias due to energy loss. In the measured ensemble, particles oriented in the direction in which the medium is shorter are over-represented as compared to those oriented in the direction in which the medium is longer. In this work we present the first semi-analytical predictions, including propagation through a realistic, hydrodynamical background, of the elliptic anisotropy for jets, obtaining quantitative agreement with available experimental data.

Jets are multi-partonic, extended objects and their energy loss is sensitive to substructure fluctuations. The ability of the medium to resolve those partonic fluctuations is determined by the physics of color coherence. We find that jet $v_2$ has a specially strong dependence on coherence physics due to the marked length-dependence of the critical angle $\theta_c$. By combining our predictions for the collision systems and center of mass energies studied at RHIC and the LHC, we show that the relative size of jet $v_2$ for jets with different cone-sizes $R$ follows a universal trend that indicates a transition from a coherent regime of jet quenching to a decoherent regime. These results suggest a way forward to experimentally reveal the role played by the physics of jet color decoherence in probing deconfined QCD matter. To this end, precise jet measurements from sPHENIX will be determinant as one explores larger values of $\theta_c$ in AuAu collisions. Our study incorporates results for OO collisions at the LHC as well, providing predictions for both jet $R_{\rm AA}$ and jet $v_2$ for different $R$.

Speaker: Adam Takacs (Heidelberg University)

09:40 First measurement of full event energy-energy correlation in high-Z tagged events in PbPb collisions in CMS

The production of a Z boson provides a clean handle to control the population of events to be studied. By selecting muonic decays of Z bosons, we can isolate the effect of the recoiling process without potential bias from requiring isolation, as is the case for photons. Di-hadron correlations can naturally separate effects from different angular scales and enable jet substructure measurements without jet reconstruction bias. Similar to the energy-energy correlator (EE) in jets where perturbative and non-perturbative regimes are separated, by studying analogous correlation in the full event, one can unravel potential larger-scale structures that may arise from the interaction of high-energy recoiling particles with the quark-gluon plasma. This talk will present the first measurement of the energy-weighted di-hadron correlation with the CMS collaboration using events tagged with a Z boson. A significant modification of EEC is observed not only in small to intermediate angular scale but also extended to the phase space that is far away from the typical range covered by jet based EEC. The result provides new insight into the jet quenching mechanism and information about the inner workings of the QGP.

Speaker: Ms Yi Chen (Vanderbilt University (US))

10:00 First evidence of the medium response to hard probes with Z-hadron correlations in PbPb and pp collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV

The first measurement of low transverse momentum ($p_\mathrm{T}$) charged hadron pseudorapidity and azimuthal angle distributions relative to $Z$ bosons in PbPb collisions at nucleon-nucleon center-of-mass energy $\sqrt{s_\mathrm{NN}} = 5.02$ TeV is presented. This study utilizes PbPb collision data recorded in 2018 with an integrated luminosity of $1.67 \pm 0.03$ nb$^{-1}$, as well as pp collision data acquired in 2017 with an integrated luminosity of $301 \pm 6$ pb$^{-1}$. For the first time in PbPb collisions, the azimuthal angle and pseudorapidity distributions of charged hadrons relative to $Z$ bosons are measured in bins of charged hadron $p_\mathrm{T}$ to search for in-medium parton shower modifications and medium recoil effects. The analysis focuses on events containing at least one $Z$ boson with $40 < p_\mathrm{T} < 350$ GeV. A significant modification in the azimuthal angle and pseudorapidity distributions for charged hadrons in the low $p_\mathrm{T}$ range, around 1 to 2 GeV, is observed compared to reference measurements from pp collisions. The results are consistent with expectations from phenomenological models, including medium recoil and the medium response to hard probes traversing the quark-gluon plasma. The data provide significant new information about the correlation between hard and soft particles in heavy ion collisions, which can be used to test predictions of various jet quenching models. In PbPb data comparisons with pp reference and theoretical models, the first evidence for medium recoil and hole effects caused by a hard probe is found.

Speaker: Yen-Jie Lee (Massachusetts Inst. of Technology (US))

10:20 First measurement of jet axis decorrelation for detecting in-medium momentum broadening in photon-tagged jets PbPb at 5.02 TeV with CMS

A search for medium-induced jet transverse momentum broadening is performed with isolated photon-tagged jet events in pp and PbPbcollisions at n5.02 TeV. The difference between jet axes as determined via energy-weight and winner-take-all clustering schemes, also known as the decorrelation of jet axes and denoted Δj, is measured for the first time in photon-tagged jet events. This observable is sensitive to both multiple scattering and large-angle scattering effects in the QGP. The pp and PbPb data samples were recorded with the CMS detector at the LHC and correspond to integrated luminosities of 1.69 nb−1 and 302 pb−1 respectively. Events are required to have a leading isolated photon with 6060 GeV in central PbPb show signs of narrowing relative to pp. The results are compared to the Hybrid, Jewel and Pyquen theoretical models, which include different mechanisms of energy loss.

Speaker: Lida Kalipoliti (LLR, École Polytechnique (FR))

08:40 → 10:40 Parallel session 29: Light and strange flavor physics & nuclei II

Convener: Marcus Bleicher

08:40 Strangeness production and polarization in fixed-target and proton-lead collisions at LHCb

With its precise vertex reconstruction and particle identification capabilities, the LHCb detector is ideally suited to study the production and polarization of strange particles. In addition, being the origin of hyperon polarization from unpolarized proton-proton and proton-nucleus collisions not yet fully understood, measurements in different collision systems and kinematic ranges must be provided. In this contribution, recent LHCb measurements of strange hyperon production and polarization in proton-lead collisions are discussed, including their implications for hadronization modification in small collision systems and for transverse-momentum-dependent parton distributions and fragmentation functions.

Speaker: Giulia Manca (Universita` degli studi di Cagliari and INFN, Cagliari, IT)

09:00 Exploring the hadronic phase with momentum and azimuthal distribution of short-lived resonances and understanding the internal structure of exotic resonances

Hadronic resonances are crucial probes to understand the various phases of matter created during relativistic heavy-ion collisions. Due to their short lifetimes, the yields of these resonances can be affected by competing rescattering and regeneration mechanisms in the final hadronic phase. Rescattering can alter the momentum of the resonance decay products, limiting their reconstruction through the invariant-mass technique, while pseudo-elastic scattering can regenerate them. Final state observables such as elliptic flow, transverse momentum spectra, and measured yields of resonances could be significantly modified due to the interaction in the hadronic phase. By contrasting the yields of longer-lived resonances, such as the ϕ-meson with shorter-lived ones, such as the K$^{*}$(892), it is possible to obtain information about the properties and timescales of the hadronic phase. This contribution will present new results on production yields, spectra, and flow harmonics for K$^{*}$(892) and ϕ(1020) in Pb–Pb collisions obtained by the ALICE Collaboration. The results will be compared with state-of-the-art models to interpret which underlying mechanism can describe the experimental observations.

In addition to probe hadronic phase, the study of resonances also offers valuable insights into the non-perturbative regime of Quantum Chromodynamics (QCD). Resonances such as the f$_0$(980) and f$_1$(1285) challenge the traditional quark model. Their structure is yet unknown as they could potentially be tetraquark states or meson-meson molecules. Proposed Glueball candidates like the f$_2$(1270), f$^{′}_2$(1525), and f$_0$(1710) also provide opportunities to explore the gluonic bound states predicted by lattice QCD. Utilizing its excellent particle identification capabilities ALICE has recently conducted detailed studies of the exotic resonance production in pp collisions data at √s = 13 and 13.6 TeV. This contribution will present new measurements of exotic resonances such as f$_0$(980), f$_1$(1285), and the glueball candidates to get more insight into their internal structure.

Speaker: Junlee Kim (CERN)

09:20 New Hypernuclei Measurements from STAR

Hypernuclei are bound states of hyperons (Y) and nucleons (N). Measurements on their yields can help us investigate their production mechanisms. In particular, the ${}^5_{\Lambda}$He and $^{4}_{\Lambda}$H(e) are bounded substantially tighter compared to the $^{3}_{\Lambda}$H. The large radius of the $^{3}_{\Lambda}$H leads to suppression in coalescence models, but not in the thermal model where the size of the nucleus does not play a role. As such, studying the $A=3-5$ hypernuclei yields allow us to extract information on the effects of hypernuclear binding on hypernuclei production in heavy-ion collisions. Meanwhile, measurements on their intrinsic properties can constrain the Y-N and Y-Y interactions, which are crucial ingredients for the equation-of-state of strange matter in dense environments.

In this talk, we will present ${}^5_{\Lambda}$He yields in Au+Au collisions at $\sqrt{s_{NN}}=3$ GeV, $^{4}_{\Lambda}$H(e) yields in Au+Au collisions at $\sqrt{s_{NN}}=3-4.5$ GeV, $^{3}_{\Lambda}$H yields in Au+Au collisions at $\sqrt{s_{NN}}=3-27$ GeV, and $^{3}_{\Lambda}$H(${}^{3}_{\bar{\Lambda}} \mathrm{\bar{H}}$) yields in Ru(Zr)+Ru(Zr) collisions at $\sqrt{s_{NN}}$ = 200 GeV. These measurements and thier yield ratios will be compared to thermal and coalescence model calculations, and implications on their production mechansims will be discussed. In addition, we will present the first measurement of $\bar{\mathrm{R}}_{3}$, the relative branching ratio of ${}^{3}_{\bar{\Lambda}} \mathrm{\bar{H}}$, which can provide information on the spin of the ${}^{3}_{\bar{\Lambda}} \mathrm{\bar{H}}$.

Speaker: yingjie zhou

09:40 Strangeness production in different collision systems and at different collision energies with the STAR experiment

Strangeness production has been suggested as a sensitive probe to the early dynamics of the deconfined matter created in heavy-ion collisions.
There are significant discussions in the field about the initial conditions, including the colliding energy and the size of the system, needed to generate a quark-gluon plasma (QGP). It is expected that the threshold can be revealed by the measurements of the colliding energy and system size dependence of baryon-to-meson enhancement. Recent datasets collected by STAR in Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7, 9.2, 11.5, 14.6, 17.3 and 19.6 GeV from the Beam Energy Scan II program and in different collision systems at $\sqrt{s_{NN}}$ = 200 GeV (d+Au, Au+Au, O+O, Zr+Zr, Ru+Ru) provide us a chance to look into the $\Omega$/$\phi$ ratios and possibly locate the threshold of QGP production.
In addition, (multi)strange hadron to pion yield versus multiplicity in different collision systems at $\sqrt{s_{NN}}$ = 200 GeV can be compared with the results from LHC, where an intriguingly smooth increase in these ratios was first reported.
Furthermore, rapidity density of (anti-)strange baryons may give insight into the baryon stopping mechanism.

In this talk, we will present new measurements of strange hadron ($K_s^0$, $\Lambda$, $\bar{\Lambda}$, $\Xi$, $\bar{\Xi}$, $\Omega$, $\bar{\Omega}$, $\phi$) production in Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7, 9.2, 11.5, 14.6, 17.3, 19.6, 200 GeV and in d+Au, O+O, Zr+Zr and Ru+Ru collisions at $\sqrt{s_{NN}}$ = 200 GeV, including transverse-momentum and rapidity spectra, nuclear modification factors, antibaryon-to-baryon ratios and baryon-to-meson ratios. In particular, precise measurements of the energy and centrality dependence of $\Omega$/$\phi$ ratios in Au+Au collisions at different energies and in different collision systems at $\sqrt{s_{NN}}$ = 200 GeV will be presented. In addition, (multi)strange hadron to pion yield versus multiplicity in different collision systems at $\sqrt{s_{NN}}$ = 200 GeV will also be reported.
These results will be compared to theoretical calculations and physics implications will be discussed.

Speaker: Weiguang Yuan (Tsinghua University)

10:00 Production, flow, and properties of hypernuclei in heavy-ions with ALICE

Hypernuclei are bound states of nucleons and hyperons. The measurement of the production of hypernuclei with mass number A=3 and 4 in heavy-ion collisions is a powerful tool to investigate the hyper-nucleosynthesis mechanism. In the coalescence model, the production yields are sensitive to the interplay between the spatial extension of the nucleus wavefunction and the baryon-emitting source size, whereas, in the statistical hadronization model, the nuclear structure does not come into play in the production. Hypernuclei span over a wide range of wavefunction radii, from about 2 fm for A=4 hypernuclei to about 10 fm for the hypertriton, making them ideal probes to test such models. In addition, the study of hypernuclei properties provides information on the nucleon-hyperon interactions, complementing the results obtained through femtoscopy correlation measurements. The strength of such interactions is a fundamental input to calculate the equation-of-state of the high-density nuclear matter found inside neutron stars.
This contribution presents recent measurements of ${}^{3}_\Lambda$H, ${}^{4}_\Lambda$H, and ${}^{4}_\Lambda$He based on the data samples collected by ALICE during the LHC Run 2 and Run 3. Besides the yield measurement, the production mechanisms can be investigated via anisotropic flow and global spin polarization measurements, comparing the behaviour of hypernuclei with their non-strange counterpart. In this contribution, we present recent flow measurements of ${}^{3}$He, ${}^{3}_\Lambda$H, and ${}^{4}$He, obtained from the large Pb-Pb data sample collected by ALICE at the LHC in Run 3. Both the yields and the flow coefficients will be discussed in the context of the state-of-the-art theoretical models.

Speaker: Janik Ditzel (Goethe University Frankfurt (DE))

10:20 Deciphering (anti)hypertriton spin from its global polarization in heavy-ion collisions

Understanding the properties of hypernuclei is crucial for constraining the nature of hyperon-nucleon ($Y\text{-}N$) interactions, which plays a key role in determining the inner structure of compact stars. The lightest (anti-)hypernuclei are the hypertriton ($^3_\Lambda\text{H}$), which consists of a pair of nucleons and a $\Lambda$ hyperon, and its anti-nucleus (${^3_{\bar{\Lambda}}}\overline{\rm H}$). Significant knowledge has recently been acquired regarding the mass, lifetime, and binding energy of $^3_\Lambda\text{H}$. However, its exact spin, whether $\frac{1}{2}$ or $\frac{3}{2}$, remains undetermined in both experimental and theoretical studies. Here, we present a novel method of using the hypertriton global polarization in heavy-ion collisions to decipher not only its total spin but also its internal spin structure. This method is based on the finding that its three different spin structures exhibit distinct beam energy dependence of its global polarization when it is produced in these collisions from the coalescence of proton, neutron and $\Lambda$. Future observations of the (anti-)hypertriton global polarization thus provide the opportunity to unveil the spin structure of (anti-)hypertriton and its production mechanism in heavy-ion collisions.

Speaker: Dr KaiJia Sun (Institute of Modern Physics, Fudan University, Shanghai, China)

08:40 → 10:40 Parallel session 30: QCD phase diagram & critical point II

Convener: Ilya Selyuzhenkov (GSI, Darmstadt)

08:40 Universal scaling near the chiral phase transition in (2+1)-flavor QCD

The chiral phase transition in (2+1)-flavor QCD is expected to be of second order if the breaking of axial anomaly remains sufficiently strong at the chiral phase transition temperature $T_c$ [1]. This is supported by lattice QCD calculations [2]. However, FRG model calculations suggest that the scaling window, in which universal scaling relations hold, may be small [3]. Furthermore, whether or not the $U_A(1)$ symmetry gets effectively restored at $T_c$ remains to be controversial. This suggest that a more detailed analysis of universal critical behavior close to $T_c$ , as well as a direct determination of the relevant universality class for the chiral phase transition [1,4] in (2+1)-flavor QCD are needed.

In this talk we present new results from a study of the scaling behavior
of an improved order parameter $M = M_\ell - H \chi_\ell$ for chiral symmetry restoration in the light 2-flavor sector of (2+1)-flavor QCD. Here $M_\ell$ and $\chi_\ell$ are multiplicatively renormalized light quark chiral condensate and chiral susceptibility, respectively. Furthermore, $H = m_l/m_s$ is the light-to-strange quark mass ratio.

We construct ratios of $M$ for two different values of the light quark mass, $m_l$ (or $H$ equivalently), and $T$ close to $T_c$ . In the scaling region we find a unique intersection point for these ratios at $T=T_c$, which allows us to determine $T_c$ and the underlying universality class of the chiral phase transition, from the knowledge of critical exponent $\delta$.

The approach followed here will allow to further constrain the influence of axial anomaly on universal critical behavior in (2+1)-flavor QCD. It also provides a new way of determining the chiral phase transition temperature $T_c$ , which strengthens the upper bound on the temperature range within which a critical end point in QCD for non-vanishing values of chemical potential can possibly be found in heavy ion experiments.

[1] R. D. Pisarski and F. Wilczek, Phys. Rev. D 29 (1984) 338
[2] H. T. Ding et al. [HotQCD], Phys. Rev. Lett. 123 (2019) 062002
[3] J. Braun et al., Phys. Rev. D 102 (2020) 056010
[4] A. Pelissetto and E. Vicari, Phys. Rev. D 88 (2013) 105018

Speaker: Dr Sabarnya Mitra (Bielefeld University)

09:00 QCD phase structure at finite isospin chemical potential and smaller-than-physical quark mass

Introduction of a nonzero isospin chemical potential in QCD leads to the emergence of a pion condensed phase at sufficiently large $\mu_I$, bounded by a second order transition line. At zero temperature the pion condensate appears at $\mu_I = m_\pi/2$. Recent numerical studies at physical quark masses show that the pion condensation boundary remains vertical up to the meeting point with the chiral crossover line. If this result remains valid when the light quark mass (and the pion mass) goes to zero, then in the chiral limit at temperatures below the chiral transition the pion condensation happens at arbitrary nonzero $\mu_I$. We report on results of a lattice QCD simulation of a 2+1 flavour QCD at nonzero isospin chemical potential, at smaller-than-physical light quark mass, that support this scenario.

Speaker: Volodymyr Chelnokov (Goethe Universität)

09:20 Baryon-Strangeness Correlations in Au+Au Collisions at RHIC-STAR

Fluctuations and correlations of conserved charges serve as sensitive observables for investigating QCD thermodynamics. In particular, baryon-strangeness correlations may offer insights on changing degrees of freedom carrying strangeness.

In this work, we present the measurements of baryon-strangeness correlations in Au+Au collisions from the beam energy scan program (BESI and BESII, 7.7-62.4 GeV) at STAR. This analysis includes newly analyzed datasets, marking the systematic investigation of the collision energy and centrality dependence of baryon-strangeness correlations. The study includes strange hadrons $K^{+}$, $\Lambda$, and $\Xi^{-}$ along with their anti-particles. Physics implications will be discussed by comparing these new results with calculations from lattice gauge theory, functional renormalization group, hadron resonance gas model, as well as hadronic transport model.

Speaker: HANWEN Feng (Central China Normal University)

09:40 Locating the QCD critical point from first principles through contours of constant entropy density

We propose a new method to investigate the existence and location of the conjectured high-temperature critical point of strongly interacting matter via contours of constant entropy density. By approximating these lines as a power series in the baryon chemical potential $\mu_B$, one can extrapolate them from first-principle results at zero net-baryon density, and use them to locate the QCD critical point, including the associated first-order and spinodal lines. As a proof of principle, we employ currently available continuum-extrapolated first-principle results from the Wuppertal--Budapest collaboration to find a critical point at a temperature and a baryon chemical potential of $T_c = 114.3 \pm 6.9$ MeV and $\mu_{B,c} = 602.1 \pm 62.1$ MeV, respectively, at expansion order $\mathcal{O}(\mu_B^2)$ [1]. We advocate for a more precise determination of the required expansion coefficients via lattice QCD simulations as a means of pinpointing the location of the critical endpoint in the phase diagram of strongly interacting matter.

[1] H. Shah et. al., arXiv:2410.16206

Speaker: Hitansh Shah

10:00 QCD deconfinement transition line up to $\mu_B=400$~MeV from finite volume lattice simulations

The QCD cross-over line in the temperature ($T$) -- baryo-chemical potential ($\mu_B$) plane has been computed by several lattice groups by calculating the chiral order parameter and its susceptibility at finite values of $\mu_B$. In this work we focus on the deconfinement aspect of the transition between hadronic and Quark Gluon Plasma (QGP) phases. We define the deconfinement temperature as the peak position of the static quark entropy ($S_Q(T,\mu_B)$) in $T$, which is based on the renormalized Polyakov loop. We extrapolate $S_Q(T,\mu_B)$ based on high statistics finite temperature ensembles on a $16^3\times 8$ lattice to finite density by means of a Taylor expansion to eighth order in $\mu_B$ (NNNLO) along the strangeness neutral line. The phase diagram is drawn up to unprecedentedly high chemical potentials.
Our results for the deconfinement temperature are in rough agreement with phenomenological estimates of the freeze-out curve in relativistic heavy ion collisions. In addition, we show that up to $\mu_B \approx 400$~MeV, the deconfinement transition gets broader at higher densities, disfavoring the existence of a deconfinement critical endpoint in this range. Finally, we examine the transition line without the strangeness neutrality condition and observe a hint for the narrowing of the crossover towards large $\mu_B$.

Speaker: Dr Chik Him Wong

10:20 Precision Measurement of Kinematic Range Scan of Fluctuations of (Net-)proton Multiplicity Distributions in Au+Au Collisions from RHIC-STAR

Higher-order cumulants of (net-)proton multiplicity distributions serve as sensitive observables for probing the QCD phase diagram and searching for the QCD critical point (CP). Critical point is a long range collective phenomenon, and within a larger kinematic range, the critical point signal is significantly enhanced, especially in the higher-order cumulants of net-proton multiplicity distributions. To explore these critical phenomena we conduct a detailed kinematic range scan of the (net-)proton number cumulants up to fourth order.

In this talk, we present the precision measurements of systematical kinematic range scans of the (net-)proton multiplicity cumulants along both rapidity and transverse momentum axes in Au+Au collisions at $\sqrt{s_{NN}}=7.7\ \mathrm{to}\ 27$ GeV from the beam energy scan program (phase-II) of the STAR experiment at RHIC. We apply both cumulative and differential scans to explore long-range correlations and examine the behaviors of cumulants within localized rapidity ($y$) and transverse momentum ($p_T$) bins. The results from 0-5% most central collisions are compared with non-critical model calculations and with data from 70-80% peripheral collisions, both of which are used as references.

Speaker: Yige Huang

10:40 → 11:10 Coffee break

11:10 → 12:50 Parallel session 31: Collective dynamics & small systems III

Convener: Hannah Elfner

11:10 Constraining the small system collectivity using d+Au and O+O collisions at $\sqrt{s_{NN}}$ = 200 GeV from STAR

The understanding of collectivity in small collision systems must address three key issues related to the initial conditions: (1) the role of nucleon and sub-nucleon fluctuations, (2) nucleon forces and emergent correlations in light ions, such as alpha clustering, and (3) the influence of longitudinal fluctuations and their impact on flow decorrelations. We present two- and four-particle cumulants of $v_2$ and $v_3$ based on new d+Au and O+O datasets collected in the year 2021 with extended forward and midrapidity acceptance of STAR. Notably, we observe a significant enhancement in both $v_2\{2\}$ and $v_2\{4\}$ in d+Au collisions compared to O+O collisions, in the highest-multiplicity events, suggesting markedly different ellipticities in these two systems. Comparisons with model calculations reveal a clear influence of sub-nucleon fluctuations and nuclear structure effects. The $v_n$ measurements are performed using two-particle correlations in rapidity ranges $|\eta|<1.5$ and $2.1<|\eta|<5.1$. The role of flow decorrelations with rapidity is investigated and the findings are compared to previous RHIC measurements and model predictions. These new findings greatly expand our understanding of small system collectivity.

Speaker: Zhengxi Yan

11:30 Measurement of Anisotropic Flow for BES-II Energies at RHIC-STAR

Anisotropic flow characterizes the shape and direction of expansion of the medium created in heavy-ion collisions. The Fourier coefficients in the expansion of particle azimuthal distribution describe the collective response of the medium to the shape of the initial collision geometry,
and are sensitive to the equation of state (EOS). A non-monotonic slope in the first-order flow coefficient $v_1$ (directed flow) is proposed as a potential signature of a first-order phase transition between the QGP and hadronic phase. The second-order coefficient $v_2$ (elliptic flow) exhibits NCQ scaling at top RHIC and LHC energies, reflecting partonic collectivity, but this scaling vanishes at $\sqrt{s_{NN}} = 3$ GeV. The third-order coefficient $v_3$ (triangular flow) is typically driven by initial-state fluctuations and is uncorrelated with the reaction plane. However, recent measurements at lower collision energies reveal a correlation between $v_3$ and the first-order event plane angle ($\Psi_{1}$), attributed to collision geometry and the potential in the responsive medium.\
\indent In this talk, we will report the measurements of $v_1$, $v_2$, and $v_3\{\Psi_{1}\}$ for light, strange, multi-strange, net-particles, and light nuclei in Au+Au collisions. These measurements span energies of $\sqrt{s_{NN}}$ = 3.2 -- 4.5 GeV in fixed-target mode, as well as 7.7 -- 19.6 GeV in collider mode, from the second phase of the Beam Energy Scan (BES-II) program at RHIC-STAR. We will present the precise measurements of $v_2$ for $\phi$ mesons and multi-strange hadrons, along with the energy dependence of NCQ scaling. The presented results aim to shed light on the onset of partonic interactions. Additionally, atomic mass number (A) scaled $v_1$ slope of light nuclei will be presented to understand their production mechanism. The experimental measurements will be compared with transport model calculations to explore the underlying physics mechanisms in the high baryon density region of the QCD phase diagram.

Speaker: SHARANG RAV SHARMA (IISER Tirupati)

11:50 Flow measurements at LHCb

Particle correlations are powerful tools for studying quantum chromodynamics in hadron collisions. In heavy-ion collisions, azimuthal angular correlations probe collective phenomena in hot, dense, nuclear media, such as QGP. In small collision systems, they could point to final-state effects or potential initial-state correlations. The LHCb experiment has the unique ability to study particle correlations in high-energy hadron collisions at forward rapidity, exploring a unique region with respect to other experiments, where longitudinal expansion becomes more important. In this contribution, recent results on collective flow from the LHCb experiment will be discussed

Speaker: Samuel Belin (Universidade de Santiago de Compostela (ES))

12:10 Fluid dynamics of charm and beauty quarks from large to small systems

Heavy quarks (i.e. charm and beauty) are powerful tools to characterize the quark-gluon plasma (QGP) produced in heavy-ion collisions. Although they are initially produced out of kinetic equilibrium via hard partonic scattering processes, recent measurements of the anisotropic flow of charmed hadrons pose the question regarding the possible thermalization of heavy quarks in the medium. By exploiting a mapping between transport theory and hydrodynamics [1], we developed a fluid-dynamic description of heavy-quark diffusion in the QCD plasma. We will present results for transverse momentum distributions and integrated yields of charm and beauty hadrons obtained with a fluid-dynamic code coupled with the conservation of a heavy-quark - antiquark current in the QGP. We will show that our calculations are in agreement with the experimental measurements in Pb-Pb collisions [2,3] and provide new predictions for Ne-Ne and O-O collisions.
This work is funded via the DFG ISOQUANT Collaborative Research Center (SFB 1225).
[1] Phys.Rev.D 106 (2022) 3, 034021
[2] Phys.Rev.D 108 (2023) 11, 116011
[3] Capellino, Dubla, Facen, Floerchinger, Grossi, Kirchner, Masciocchi, in preparation

Speaker: Federica Capellino (GSI - Helmholtzzentrum fur Schwerionenforschung GmbH (DE))

12:30 Universality of scaled particle spectra in ultrarelativistic heavy-ion collisions

We find a remarkable universality in the experimentally observed transverse momentum spectra in ultrarelativistic nuclear collisions, obtained by removing the global scales of total particle number and mean transverse momentum. This scaling behavior breaks down at large transverse momentum and for very small systems, such as those produced in p-p collisions. We further demonstrate that this universal scaling behavior is compatible with hydrodynamic predictions and, thus, may indicate the onset of a hydrodynamic regime. Our results pave the way for further theoretical and experimental investigations of this novel scaling phenomenon to bring to light the collective and non-collective behavior encoded in the transverse particle spectrum of different collision systems.
This presentation is based on 2406.15208.

Speaker: Dr Fernando Gardim (Federal University of Alfenas)

11:10 → 12:50 Parallel session 32: Correlations & fluctuations III

Convener: Horst Stoecker (GSI)

11:10 Recent femtoscopy and polarization studies with the CMS experiment

This presentation discusses recent results on femtoscopic correlations involving $D^0$ ($\bar{D0}$) mesons and hyperon polarization in pPb collisions at $\sqrt{s_{NN}}=8.16$ TeV. The femtoscopic measurements investigate the strong interaction for pair combinations of particles, allowing to extract the scattering observables accessible through their correlation functions. The results may contribute to the search for new exotic bound states of matter, such as molecular states arising from strong interactions. The results for charged hadrons produced in PbPb collisions at $\sqrt{s_{NN}}=5.02$ TeV will also be discussed, in which the Levy-type of sources are employed to extract the correlation strength parameter \lambda, the Levy index \alpha, and the Levy scale parameter $R$, as a function of the pair transverse mass $m_T$, for various centrality ranges. In addition, the first measurements of $\Lambda$ and $\bar{\Lambda}$ particles’ local polarization in pPb collision at $\sqrt{s_{NN}}= 8.16$ will be presented. These results pose a challenge to the current theoretical implementation of spin polarization in heavy ion collisions and offer new insights into the origin of spin polarization in hadronic collisions at LHC energies.

Speaker: Isabela Maietto Silverio (UNESP - Universidade Estadual Paulista (BR))

11:30 Residual 3rd-body Coulomb Effect on Identical Charged Pion Correlations in Au+Au Collisions at STAR

Measurements of identical pion femtoscopy offer insights into collision dynamics, such as collective expansion, geometry of the collision zone at freeze-out, etc. In addition to the quantum interference among the pion pairs, Coulomb interactions between the pair and the net positive charge in the emitting source affect the final measurements as well. Furthermore, due to the imbalance of protons and neutrons inside the colliding nuclei, initial isospin, which also affects the correlation functions, plays an important role in determining the Equation of State of the medium in collisions at high baryon density.

To extract the effect of the isospin, a systematic analysis of the identical charged pion correlations from $\sqrt{s_\mathrm{NN}}$ = 3.0, 3.2, 3.5, 3.9, 4.5, 5.2, 7.7 GeV Au+Au collisions collected by the STAR experiment has been carried out. A new procedure has been developed to remove the residual effect from the 3rd-body Coulomb force and to extract the contribution from isospin. In this talk, we will report collision energy dependence of the source size parameters extracted from the positive and negative charged pion correlation functions after removing the 3rd-body Coulomb effect. Transport model UrQMD calculations with realistic experimental cuts will be used to aid the discussions.

Speaker: Vinh Luong (Joint Institute for Nuclear Research)

11:50 Measurements of Light Nuclei ($d$, $t$, $^{3}He$) and $\Lambda$ correlation in Au+Au collisions at $\sqrt{s_{{NN}}}=$ 3 GeV from STAR

Heavy-ion collisions offer a new way to understand hyperon-nucleon ($Y$-$N$) interactions. The two-particle correlation in pair relative momentum, which reveals valuable information about the space-time evolution of the particle-emitting source and final state interactions involving hyperons, is the primary observable of interest. The measurements of correlations between light nuclei ($d$, $t$, $^{3}He$) and $\Lambda$ can shed light on internal structure and binding energy of hypernuclei as well as the many-body interactions among hyperon and nucleons which are important for understanding the inner structure and equation of state of neutron stars.
In this talk, we present the first measurement of $\it d$-$\Lambda$, $\it t$-$\Lambda$ and $^{3}He$-$\Lambda$ correlations with high statistics data of $\sqrt{s_{_{\rm NN}}}=$ 3 GeV Au+Au collisions from the STAR Beam Energy Scan II. The correlation functions are analyzed within the Lednicky-Lyuboshitz formalism in order to characterize the emission source size, the scattering length, and the effective range of light nuclei-$\Lambda$ interactions. The extracted parameters will be compared to those from other baryon correlations and various effective theory model calculations. Finally, physics implications on final state interactions involving hyperons and the inner structure of the light hypernuclei will be discussed.

Speaker: Xialei Jiang (Central China Normal University)

12:10 Tilted emitting source in the heavy-ion collisions

The three-dimensional structure of the initial state in heavy-ion collisions have become important topic of interest in the community, since it provides insights into the origins of anisotropic flow, the breaking of boost invariance, longitudinal de-correlation, and particle correlations observed after kinetic freeze-out. One of the important properties of the 3D initial state is the tilt of the particle-emitting source away from the beam direction—a phenomenon particularly pronounced at collision energies of a few GeV and lower. This tilt, rooted in the initial collision geometry’s three-dimensional structure, is relevant for understanding phenomena such as directed flow and particle polarization. However, a direct extraction of tilt from such measurements remains challenging.

By employing azimuthally-sensitive femtoscopy (asHBT), we explore the relationship between the tilt parameter measured experimentally and the predicted tilt of freeze-out coordinates from UrQMD model [1]. This approach allows us to investigate the tilt of the pion-emitting source in Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7, 14.5, and 27 GeV using asHBT techniques.

While tilt might naively be expected to correlate strongly only with collision centrality, our findings indicate a more pronounced dependence on the momentum of particle pairs. This momentum dependence invites a reevaluation of directed flow: rather than attributing it to a uniform tilt of the emission source, we propose that distinct regions of homogeneity with varying orientations may underlie these observations.

Our approach offers a framework that the STAR experiment can employ to perform a precision extraction of tilt using the high-statistics BES-II data—measurements of which are currently underway and whose prospects we discuss here. By accurately determining tilt angles across different particle types and momenta, we aim to provide a more precise description of directed flow across species and momenta. This method offers a potentially clearer and more detailed framework for understanding directed flow and other phenomena influenced by the three-dimensional initial state.

[1] Yevheniia Khyzhniak and Michael Annan Lisa. “Pair momentum depen-
dence of tilted source in heavy ion collisions”. In: (Oct. 2024). arXiv:
2410.15134 [nucl-th].

Speakers: Michael Lisa, Yevheniia Khyzhniak

12:30 The Fluctuating Boltzmann Equation and Short-Range Charge Correlations in Relativistic Heavy-Ion Collisions

Short-range correlations among hadrons in relativistic heavy-ion experiments are typically regarded as non-hydrodynamic and subtracted from the data. However, the fluid nature of the quark-gluon plasma (QGP) gives rise to short-range correlations stemming from hydrodynamic fluctuations. These correlations, driven by fluctuation-dissipation relations near local equilibrium, provide key insights into the transport properties of the QGP and can become particularly significant in the vicinity of the QCD critical region.

In this talk, we present a theoretical framework for calculating two-particle correlations of charged hadrons. Using the Schwinger-Keldysh formalism, we first derive an evolution equation for four-point functions of fields in complex scalar field theory. This equation can be interpreted using the Boltzmann-Langevin Equation (BLE), which predicts the two-particle correlations in momentum space amongst the charges of a conducting fluid. Our results demonstrate how measuring these short-range charge correlations in heavy ion collisions can constrain the transport properties of the QGP in detail.

Speaker: Li Yan (Fudan University)

11:10 → 12:50 Parallel session 33: Electromagnetic probes II

Convener: Alberica Toia (Goethe University Frankfurt (DE))

11:10 Direct photon production from small to large collision systems with ALICE

Electromagnetic probes are a unique tool for studying the space-time evolution of the hot and dense matter created in ultra-relativistic heavy-ion collisions. Photons and dielectron pairs are emitted during the entire evolution of the medium created in such collisions, providing access to direct-photon production that includes thermal radiation from the early hot stages of the collision. The measurement of direct photons in minimum-bias pp collisions serves as a crucial baseline for the studies in heavy-ion collisions, whereas pp collisions with high charged-particle multiplicities allow the search for interesting phenomena such as the possible presence of QGP in small systems.

This talk will present the final LHC Run 2 ALICE results on the direct-photon production in pp and Pb--Pb collisions. In central Pb–Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV, results from virtual and real direct photons, as well as direct photon Bose-Einstein correlations are shown, enabling a precise direct photon spectrum over a broad range of transverse momentum. To study the possible onset of the formation of a hot medium, we also report the results on the direct-photon production in pp collisions at $\sqrt{s}$ = 13 TeV as a function of charged-particle multiplicity. The results are compared to theoretical models that include a contribution from a thermalised source.

Speaker: Joshua Leon Konig (Goethe University Frankfurt (DE))

11:30 Early EM fields in PbPb collisions via their effect on dimuon decays of Z bosons and top quark production with CMS

We present a selection of recent measurements of electromagnetic probes conducted by CMS to probe initial-state aspects of the heavy ion collision. The Z boson has a very short lifetime and can decay to dimuon final states that can be precisely measured. These properties make it a valuable tool for studying the initial state of heavy ion collisions. It has been proposed that the momentum of muons resulting from a Z decay, and therefore the experimentally observed mass and width of the Z boson, may be modified by the presence of the immense initial-state electromagnetic fields that are thought to be created in heavy ion collisions. Using the CMS experiment, we report the first search for a modification of the observed Z boson mass and width in PbPb collisions at $\sqrt{s_{{\mathrm{NN}}}} = 5.02$ TeV relative to those observed in pp collisions. In addition, measurements of Z boson yields and azimuthal anisotropy, $v_2$, will be discussed. The centrality and rapidity dependence of the measurements are compared to theoretical models. Taken together, these measurements provide new insights into the role of initial electromagnetic fields, nuclear parton distribution functions, and initial-state geometry in heavy ion collisions. The production of top quarks in heavy ion collisions serves as a novel tool for investigating nuclear parton distribution functions at high Bjorken-x. Although being a quark, the top has a short lifetime, decaying predominantly to a W boson and b quark pair, before hadronizing. Leptonic final states from the subsequent W boson decay are thus effectively electroweak probes of the medium they traverse before reaching the detector. The CMS collaboration has reported evidence of top quark pair ($\mathrm{t\bar{t}}$) production using data from lead-lead (PbPb) collisions during Run 2. This talk will present the first measurement of $\mathrm{t\bar{t}}$ production utilizing $1.63~\mathrm{nb}^{-1}$ of PbPb data collected by CMS at 5.36 TeV in 2023 and derived using kinematic variables from leptons and jets.

Speaker: Jose Francisco Gonzalez (University of California Davis (US))

11:50 Thermal dielectron measurements with the STAR experiment

Thermal dielectrons, which are produced throughout the entire evolution of relativistic heavy-ion collisions and are not involved in strong interactions, are suggested as the ideal probes of the hot and dense medium. By measuring the mass spectra of thermal dielectrons, one can extract the average temperature of the medium in different stages.Since the thermal dielectron emission rate is proportional to the medium’s electromagnetic spectral function, the integrated yields offer valuable insights into the microscopic interaction mechanism between the electromagnetic current and the medium. Additionally, the azimuthal angle distribution of the single decayed electrons in the dielectron rest frame provides means to explore the polarization of thermal dielectrons.

In this talk, we will report on STAR measurements of thermal dielectrons produced in Au+Au collisions at BES-II energies ($\sqrt{s_{\text{NN}}}=$ 7.7 - 19.6 GeV), as well as in Ru+Ru and Zr+Zr collisions at $\sqrt{s_{\text{NN}}}=$ 200 GeV. The presented results will cover the electron angular distributions in the dielectron rest frame, thermal dielectron invariant mass spectra, excess yields, and temperature measurements.

Speaker: Jiaxuan Luo

12:10 Electromagnetic tomography of Quark-Gluon Plasma at finite baryon densities

Relativistic heavy-ion collisions at the RHIC Beam Energy Scan program and SPS energies probe the QCD phase diagram at finite baryon densities. Precise photon and dilepton measurements serve as unique tomographic probes for the properties of hot nuclear matter. In this work, we study the electromagnetic radiation from relativistic heavy-ion collisions from 7.7 to 200 GeV. The dynamical evolution of the QCD medium is calibrated with hadronic data in the RHIC Beam energy scan program based on a recent (3+1)D Bayesian analysis [1, 2]. We convolute the full model posterior distribution from the Bayesian analysis to make robust predictions for the electromagnetic observables with the propagated theoretical uncertainties. We will show how photon and dilepton observables, especially their rapidity dependence, can impose complementary constraints on the QGP dynamics and help to deduce its transport properties at finite baryon densities.

[1] C. Shen, B. Schenke and W. Zhao, "Viscosities of the Baryon-Rich Quark-Gluon Plasma from Beam Energy Scan Data," Phys. Rev. Lett. 132, no.7, 072301 (2024)

[2] S. A. Jahan, H. Roch and C. Shen, "Bayesian analysis of (3+1)D relativistic nuclear dynamics with the RHIC beam energy scan data," arXiv:2408.00537 [nucl-th].

Speaker: Xiang-Yu Wu (McGill University)

12:30 Dilepton polarization as a signature of plasma anisotropy

We propose the angular distribution of lepton pairs produced in ultrarelativistic heavy-ion collisions as a probe of thermalization of the quark-gluon plasma. We focus on dileptons with invariant masses large enough that they are produced through quark-antiquark annihilation in the early stages of the collision. The angular distribution of the lepton in the rest frame of the pair then reflects the angular distribution of quark momenta. At early times, the transverse pressure of the quark-gluon plasma is larger than its longitudinal pressure as a result of the fast longitudinal expansion, which results in an oblate lepton distribution. By contrast, direct (Drell-Yan) production by quarks and antiquarks from incoming nuclei, whose momenta are essentially longitudinal, results in a prolate distribution. As the invariant mass increases, Drell-Yan gradually becomes the dominant source of dilepton production, and the lepton distribution evolves from oblate to prolate. The invariant mass at which the transition occurs is highly sensitive to the equilibration time of the quark-gluon plasma or, equivalently, the shear viscosity over entropy ratio %\eta/s% in the early stages of the collision.

Speaker: Maurice Coquet (Centre National de la Recherche Scientifique (FR))

11:10 → 12:50 Parallel session 34: Physics of ultraperipheral collisions II

Convener: Daniel Tapia Takaki (University of Kansas)

11:10 Constraining nuclear PDFs at low-x with the first measurements of the D0 production in ultraperipheral heavy ion collisions with CMS

In this talk, we present the first measurement of $\mathrm{D}^0$ photoproduction in heavy ion ultraperipheral collisions (UPCs) using the data collected by CMS experiment during 2023 PbPb collisions at LHC. The measured production cross sections are presented as a function of the $\mathrm{D}^0$ transverse momentum (2$< p_{T} <$12 GeV/c) and rapidity (-2< y <2). Additionally, we will present new results from a much larger data sample from 2024 PbPb UPCs, which could significantly improve the data precision and extend the measurements to zero $p_T$. The results are compared to theoretical calculations that exploit different modeling of the nuclear parton distribution functions (nPDFs) and provide new constraints into the properties of nuclear matter at low x down to $x~10^{-4}$ and for Q^2 ranging from O(10) to O(100) GeV$^2$, in absence of significant final-state effects.

Speaker: Yu-Chen (Janice) Chen (Massachusetts Institute of Technology)

11:30 First energy-dependent measurement of incoherent J/Psi photoproduction in PbPb UPCs at 5.02 TeV at CMS

The study of $\rm{J}/\psi$ photoproduction in ultraperipheral collisions (UPCs) offers a powerful probe of gluon dynamics in heavy nuclei. In UPCs, photons can interact with the nucleus coherently (involving the entire nucleus) or incoherently (with individual constituents). While coherent processes provide insight into the average gluon density, incoherent photoproduction is uniquely sensitive to the local gluon density fluctuations at both nucleonic or subnucleonic levels. In this talk, we present the first energy-dependent measurement of the incoherent $\rm{J}/\psi$ photoproduction cross section in PbPb UPCs at the LHC with the CMS experiment. Employing forward neutron tagging technique, we explore a wide photon-nucleon center-of-mass energy range from 40 to 400 GeV, probing fluctuating gluon fields over a broad Bjorken-$x$ range of $x <10^{-4}$. Furthermore, we report the cross section ratio of incoherent to coherent $\rm{J}/\psi$ production and evaluate the nuclear suppression factor for the incoherent production as a function of $x$. By comparing state-of-the-art theoretical predictions, these results offer unique insights into gluon fluctuations and have important implications for understanding nuclear gluon distributions and saturation phenomena.

Speaker: Xiao Huang (Rice University (US))

11:50 Photon-induced $J/\psi$ production and polarization effects in isobar collisions at STAR

In relativistic heavy-ion collisions, $J/\psi$ photoproduction serves as a sensitive probe for studying the gluonic structure in heavy nuclei. Differential measurements of photon-induced $J/\psi$ production provide important constraints on gluon distribution functions and sub-nucleonic shape fluctuations. The linear polarization of photons involved in these processes enables imaging of the nucleus via spin interference effect in vector meson photoproduction, which can be quantified by measuring the azimuthal angular modulation between the $J/\psi$ momentum and its decay daughters' momenta. Moreover, the decay daughters of these vector mesons inherit polarization of the photons, which could be used to access the initial collision geometry. Thus, measurement of differential cross sections of J/$\psi$ photoproduction and the azimuthal anisotropy of their decay daughters offer a novel and direct probe into both the gluonic structure and the initial collision geometry.

In this contribution, we will present the differential cross sections of photon-induced coherent $J/\psi$ in Ru+Ru and Zr+Zr ultra-peripheral collisions (UPCs) at $\sqrt{s_{NN}}$ = 200 GeV. These cross-section measurements will also be compared with existing d+Au and Au+Au data to investigate system size dependencies. Furthermore, we will report the azimuthal angular modulation measurements of the photon-induced $J/\psi$ in isobaric UPCs. Finally, we will present the measurements of the azimuthal anisotropy of electrons decayed from photon-induced $J/\psi$ with respect to the event plane in non-central collisions from the same collision systems. Physics implications of these results will be discussed together with model comparisons.

Speaker: Kaiyang Wang (University of Science and Technology of China)

12:10 Hard probes in ultraperipheral collisions at LHCb

Measurements of quarkonia production in peripheral and ultraperipheral
heavy-ion collisions are sensitive to photon-photon and photon-nucleus interactions, the partonic structure of nuclei, and to the mechanisms of vector-meson production. LHCb has studied production of the J/ψ and ψ(2s) charmonium states in peripheral and ultra-peripheral collisions using PbPb data at forward rapidity, obtaining the highest precision currently accessible. In this contribution, quarkonium measurements, along with new studies of charm pair production, will be discussed and compared with the latest theoretical models and with results from other experiments. Future UPC measurements with the upgraded LHCb detector in Run 3 will also be discussed

Speaker: Krista Lizbeth Smith (Pusan National University (ROK))

12:30 Nuclear suppression in diffractive vector meson production within the color glass condensate framework

Diffractive vector meson production is a golden channel in the search for gluon saturation. The color glass condensate (CGC) framework has been successfully applied to describe diffractive vector meson production in $e+p$ collisions at HERA. Predictions for $\gamma+Pb$ collisions realized in ultraperipheral collisions at the LHC are however overestimating the experimental data at large center of mass energy, implying that the calculated saturation effects are not strong enough to describe the nuclear suppression. In this work we perform a combined Bayesian analysis of $J/\psi$ production data from both $\gamma+p$ and $\gamma+Pb$ collisions within the CGC framework based on an impact parameter dependent McLerran-Venugopalan model combined with JIMWLK evolution to describe the center of mass energy dependence. The combined fit confirms the challenges of the framework to describe both $\gamma+p$ and $\gamma+Pb$ data simultaneously hinting at shortcomings of the framework. We discuss major sources of uncertainty and suggest possible solutions including model extensions. We further present predictions for intermediate systems such as $\gamma+O$ collisions whose measurement in UPCs could provide further insight into the $A$ dependence of the $J/\psi$ production cross section.

Speaker: Dr Hendrik Roch (Wayne State University)

11:10 → 12:50 Parallel session 35: QCD phase diagram & critical point III

Convener: Dr Anar Rustamov (GSI Helmholtzzentrum für Schwerionenforschung GmbH)

11:10 Proton High-order Cumulants Results from the STAR Fixed-Target Program

Fluctuations of conserved charges in heavy-ion collisions are expected to be sensitive to a critical point in the phase diagram of QCD matter [1, 2]. Such a critical point is increasingly predicted to be located in the high baryon chemical potential ($\mu_B$) region around $\mu_B$ = 500 - 650 MeV [3–8]. In 2018, the STAR Experiment started collecting data in a fixed-target configuration in order to map the high baryon chemical potential region of the phase diagram ($\mu_B$ = 420 - 720 MeV). Critical fluctuations may be observed by measuring various orders of cumulants, $C_n$, of the distributions of baryon number. The collision-energy dependence of net-proton $C_4$/$C_2$ from STAR’s measurements in Beam Energy Scan I hinted at a possible non-monotonic deviation from the non-critical baseline in Au+Au collisions from $\sqrt{s_{NN}}$ = 7.7 GeV to 19.6 GeV, and the first published result from the fixed-target program, in Au+Au collisions at $\sqrt{s_{NN}}$ = 3.0 GeV is consistent with the non-critical baseline [9–13]. We report here new results on proton-number high-order cumulants from STAR’s Fixed-Target Program. Implications for the QCD phase diagram and critical-point search will be discussed.

[1] M. A. Stephanov, Phys. Rev. Lett. 102, 032301 (2009).
[2] M. A. Stephanov, Journal of Physics G: Nuclear and Particle Physics 38, 124147 (2011).
[3] W.-j. Fu, J. M. Pawlowski, and F. Rennecke, Phys. Rev. D 101, 054032 (2020).
[4] F. Gao and J. M. Pawlowski, Physics Letters B 820, 136584 (2021).
[5] P. J. Gunkel and C. S. Fischer, Phys. Rev. D 104, 054022 (2021).
[6] J. Goswami, D.A. Clarke, P. Dimopoulos, F. Di Renzo, C. Schmidt, S. Singh, and K. Zambello, EPJ Web Conf. 296, 06007 (2024).
[7] A. Sorensen and P. Sorensen, (2024), arXiv:2405.10278 [nucl-th] .
[8] M. Hippert et al., (2023), arXiv:2309.00579 [nucl-th] .
[9] M. S. Abdallah et al. (STAR Collaboration), Phys. Rev. C 107, 024908 (2023).
[10] M. S. Abdallah et al. (STAR Collaboration), Phys. Rev. Lett. 128, 202303 (2022).
[11] M. S. Abdallah et al. (STAR Collaboration), Phys. Rev. Lett. 127, 262301 (2021).
[12] J. Adam et al. (STAR Collaboration), Phys. Rev. Lett. 126, 092301 (2021).

Speaker: Zachary Sweger (University of California, Davis)

11:30 The antiproton puzzle, QCD critical point, and fireball properties in heavy-ion collisions

We contrast the behavior of ordinary and factorial cumulants of proton number and point out that the latter has numerous advantages over the former, in particular with regards to the critical point search in heavy-ion collisions. The new RHIC BES-II data on factorial cumulants of protons is analyzed, indicating clear deviation of two- and three-particle proton correlations from the non-critical baseline at collision energies below 10 GeV. Multiple scenarios for this deviation are explored, including the possibility of critical fluctuations and the implications for the QCD phase diagram.

We also illustrate that the acceptance dependence of the appropriately scaled factorial cumulants, $\hat{C}_n/(\hat{C}_1)^n$, is flat if all correlations are driven by long-range correlations only, making it a clean probe of short-range correlations. Furthermore, global baryon conservation and volume fluctuations predict identical $\hat C_2/(\hat C_1)^2$ of proton and antiprotons at given collision energy. However, the analysis of RHIC BES-I data reveals a remarkably strong splitting between scaled factorial cumulants of protons and antiprotons, which is at odds with the expectation based on a single-fluid hydrodynamics picture. We argue that such a splitting indicates incomplete equilibration between stopped and produced matter, which can be further probed with upcoming high-statistics data from RHIC BES-II with extended acceptance coverage.

Speaker: Dr Volodymyr Vovchenko (University of Houston)

11:50 First order transition region of an equation of state for QCD with a critical point

In addition to signals for the critical point, evidence for a first order phase transition would indicate a nontrivial structure within the QCD phase diagram. Moreover, while not a direct measurement of the critical point, the presence of a first order transition would imply its existence. This motivates the need to understand signatures of this first order transition in addition to directly studying the effect of a critical point. To this effect, we map the mean-field Ising model equation of state onto the QCD phase diagram, and reconstruct the full coexistence region in the case of a first order phase transition [1]. Beyond the coexistence line, we maintain access to the spinodal region in the phase diagram, thus providing a description of metastable and unstable phases of matter. Thus, we describe the super-heated hadronic phase and the super-cooled quark-gluon plasma, which is useful for hydrodynamic simulations of the fireball created in a heavy-ion collision at low collision energy, where a first order phase transition is expected. We discuss the features of the pressure and other thermodynamic observables as functions of temperature and baryonic chemical potential, in particular their behavior in the coexistence region. Finally, we compare our equation of state to 3D Ising model ones available in the literature.

1. J.M. Karthein, V. Koch, C. Ratti, arXiv:2409.13961, PRD (under review)

Speaker: Jamie Karthein (MIT)

12:10 Non-Fluctuational Signature of the QCD Critical Point

We study the influence of the QCD critical point on the trajectories describing the evolution of the expanding fireball in heavy-ion collisions. We find that the lines of constant specific entropy (entropy per baryon) on the phase diagram are drawn towards a special point on the coexistence (first-order) line where the specific entropy reaches a maximum [1]. We study the interplay between this feature of the isentropic trajectories and the freezeout curve. If the critical point is located near the freezeout curve, a region inaccessible by isentropic expansion appears on the freezeout curve. This behavior of the isentropes could provide a novel, non-fluctuational signature of the critical point requiring significantly lower statistics than the fluctuational signatures.

[1] Maneesha Sushama Pradeep, Noriyuki Sogabe, Mikhail Stephanov, and Ho-Ung Yee, Phys. Rev. C 109, 064905 (2024).

Speaker: Noriyuki Sogabe (University of Illinois Chicago)

12:30 Higher order net-baryon number cumulants and baryon-strangeness correlations: Comparing QCD results on the pseudo-critical line with RHIC-BES II results on the freeze-out line

Non-monotonic behavior in the RHIC net proton number fluctuation data could signal the presence of a QCD critical point if they probe thermal conditions at the time of freeze-out. However, the connection between higher-order baryon number cumulants calculated in equilibrium QCD thermodynamics and higher-order net proton number cumulants may not be straightforward. Moreover, correlations between conserved net baryon number and net strangeness number have been suggested as sensitive probes for detecting changes in the degrees of freedom that carry strangeness at low temperatures (hadrons) and high temperatures (quarks) [1]. A non-interacting hadron resonance gas (HRG) model can explain baryon-strangeness correlations calculated in QCD at low temperatures and small chemical potentials, especially when additional strange hadrons are included (QMHRG2020).
In this presentation, we will show the ratio of first-order to second-order baryon number fluctuations and compare them with the RHIC BES-II results for proton number fluctuations. We will argue that a simple non-interacting HRG model cannot explain the RHIC BES-II results at higher densities, particularly at lower beam energies. Additionally, we will compare higher-order baryon number fluctuations with lattice QCD results and argue that these results rule out the possibility of a QCD critical point in the BES-II collider mode. We will also discuss the potential influence of corrections arising from global charge conservation in heavy-ion collisions [2]. Finally, we will compare our results on correlations between conserved net baryon number and net strangeness number at finite chemical potential [3] with preliminary results from RHIC BES-II.
[1] V. Koch et al, Baryon-strangeness correlations: a diagnostic of strongly interacting matter, Phys. Rev. Lett. 95, 2005, 182301
[2] V. Vovchenko et al, Connecting fluctuation measurements in heavy-ion collisions with the grand-canonical susceptibilities, Phys.Lett.B 811 (2020) 135868
[3] D. Bollweg et al, Strangeness-correlations on the pseudocritical line in (2+1)-flavor QCD, Phys.Rev.D 110 (2024), 054519

Speaker: Jishnu Goswami (Bielefeld University)

12:50 → 19:20 Excursions

13:30 → 17:00 IAC Meeting

17:30 → 20:30 IAC Dinner

Thursday 10 April

09:00 → 10:40 Parallel session 10: QCD phase diagram & critical point IV

Convener: Joachim Stroth

09:00 Next to leading order estimate for the critical endpoint from contours of constant observables with lattice QCD

A recent suggestion for finding the QCD critical endpoint [arXiv:2410.16206]
is to investigate entropy contours on the QCD phase diagram. We generalize this
idea to other lines of constant observables, like that of strangeness
susceptibility. Such quantities can not be directly calculated in lattice QCD
because of the infamous sign problem. We present an analytical continuation of
various contours from vanishing and imaginary chemical potentials. This way we
can constrain possible locations of the QCD critical endpoint. We use 4stout
lattices with a large volume which are tuned for strangeness neutrality.

Speaker: Jana N. Guenther (University of Wuppertal)

09:20 Critical point and Columbia plot from functional QCD

In the past year, results for the location of the critical point from different
approaches seem to converge towards previous predictions from functional QCD.
We summarise recent theoretical results of the latter approach on the
Columbia plot [1], the QCD phase diagram and the location of the critical
point at finite baryon chemical potential [3]. We highlight our efforts to
systematically gauge the approach by making contact with lattice results
at zero and imaginary chemical potential [2] in various regions of the Columbia
plot [1]. We furthermore discuss steps towards making contact with experiment
via baryon number fluctuations [4].

[1] J. Bernhardt and C. S. Fischer, Phys. Rev. D 108 (2023) no.11, 114018
[2] J. Bernhardt and C. S. Fischer, Eur. Phys. J. A 59 (2023) no.8, 181
[3] P. J. Gunkel and C. S. Fischer, Phys. Rev. D 104 (2021) no.5, 054022
[4] P. Isserstedt, M. Buballa, C. S. Fischer and P. J. Gunkel, Phys. Rev. D 100 (2019) no.7, 074011

Speaker: Prof. Christian Fischer (University of Giessen, Germany)

09:40 Search for the critical endpoint in high-statistics lattice QCD simulations

One method to estimate the position of the critical endpoint of QCD is to model the free energy as a rational function of the baryon chemical potential $\mu_B$ and determine the Lee-Yang edge singularities. Using high-statistics simulations on 4HEX improved staggered $16^3\times 8$ lattices by the Wuppertal-Budapest Collaboration we estimate the location of the closest singularity in the QCD phase diagram. Using this lattice setup we are able to reach an unprecedentedly low temperature of $T=100$MeV in our simulation dataset. To understand the true predictive power of such an approach we analyze the systematic uncertainties of such an approach in detail. We compare various ansätze, including formulations that preserve baryon charge quantization by forcing the appropriate periodicity in the imaginary chemical potential. The parameters can be constrained by the cumulants of the net baryon density calculated with lattice simulations at $\mu_B^2\leq 0$. Thus, we also compare single point and multipoint Padé approximations.

Speaker: Alexander Adam (Bergische Universität Wuppertal)

10:00 Quantifying fluctuation signatures of the QCD critical point using maximum entropy freeze-out

A key question about the QCD phase diagram is whether there is a critical point somewhere on the boundary between the hadron gas and quark-gluon plasma phases, and if so where. Heavy-ion collisions offer a unique opportunity to search for signatures of such a critical point by analyzing event-by-event fluctuations in particle multiplicities. To draw meaningful conclusions from experimental data, a theoretical framework is needed to link QCD thermodynamics with the particle spectra and correlations observed in detectors. The unknown Equation of State (EoS) of QCD near a critical point can be related to the universal Gibbs free energy of the 3D Ising model using four non-universal mapping parameters whose values are determined by the microscopic details of QCD. We use maximum entropy freeze-out of fluctuations to make estimates for the factorial cumulants of proton multiplicities, assuming thermal equilibrium, for a family of EoSs with a 3D Ising-like critical point, varying the microscopic inputs that determine the location and strength of the critical point. We quantify the effect of the non-universal mapping parameters, the distance between the critical point and the freeze-out curve, and the contribution of decay protons on the factorial cumulants of proton multiplicities.

Speaker: Maneesha Sushama Pradeep

10:20 Constraining the Phases of Hot QCD Matter via Precision Measurement of the Speed of Sound with the CMS Experiment

Ultrarelativistic nuclear collisions have revealed the existence of a hot, dense medium exhibiting collective flow with minimal viscous dissipation, indicative of a strongly interacting quark-gluon plasma. The CMS Collaboration previously measured the speed of sound in this medium, determining a value of approximately 0.241 at an effective temperature of 219 MeV in ultracentral 5.02 TeV PbPb collisions at the LHC, in excellent agreement with lattice quantum chromodynamics (QCD) predictions for a deconfined QCD matter. In this talk, we present new results from measurements using 5.36 TeV PbPb data collected during LHC Run 3 (2023) and 8.16 TeV pPb data from Run 2. The new larger data samples significantly improve precision, enabling more detailed differential studies. We explore the temperature dependence of the speed of sound by analyzing data at different collision energies and the mean transverse momentum at various rapidities. Additionally, the impact of different centrality estimators on the extraction of the speed of sound is examined in detail. These new results provide critical insights into the thermal properties of the quark-gluon plasma and offer stringent tests of lattice QCD predictions.

Speaker: Wei Li (Rice University (US))

09:00 → 10:40 Parallel session 6: Collective dynamics & small systems IV

Convener: Dr Jan Steinheimer (Frankfurt Institute for Advanced Studies)

09:00 Probing QCD collectivity at the smallest scales through photonuclear collisions and high-multiplicity jets in pp collisions at CMS

Recent CMS data revealed intriguing long-range correlations within high-multiplicity jets produced in proton-proton collisions, suggesting the potential onset of collective behavior, typically associated with heavy-ion collisions, at much smaller scales. Two-particle correlations in the “jet frame” show a surprising rise in elliptic flow harmonics, v2, at large pseudorapidity separations (Δη > 2), in jets with high charged-particle multiplicities, a trend not reproduced by event generators like Pythia and Sherpa. In this talk, we present the transverse momentum and ∆η* dependence of elliptic and triangular flow, across a wide range of jet multiplicity and transverse momentum. We further explore the role of jet substructure in driving the observed long-range azimuthal correlations, examining its potential connection to the geometry of the initial-state jet system. Recent LHC studies also show signs of collectivity in high-multiplicity photonuclear collisions, where a quasi-real photon from one nucleus interacts with the other nucleus. In this talk, we also present new CMS measurements of two- and multi-particle correlations from photonuclear collisions using PbPb data at $\sqrt{s_{NN}} = 5.36$ TeV collected during the LHC Run 3. By leveraging the full detector acceptance, we explore wide pseudorapidity gaps up to 7–8 units, which significantly reduces non-collective contributions and enhances the search for long-range near-side ridge effects. Elliptic and triangular azimuthal anisotropies are extracted as functions of particle transverse momentum and event multiplicity.

Speaker: Xiaoyu Liu (Rice University (US))

09:20 First measurement of high $p_\mathrm{T}$ azimuthal anisotropy using subevent cumulants in small system collisions at CMS

Measurements at the LHC have provided evidence for collective behavior in high-multiplicity pp and pPb collisions through multiparticle correlation techniques. However, no conclusive evidence of jet quenching—characterized by the energy loss of high-$p_\mathrm{T}$ partons traversing the medium—have been observed in these smaller systems. This paradox raises an intriguing question: How can a medium that exhibits hydrodynamic-like behavior and substantially modifies the distribution of final-state hadrons have little to no effect on high-$p_\mathrm{T}$ particles? To explore this, we present a comprehensive analysis of differential Fourier coefficients ($v_{n}$) as a function of particle transverse momentum ($p_\mathrm{T}$) and event multiplicity in pp and pPb collisions, recorded by CMS at 13 TeV and 8.16 TeV, respectively. In particular, first measurements of $p_\mathrm{T}$-differential multiparticle cumulants using the subevent method are reported, probing an extended phase space region up to high $p_\mathrm{T}$ values. Furthermore, we compare the results across pp, pPb, and PbPb collisions within similar multiplicity ranges. This comparison will help assess similarities and differences in the medium's interaction with high-$p_\mathrm{T}$ particles in these three collision types.

Speaker: Rohit Kumar Singh (Indian Institute of Technology Madras (IN))

09:40 sPHENIX measurements of collective behavior in small and large systems

sPHENIX is a next-generation collider detector at RHIC which was successfully commissioned in 2023 and 2024 and took physics data in p+p and Au+Au collisions. In addition to its high-p$_T$ jet and heavy flavor physics program, sPHENIX is a general purpose collider detector with excellent capabilities for exploring collective phenomena in small and large systems. This is due to its large pseudorapidity and full-azimuth acceptance, dedicated forward detectors such as the sPHENIX Event Plane Detector (sEPD), and large data samples collected in RHIC Run-24. This talk presents new measurements of collective phenomena in Au+Au collisions using the sEPD, and in p+p collisions through two-particle correlation and cumulant methods. These measurements can help shed light on the nature of collective behavior at RHIC energies, especially in small systems, with a greatly expanded statistical and kinematic reach.

Speaker: Hao-Ren Jheng (Massachusetts Inst. of Technology (US))

10:00 Longitudinal dynamics of large and small systems from a 3D Bayesian calibration with RHIC top energy data

Over the past several years, significant progress has been made in leveraging rapidity-dependent measurements from colliders to gain insight about the early and late-time dynamics of relativistic nuclear collisions. In this study, we perform a systematic model-to-data comparison using simulations of Au-Au and $d$-Au collisions and a broad range of measurements from the PHENIX, STAR, PHOBOS, and BRAHMS Collaborations spanning nearly two decades of RHIC operations. In particular, we perform fully 3D multi-stage simulations including rapidity-dependent energy deposition with global energy conservation, along with relativistic viscous hydrodynamics and a hadronic afterburner [1]. We calibrate on rapidity- and pT-differential observables–examining the respective constraints on initial state and transport properties–and highlight the tension between them within our model. We quantify the additional constraints provided by rapidity dependent measurements and we emphasize the similarity in posteriors from separate calibrations of large and small systems. We use our calibrated model to make predictions of observables in additional smaller systems in $p$-Au and $^3$He-Au, as well as statistically-sensitive or proposed measurements in the calibrated systems. Furthermore, we predict longitudinal de-correlations, highlighting the dependence of flow measurements on the rapidity of the regions of interest and reference.

[1] C. Shen and B. Schenke, "Longitudinal dynamics and particle production in relativistic nuclear collisions," Phys. Rev. C 105, no.6, 064905 (2022) [arXiv:2203.04685 [nucl-th]]

Speaker: Andi Mankolli (Vanderbilt University (US))

10:20 Hydrodynamization and thermalization in heavy-ion collisions from a kinetic theory perspective

Understanding the applicability of fluid-dynamical models to describe the hot and dense matter produced in the early stages of hadronic collisions is a fundamental problem in the field. In particular, it is not clear to what degree this hydrodynamization process requires proximity to a local equilibrium state. In this contribution, we study this problem in kinetic theory considering an ultrarelativistic gas undergoing strong longitudinal expansion, assuming Bjorken flow. We solve the Boltzmann equation using a relativistic generalization of the method of moments [1], where the single-particle distribution is reconstructed via a moment expansion. We first demonstrate that this traditional moment expansion actually diverges for this system and that this series must be resummed [2]. We then compare the exact resummed solution of the single-particle distribution function with solutions of the Boltzmann equation in the hydrodynamic limit and verify that the system displays considerable deviations from local equilibrium, even though the energy-momentum tensor is well described by fluid dynamics. We further show the 14-moment approximation [3], widely employed in simulations of heavy ion collisions, does not provide a good approximation for the momentum distribution of this system. We quantity this effect computing the emission of photons in the quark-gluon plasma [4] and verify if this deviation from equilibrium can be observed.

[1] G. S. Denicol, H. Niemi, E. Molnár, and D. H. Rischke. Phys. Rev. D 85, 114047 (2012). [Erratum: Phys. Rev. D 91, 039902 (2015)].
[2] C. V. P. de Brito, D. Wagner, G. S. Denicol, and D. H. Rischke. arXiv:2411.06267 [nucl-th].
[3] W. Israel and J. M. Stewart. Ann. Phys. 118, 341 (1979).
[4] C. Shen, J.-F. Paquet, U. Heinz, and C. Gale. Phys. Rev. C 91, 014908 (2015).

Speaker: Caio Brito (Universidade Federal Fluminense)

09:00 → 10:40 Parallel session 7: Heavy flavor & quarkonia IV

Convener: Vincenzo Greco

09:00 sPHENIX measurement of the $\Lambda_c$ / $D^0$ ratio in p+p collisions at RHIC

sPHENIX is a next-generation experiment at RHIC for jet and heavy-flavor physics which was fully commissioned during 2023 and 2024. Using its novel streaming-readout-capable, precision tracking system, sPHENIX collected 100 billion unbiased p+p collisions, and a further sample of minimum-bias Au-Au collisions, in Run-24. A key measurement of the sPHENIX heavy flavor physics program is the comparison of $\Lambda_c$ to $D^0$ differential yields in both Au+Au and p+p collisions, which probes questions related to the hadronization of heavy-flavor baryons compared to mesons in the Quark-Gluon Plasma medium and in vacuum. At RHIC energies, there is no previous measurement of the $\Lambda_c$ / $D^0$ baseline in p+p collisions, modern Monte Carlo event generators give widely different predictions, and the ratio in Au+Au is only poorly known. This talk presents a measurement from sPHENIX of the $\Lambda_c$/ $D^0$ ratio in p+p collisions and the status of the measurement in Au+Au collisions.

Speaker: Joe Osborn (Brookhaven National Laboratory)

09:20 Charm quark and QGP interactions through the spectra and anisotropic flow of D0 over the widest pT interval using Event-Shape Engineering at CMS

The charm quark is formed almost exclusively during the initial stages of the collision, and a significant fraction of the charm quarks fragment into the $D^{0}$ meson, the lightest open-charm hadron. We can gain insights into the interactions between the charm quark and the quark-gluon plasma (QGP) medium by studying the production and the flow of $D^{0}$ meson in heavy-ion collisions. We study the effect of the initial shape of the collision system on the elliptic flow ($v_{2}$) of promptly produced $D^{0}$ using event-shape engineering (ESE) in PbPb collisions at 5.02 TeV, at CMS. A correlation between the initial shape anisotropy with the $D^{0}$ $v_{2}$ would suggest that the flow gets driven by the interactions between the charm quark and the QGP and comparison with theoretical predictions allow us to unravel the mechanism behind the generation of $v_{2}$. We also study the anisotropic flow of nonprompt $D^{0}$ produced due to the decay of the bottom quarks and the nuclear modification factor ($\mathrm{R_{AA}}$) for prompt and nonprompt $D^{0}$. These studies provide further insights into the QGP interactions of heavy quarks like charm and bottom in heavy-ion collisions in different $p_{T}$ regions ranging from 1--40 GeV/c, the widest ever performed with ESE, and centralities between 0--50\%.

Speaker: Soumik Chandra (Purdue University (US))

09:40 Investigating Charm Quark Interactions and Hadronization in PbPb Collisions through $\Lambda_c^+$ and $D_s^\pm$ flow Measurements

Charm quarks serve as a sensitive probe of the Quark Gluon Plasma providing direct insights into its formation, evolution, and properties. We present the first-ever measurement of the elliptic flow of charm baryons, specifically prompt $\Lambda_c^+$, in lead-lead (PbPb) collisions at a center-of-mass energy of 5.36 TeV, using the CMS experiment. We also present the elliptic and triangular flow of prompt $D_{s}^{\pm}$ mesons with new standard of precision. The broader kinematic coverage and the first-ever $v_3$ measurement of $D_{s}^{\pm}$ mesons provide new insights into charm quark flow generation mechanisms and the impact of hadronization on flow. By comparing the flow of prompt $\Lambda_c^+$ and prompt $D_{s}^{\pm}$ to $D^0$ mesons in the same kinematic range, we investigate charm quark interactions with the medium and hadronization mechanisms. Additionally, we explore the charm quark hadronization mechanism further by measuring the $\Lambda_c^+/D^0$ yield ratio across pp, PbPb, and pPb collisions. These results offer a unique opportunity to explore the influence of initial-state effects on heavy quark dynamics. Additionally, comparison with theoretical models provides critical validation of the underlying physics mechanisms, and provide deeper insights into charm quark dynamics in heavy ion collisions.

Speaker: Nihar Ranjan Saha (Indian Institute of Technology Madras (IN))

10:00 Pre-equilibrium charm quark dynamics and their impact on D-Meson observables

Charm quarks offer valuable insights into the properties and evolution of the QCD medium as they are generated in the initial moments of energetic heavy-ion collisions at Relativistic Heavy-Ion collider (RHIC) and Large Hadron Collider (LHC). In particular, they can carry signals from the pre-equilibrium stage since the very high energy density of this stage can result in significant charm-medium interaction. Using a hybrid simulation framework that combines the IP-Glasma model for initial-state with viscous hydrodynamic evolution MUSIC to model the medium, we study charm quark propagation through different stages of the collision, from pre-equilibrium to hadronization. Heavy quark initial production is generated using PYTHIA and their interactions within the pre-equilibrium and hydrodynamic medium are modeled through Langevin dynamics in MARTINI. Our results reveal that both the nuclear modification factor and anisotropic flow coefficients of D-mesons in Pb+Pb collisions at $\sqrt{s_{NN}} = 5.02$ TeV are sensitive to the momentum-dependent energy loss of charm quarks in the initial stages. Furthermore, we demonstrate that hadronization mechanisms—specifically fragmentation and coalescence—play a crucial role in shaping the final-state charm observables.

Speaker: Bjoern Schenke (Brookhaven National Lab)

10:20 Testing the QCD formation time with reconstructed parton splittings

Heavy ion collisions provide a unique opportunity to access the space-time ordering of parton branching processes, since parton showers interact with a spatially-extended dense medium. However, the challenges of robustly isolating the formation time of a splitting phenomenologically has prevented formation time-dependent modification effects from being accessed experimentally. We address these challenges using heavy flavour, which offers an additional handle on tracing the propagation of individual quarks through the medium. We demonstrate how modified and groomed versions of the Cambridge/Aachen or FlavourCone jet finding algorithms applied on $g \to c\bar{c}$ splittings can be used to reconstruct the kinematics of such splittings with high fidelity using either final state partons or hadrons. We construct the parton formation time distribution, which has characteristic distortions arising from medium effects. This effect can be used to construct ratios of $\tau_f$-distributions in which the sensitivity of parton branching to the QCD formation time becomes experimentally accessible.

Speaker: Jasmine Therese Brewer (University of Oxford (GB))

09:00 → 10:40 Parallel session 8: Jets V

Convener: Marco Van Leeuwen (Nikhef National institute for subatomic physics (NL))

09:00 Energy correlators for gluon splitting to heavy quarks

Energy correlators inside of high energy jets provide a powerful tool to image the intrinsic and emergent angular scales of QCD. They have the potential to provide unprecedented insight on the interplay between vacuum scales inside of a jet and its medium modification. Energy correlators of jets containing a gluon splitting to heavy quarks can provide unique experimental access to two- and three-point correlators with known parton flavors through jets tagged with heavy hadrons. We demonstrate how these correlators open up important new avenues in phenomenology. The interplay between the quark mass and medium effects gives the correlator a distinct formation time-dependent medium modification at small angles, while the clean access to the splitting has the potential to unveil effects from an anisotropic, flowing medium.

Speaker: João Martins da Silva (LIP - Lisboa / ULisboa - IST)

09:20 A generalized picture of colour decoherence in dense QCD media

One of the most striking modifications induced by the QGP on a QCD parton shower is the breaking of angular-ordering. This was established in a series of pioneering papers almost 15 years ago by studying the radiation pattern off a QCD antenna in the presence of a medium [1,2]. These effects were captured by a single critical angle solely depending on the medium properties. Constraining the values of $\theta_c$, and more generally the physics of colour decoherence, is one of the targets of the heavy-ion experimental program at the LHC.

In this talk, we will demonstrate that the notion of a critical angle that exclusively depends on the medium properties no longer holds after accounting for medium effects during the formation of the antenna. This generalisation of the results of Refs. [1,2] shows that each splitting in an in-medium parton shower experiences colour decoherence in a different fashion. In addition, the total rate of emissions is enhanced with respect to previous calculations. This result has important implications for the experimental quest of determining medium modifications to parton evolution, as well as in the formulation of in-medium parton showers.

[1] Phys. Rev. Lett. 106 (2011) 122002
[2] JHEP 08 (2011) 015

Speaker: Xoán Mayo López (Universidade de Santiago de Compostela - IGFAE)

09:40 sPHENIX measurements of high-pT neutral mesons and isolated photons in p+p collisions

The sPHENIX experiment is a next-generation collider detector at RHIC designed for rare jet and heavy flavor probes of the Quark-Gluon Plasma. The experiment includes a large-acceptance, granular electromagnetic calorimeter (EMCal) and very high-rate data acquisition plus trigger system. In RHIC
Run-24, sPHENIX sampled 107/pb of p+p collision data at 200 GeV using an efficient high-pT photon trigger. This dataset represents a nearly-tenfold increase of the luminosity times acceptance compared to previous EMCal-based high-pT neutral meson measurements for this collision energy at RHIC. This talk presents the first sPHENIX EMCal measurements in p+p collisions, including neutral pion and eta meson production over a significantly extended kinematic range, where the eta decay photons remain well-separated in the granular EMCal out to very high pT, and isolated photon production, with an isolation requirement based on the full calorimeter system for the first time at RHIC. These provide important pQCD baselines for future measurements of single hadron suppression at very high pT in Au+Au collisions. Additionally, the transverse single spin asymmetry for neutral mesons serves as a foundational measurement for the sPHENIX Cold QCD physics program.

Speaker: Jaebeom Park

10:00 Heavy-flavor and exotic hadron production in jets at LHCb

Jets offer a unique laboratory for studying the production of heavy-flavor hadrons and quarkonia, including exotic hadron candidates.
With a full range of precision vertexing, tracking, and particle ID capabilities covering forward rapidity, the LHCb experiment is especially well suited to measurements of heavy-flavor and exotic hadron production. This talk will present recent LHCb studies of exotic hadron production in jets, as well as studies of heavy-flavor-jet substructure.

Speaker: Ezra Lesser (CERN)

10:20 The first measurement of energy-energy correlator of jets in PbPb collisions at CMS

Energy-energy correlators can isolate physics of different angular scales, which has attracted a lot of interest recently to study it in heavy ion environments. Any modification from proton-proton reference can reveal hints about the inner workings of the quark-gluon plasma. In this presentation, we will present the first measurement of the energy-energy correlator of jets in heavy ion collisions using lead-lead data at 5.02 TeV collected by CMS. We observe significant modifications over the pp reference and discuss the implications of these observations. We also present the possibility of extending to 3-point correlators, accessing a new phase-space for disentangling the varied mechanisms of energy loss resulting from the concurrent evolution of the jet and plasma.

Speaker: Jussi Viinikainen (Vanderbilt University (US))

09:00 → 10:40 Parallel session 9: New theoretical developments III

Convener: Guy Moore

09:00 The space of transport coefficients allowed by causality

As an effective theory, relativistic hydrodynamics is fixed by symmetries up to a set of transport coefficients. A lot of effort has been devoted to explicit calculations of these coefficients. Here we adopt a more general approach, deploying bootstrap techniques to rule out theories that are inconsistent with microscopic causality. What remains is a universal convex geometry in the space of transport coefficients, which we call the hydrohedron. The landscape of all consistent theories necessarily lies inside or on the edges of the hydrohedron. We analytically construct cross-sections of the hydrohedron corresponding to bounds on transport coefficients that appear in sound and diffusion modes’ dispersion relations for theories without stochastic fluctuations.

Speaker: Michal Heller

09:20 Covariant Formulation of Semi-Classical Spin Hydrodynamics and Equilibration of Spin: Linear and Nonlinear Perspectives

Spin hydrodynamics is the extension of standard relativistic hydrodynamics with the total angular momentum considered as an independent conserved charge, where the total angular momentum is often expressed as a surface integral of a rank-3 tensor.
This rank-3 tensor is then decomposed into orbital and spin parts, which, by using the definition of orbital angular momentum in Cartesian coordinates and the energy-momentum tensor conservation, leads to the equation of motion for the spin tensor.
This treatment is not covariant under coordinate transformations, even in flat spacetime.
We address this issue by redefining the orbital and total angular momentum in a covariant way, which can also be extended to curved spacetime in cases where the Belinfante and Hilbert definitions of the energy-momentum tensor are equivalent.
Then, we review the constitutive equations of semi-classical spin hydrodynamics developed from a systematic expansion in the reduced Planck constant, $\hbar$.
This formulation is inspired by, but not necessarily based on, quantum kinetic theory.
Up to the first order in $\hbar$, there is no back-reaction from the spin sector to fluid dynamics, and, therefore, solutions to standard hydrodynamics act as an input for the equations of motion for the spin tensor.
We consider three simple solutions to the fluid’s equation of motion: hydrostatic, linearized hydrodynamics, and conformal Bjorken flow.
We demonstrate that, up to first-order in $\hbar$, the spin waves are not affected by the fluid waves, and therefore, the results from the hydrostatic case and linearized hydrodynamics are the same.
This shows that our results for the damping of spin waves are not modified by slight fluid perturbations.
For the case of conformal Bjorken flow, we first review the consequences of conformal invariance in spin hydrodynamics and, in particular, show that conformal invariance should be considered only in the limit where $\hbar$ vanishes.
Then, we use the attractor solution to find the evolution of the spin potential.
We show that the spin potential in this nonlinear regime is damped in a similar fashion to the linear regime.
These results strengthen our previous findings on the possible long equilibration time for spin, and support a dynamical treatment of spin degrees of freedom.

Speaker: Masoud Shokri (Goethe University, ITP)

09:40 Hot and dense perturbative QCD in a very strong magnetic background

We compute the pressure, chiral condensate and strange quark number susceptibility from first principles within perturbative QCD at finite temperature and very high magnetic fields up to two-loop and physical quark masses. We also discuss cold and dense quark matter in the presence of a very strong magnetic field using perturbative QCD at finite density. The effectively negligible contribution of the exchange diagram allows for building a simple analytic model for the equation of state for pure quark magnetars at extremely large fields. Finally, we consider the Taylor coefficients for the pressure in magnetic QCD. Our results for the chiral condensate, strange quark number susceptibility and Taylor coefficients can be directly compared to recent lattice QCD data away from the chiral transition. Even though current lattice results do not overlap with the region of validity for perturbation theory, perturbative results seem to be in the same ballpark.

Speaker: Eduardo Fraga (Universidade Federal do Rio de Janeiro)

10:00 NLO calculations at small x

This talk reviews recent progress in next-to-leading order calculations for dilute-dense-processes in the small-x gluon saturation regime, especially for Deep Inelastic Scattering observables. A systematical formalism for such observables is provided by a combination of light cone perturbation for the dilute probe, combined with a Color Glass Condensate picture for eikonal scattering off the dense gluonic target. In particular we focus here on the recent calculation [1,2] of the diffractive DIS structure function, which is expected to be more sensitive to gluon saturation than inclusive DIS observables.

[1] G. Beuf, T. Lappi, H. Mäntysaari, R. Paatelainen and J. Penttala, "Diffractive deep inelastic scattering at NLO in the dipole picture," JHEP 05 (2024), 024, [arXiv:2401.17251 [hep-ph]].

[2] G. Beuf, H. Hänninen, T. Lappi, Y. Mulian and H. Mäntysaari, "Diffractive deep inelastic scattering at NLO in the dipole picture: The qqg contribution," Phys. Rev. D 106 (2022) no.9, 094014, [arXiv:2206.13161 [hep-ph]].

Speaker: Tuomas Lappi

10:20 Extending the fluid-dynamic description to the initial state

Over the past two decades, research has shown that various observables measured in heavy-ion collisions can be effectively described using relativistic fluid dynamics across different collision systems and energies. However, a common challenge in these studies is the modeling of the transition from the initial state to the fluid-dynamic phase. While the collision likely involves complex, far-from-equilibrium dynamics, it is possible that a second-order fluid theory can adequately capture its softer features. If successfully completed, this approach could lead to a comprehensive dynamical description of a heavy-ion collision, where the only free parameters are related to the thermodynamics and the transport properties of quantum chromodynamics.
In our work (arXiv:2410.08169), we investigate this possibility. We discuss how to characterize the state prior to the collision within this framework, the implications of relativistic causality on the equations of motion, the entropy production from shear and bulk viscous dissipation during the initial longitudinal dynamics, and how this can inform sensible initial conditions for subsequent transverse expansion.

Speaker: Andreas Kirchner

10:40 → 11:10 Coffee break

11:10 → 12:50 Plenary session 4: Heavy flavor & quarkonia

Convener: Min Jung Kweon (Inha University (KR))

11:10 Experiment overview of open heavy flavor as probes for deconfinement

Speaker: Dr Gian Michele Innocenti (Massachusetts Inst. of Technology (US))

11:35 EPJ featured talk: Theory of open heavy-flavor as probes for deconfinement

Speaker: Salvatore Plumari

12:00 Experiment overview of quarkonia as probes for deconfinement

Speaker: Michael Winn (Université Paris-Saclay (FR))

12:25 Theory of quarkonia as probes for deconfinement

Speaker: Xiaojun Yao (University of Washington)

12:50 → 14:20 Lunch

14:20 → 16:00 Plenary session 5:Jets

Convener: Barbara Jacak (University of California Berkeley (US))

14:20 Experimental measurement of jet substructure and energy-energy correlators

Speaker: Wenqing Fan (University of Houston (US))

14:45 Theory overview of jet substructure and energy-energy correlators

Speaker: Alba Soto Ontoso (Universidad de Granada (ES))

15:10 Experimental measurement of medium response and hard-soft correlation

Speaker: Christopher Mc Ginn (MIT)

15:35 Theory overview of medium response and hard-soft correlation

Speaker: Dr ismail soudi (University of Jyvaskyla)

16:00 → 16:30 Coffee break

16:30 → 18:30 Plenary session 6: Chirality — Collective dynamics & small systems

Convener: Xin-Nian Wang (Central China Normal University)

16:30 Experimental measurements of chiral effects, spin polarization and alignment in heavy-ion collisions

Speaker: Zhiwan Xu (Los Alamos National Laboratory)

17:00 Theory of spin polarization and alignment and chiral effects in heavy-ion collisions

Speaker: Shi Pu

17:30 Collective dynamics in small systems: a critical experimental overview

Speaker: Blair Daniel Seidlitz (Columbia University (US))

18:00 Critical overview of collective dynamics, particularly in small systems and what can be learned with current and future programs

Speaker: Govert Hugo Nijs (CERN)

19:30 → 21:00 Concert

21:00 → 22:00 Informal get-together on diversity and inclusion in physics (closed session)

Friday 11 April

09:00 → 10:30 Plenary session 7: Initial state of hadronic and electron-ion collisions & nuclear structure

Convener: Jiangyong Jia (Stony Brook University (US))

09:00 Overview of the latest developments in understanding the initial state and thermalization

Speaker: Kirill Boguslavski (TU Wien (AT))

09:30 Latest developments in high-energy QCD to understand the initial state in heavy-ion collisions

Speaker: Farid Salazar (Institute for Nuclear Theory, University of Washington)

10:00 Probing nuclear structure in heavy-ion collisions from small to large systems

Speaker: You Zhou (Niels Bohr Institute (DK))

10:30 → 11:00 Coffee break

11:00 → 12:30 Plenary session 8: QCD phase diagram & critical point

Convener: Volker Koch

11:00 BES-II data and critical assessment with respect to QCD critical point search

Speaker: Shinichi Esumi (University of Tsukuba (JP))

11:30 Equation of state, QCD phase diagram: critical assessment and predictions from lattice QCD

Speaker: Bastian Brandt (University of Bielefeld)

12:00 Equation of state, QCD phase diagram: critical assessment and predictions from functional methods

Speaker: Fabian Rennecke (Justus Liebig University Giessen)

12:30 → 14:00 Lunch

14:00 → 16:00 Plenary session 9: Correlations & fluctuations — New theoretical developments — Physics of ultraperipheral collisions — QCD matter in astrophysics

Convener: Prof. Jacquelyn Noronha-Hostler (University of Illinois Urbana Champaign)

14:00 Probing bulk dynamics of the QGP with correlations and fluctuations

Speaker: Mesut Arslandok (Yale University (US))

14:30 Quantum computing for heavy-ion physics: near-term status and future prospects

Speaker: Joao Lourenco Henriques Barata

15:00 Experiment overview of physics of ultraperipheral collisions

Speaker: Agnieszka Ewa Ogrodnik (Charles University (CZ))

15:30 Progress in dense QCD and applications to neutron stars

Speaker: Yuki Fujimoto (University of California, Berkeley / RIKEN)

16:00 → 16:30 Coffee break

16:30 → 18:30 Plenary session 10: Electromagnetic probes — Light and strange flavor physics & nuclei

Convener: Dr Laura Tolos

16:30 Experiment overview of electromagnetic radiation in heavy-ion collisions

Speaker: Horst Sebastian Scheid (CERN)

17:00 Theory overview on electroweak emission from heavy-ion collisions

Speaker: Greg Jackson (SUBATECH (CNRS/IN2P3))

17:30 Femtoscopy measurements to understand hadron interactions at the LHC

Speaker: Valentina Mantovani Sarti (Technische Universitaet Muenchen (DE))

18:00 Production of light and strange flavor hadrons and loosely bound nuclei

Speaker: Benjamin Donigus (Johann-Wolfgang-Goethe Univ. (DE))

19:00 → 22:00 Conference dinner

Saturday 12 April

09:00 → 10:00 Plenary session 11: Detectors & future experiments

Convener: Piotr Gasik (GSI - Helmholtzzentrum fur Schwerionenforschung GmbH (DE))

09:00 The ultimate pixel detector for tracking and vertexing in ultra-relativistic nuclear collisions

Speaker: Magnus Mager (CERN)

09:30 Machine Learning in Analysis Techniques for Future Physics

Speaker: Hannah Bossi (Massachusetts Inst. of Technology (US))

10:00 → 11:00 Plenary session 12: Summary talks

Convener: Helen Caines (Yale University (US))

10:00 Summary: experiment

Speaker: Jana Bielcikova (Czech Academy of Sciences (CZ))

10:30 Summary: theory

Speaker: Hannah Elfner

11:00 → 11:30 Coffee break

11:30 → 13:30 Awards & flash talks

Convener: Dirk Rischke

13:30 → 14:00 Adjourn

Convener: Dirk Rischke