18th International Conference on QCD in Extreme Conditions

27 Jul 2022, 08:15 → 29 Jul 2022, 17:55 Europe/Zurich

Alexander Rothkopf (University of Stavanger), Jens Oluf Andersen (NTNU), Pedro Costa

The 18th International Conference on QCD in Extreme Conditions (XQCD 2022) will be organized by The Norwegian University of Science and Technology in Trondheim, Norway, from July 27 to July 29 2022. XQCD is a series of international workshop-style conferences, held annually, which aims at covering recent advances in the theory and phenomenology of QCD under extreme conditions of temperature and/or baryon density, together with related topics.

We will have 6 invited talks and approximately 30 contributed talks covering the topics

QCD at Finite Temperature and Density

Heavy Ion Collisions Phenomenology

Phase Diagram of Strongly Interacting Matter

Properties of the Quark-Gluon Plasma

Properties of Strongly Interacting Gauge Theories

The Sign Problem in Lattice QCD

QCD in External Fields

Neutron star

 

Invited speakers

 

Gergely Endrodi (Bielefeld U)

Robert Pisarski (Brookhaven)

Claudia Ratti (Houston U)

Denes Sexty (Graz U)

Misha Stephanov (Chicago U)

Aleksi Vuorinen (Helsinki U)

 

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Our sponsors are

 

 https://www.epj.org/

 

Wednesday 27 July

Registration: Outside Auditorium R2 On floor U1 in Realfagbygget

Welcome

Convener: Jens Oluf Andersen (NTNU)

Session: Lattice and hot QCD

Convener: Jon-Ivar Skullerud

1 Recent results on hot and dense matter from the lattice

Speaker: Claudia Ratti

Talk_Trondheim.pdf

2 QCD in extreme conditions on the lattice

The thermodynamics of strongly interacting matter in extreme conditions can be investigated via lattice QCD simulations. In this talk, I will give an overview of recent developments in this field, focusing on the impact of temperature, background electromagnetic fields as well as an isospin asymmetry between the light quark chemical potentials. Selected applications of the results for off-central heavy-ion collisions and the evolution of the early Universe will also be discussed.

Speaker: Gergely Endrodi

xqcd_2022.pdf

10:30 Coffee break

Session

Convener: Claudia Ratti

3 Hadrons at high temperature: a lattice update

We present the most recent results from the FASTSUM collaboration for hadron properties at high temperature from anisotropic lattice QCD. This includes the temperature dependence of the light and charmed meson and baryon spectrum, as well as properties of heavy quarkonia.

Speaker: Jon-Ivar Skullerud

XQCD22.pdf

4 Lattice study of electromagnetic conductivity of quark-gluon plasma at finite baryon density

In this talk we present our study of the electromagnetic conductivity in dense quark-gluon plasma obtained within lattice simulations with Nf = 2 + 1 dynamical quarks. We employ stout improved rooted staggered quarks at the physical point and the tree-level Symanzik improved gauge action. The simulations are performed at imaginary chemical potential. To reconstruct electromagnetic conductivity from current-current correlators, we employ the Tikhonov regularisation method as well as the modified Backus-Gilbert method, computing the convolution of the spectral density with the target function. Our results are analytically continued to real values of baryon chemical potential. Our study indicates that electromagnetic conductivity of quark-gluon plasma rapidly grows with the real baryon density.

Speaker: Manuel Naviglio

XQCD_MN.pdf

5 Heavy quark diffusion coefficient from the lattice

The heavy quark diffusion coefficient is encoded in the spectral functions of the chromo-electric and the chromo-magnetic correlators, of which the latter describes the T/M contribution. We study these correlators in the deconfined phase of SU(3) gauge theory on the lattice using the gradient flow. We perform both continuum and zero flow time limits to extract the heavy quark diffusion coefficient at a few different temperatures.

Speaker: Viljami Leino (Technical University Munich)

xqcd_version1_VLeino.pdf

12:15 Group photo

Main building

12:45 Lunch Break

Session

Convener: Jacobus Verbaarschot

6 Quark-antiQuark potential from Wilson Line Correlators at finite temperature: A comparison between different methods

We present results for the analysis of the energy and spectral width of the quark-antiQuark potential obtained from wilson line correlators at finite temperature using 2+1 flavor HISQ configurations ms/ml=20. We extract the energy and spectral width using 4 different methods: Zero temperature subtraction, Pade fit, Bayesian Reconstruction and HTL motivated fit. We compare the results and discuss the pros and cons of each method.

Speaker: Rasmus Larsen (University of Stavanger)

Rasmus_Larsen_XQCD_2022.pdf

7 Lattice QCD with an inhomogeneous magnetic field background

The magnetic fields generated in non-central heavy-ion collisions are among the strongest fields produced in the universe, reaching magnitudes comparable to the scale of strong interactions. Backed by model simulations, the resulting field is expected to be spatially modulated, deviating significantly from the commonly considered uniform profile. In this work, we present the next step to improve our understanding of the physics of quarks and gluons in heavy-ion collisions by adding an inhomogeneous magnetic background to our lattice QCD simulations. We simulate $2+1$ staggered fermions with physical quark masses for a range of temperatures covering the QCD phase transition. We assume a $1/\cosh(x)^2$ function to model the field profile and vary its strength to analyze the impact on the chiral condensate and the Polyakov loop. These order parameters show non-trivial spatial features due to the interplay between the sea and the valence effects as the system approaches the crossover temperature. We use these quantities to draw the phase diagram in the $T$-$B$ plane and understand the implications of $B$ to QCD physics. We also find that in this set-up, the system develops steady electric currents which flow in equilibrium. We use these currents to present our new method of obtaining the magnetic susceptibility of the QCD medium and compare it to previously established methods.

Speaker: Adeilton Dean Marques Valois (Bielefeld University)

XQCD_deanvalois.pdf

8 Extending AMY Shear Viscosity Calculations to Finite Baryon Chemical Potentials

Transport coefficients, such as viscosity, can be calculated theoretically in weakly coupled quantum field theory, and present interesting information about hydrodynamic models of heavy-ion collisions. We present the results for shear viscosity calculations at leading-log in QCD in a regime of high baryon density, where the chemical potentials are greater than the temperature, which is a very unknown region of the QCD phase diagram. For that, we extend the results obtained by Arnold, Moore, and Yaffe. Such conditions of temperature and baryon density are found in medium-energy heavy-ion collisions and in the nuclei of neutron star mergers.

Speaker: Ms Isabella Danhoni

XQCD.pdf

15:30 Coffee break

Session

Convener: Rasmus Larsen (University of Stavanger)

9 Studying explicit $U(1)_A$ symmetry breaking in hot and magnetised two flavour non-local NJL model constrained using lattice results

We study the two-flavour non-local Nambu\textemdash Jona-Lasinio (NJL) model in the presence of a magnetic field and explore the chiral crossover in presence of a non-local form of the 't Hooft determinant term [1]. Its coupling is governed by a dimensionless parameter $c$. This term is responsible for the explicit breaking of $U(1)_A$ symmetry. We have attempted a systematic analysis of the model parameters by fitting to self-consistent lattice QCD calculations. Three parameters of the model are fixed by $eB=0$ results from published lattice QCD on the chiral condensate, the pion decay constant ($F_\pi$), and the pion mass ($m_\pi$). The difference of the $u$ and $d$ quark condensates in the presence of a magnetic field ($eB$) is quite sensitive to $c$ and we fix $c$ using published lattice QCD results for this observable. We see no evidence that $c$ depends on $eB$. The crossover temperature decreases with increasing $eB$ only for condensate values at the lower end of the allowed values (as already seen in~\cite{Pagura:2016pwr}) and $F_\pi$ at the upper end of the allowed values. We further check our model predictions by calculating the topological susceptibility with the fitted $c$ values and comparing it with lattice results. Since the topological susceptibility is related to the extent of the $U(1)_A$ symmetry breaking, we find that it is sensitive to the value of $c$.

M. S. Ali, C. A. Islam and R. Sharma
PRD 104, no.11, 114026 (2021) [arXiv:2009.13563].

Speaker: Chowdhury Aminul Islam

slides_XQCD_cai.pdf

slides_XQCD_cai.pptx

10 Open heavy flavour in a hot bath

We present the properties of open heavy mesons in hot mesonic matter based on a self-consistent theoretical approach that takes into account chiral and heavy-quark spin-flavour symmetries. The heavy-light meson-meson unitarized scattering amplitudes in coupled channels incorporate thermal corrections as well as the dressing of the heavy mesons with the self-energies [1, 2]. As a result, the open heavy-flavour ground-state spectral functions broaden and their peak is shifted towards lower energies with increasing temperatures. This has strong implications for the excited mesonic states generated dynamically in this heavy-light molecular model. In addition, we show the meson Euclidean correlators calculated using the thermal ground-state spectral functions obtained within our approach and compare them with recent calculations of lattice correlators [3].

[1] G. Montaña, A. Ramos, L. Tolos and J. M. Torres-Rincon, Phys. Lett. B 806 (2020), 135464 doi:10.1016/j.physletb.2020.
[2] G. Montaña, A. Ramos, L. Tolos and J. M. Torres-Rincon, Phys. Rev. D 102 (2020) 9, 096020 doi:10.1103/PhysRevD.102.096020
[3] G. Montaña, O. Kaczmarek, L. Tolos and A. Ramos, Eur. Phys. J. A 56 (2020) 11, 294 doi:10.1140/epja/s10050-020-00300-y

Speaker: Laura Tolos

tolos_xQCD.pdf

IAC meeting

Conveners: Prof. Aleksi Vuorinen (University of Helsinki), Jacobus Verbaarschot, Michael Strickland (Kent State University)

Poster session

dd

Thursday 28 July

Session: Dense QCD and stars

Convener: Jean-Loic Kneur (Univ. Montpellier)

11 Searching for quark matter: from loops to neutron-star cores

I will describe recent advances in the study of dense quark matter, expected to be present inside the cores of massive neutron stars. I will concentrate on two somewhat differing topics: first, on perturbative studies of the bulk thermodynamic properties of unpaired quark matter, and second, on the application of these results to the model-independent determination of the neutron-star-matter equation of state. I will argue that with recent improvements in ab-initio calculations at low and high density as well as in astrophysical measurements, we are close to being able to pinpoint the properties of matter inside neutron-star cores to such an extent that a reliable phase identification will soon become possible.

Speaker: Prof. Aleksi Vuorinen (University of Helsinki)

XQCD_2022.pptx

12 Complex Langevin simulations for finite density QCD

Lattice simulations of non-zero density QCD introduce the so-called sign problem, which invalidates importance sampling methods. We use the Complex Langevin equation (CLE) to circumvent the sign problem. Recent results regarding the phase diagram and thermodynamics of QCD using Complex Langevin simulations will be reviewed. Theoretical developments about 'boundary terms' are also discussed: in some cases one observes the convergence of the CLE to incorrect results. A cheap observable is proposed which allows unambigous detection of the correctness of the simulations. Preliminary results of measurements of these 'boundary term' observables in full QCD are presented.

Speaker: Denes Sexty

sexty_xqcd2022.pdf

10:30 Coffee break

Session

Convener: Dr Violetta Sagun (University of Coimbra)

13 Strange Quark Matter from Baryons

Cold and dense matter can be explored in a systematic way both in the high-density (perturbative
QCD) and low-density (Chiral EFT) regime. However, the path connecting them is yet to be discov-
ered. As a result, these descriptions are usually extrapolated into the intermediate density regime
and then connected at some transition point. In this work I will present a model that has features
of both, but within a unified description. The model contains hadronic degrees of freedom and is
calibrated using nuclear matter properties; yet it exhibits a phase transition towards a “quark mat-
ter” phase that has approximately restored chiral symmetry, strangeness, and asymptotes to the
conformal limit of the speed of sound. While this model can describe different qualitative scenar-
ios regarding the phase transition and the strangeness onset, empirical constraints significantly
narrow down the allowed parameter range. Moreover, hybrid stars above two solar masses are
predicted, exhibiting a stiff “quark matter” core. This approach has implications for the hyperon
puzzle and is also crucial for future exploration of inhomogeneous phases and the surface tension
between hadron and quark phases.

Speaker: Mr Savvas Pitsinigkos (University of Southampton)

SQM_XQCD.pdf

14 QCD equation of state via the complex Langevin method

We present results on the phase diagram of Quantum Chromodynamics (QCD) with two light quark flavours at finite chemical potential from first principle lattice simulations. To circumvent the sign problem we use the complex Langevin method. The pion mass is of approximately 480 MeV. We report on the pressure, energy and entropy equations of state. A particular emphasis is put on the “cold” regions of the phase diagram and the observation of the Silver Blaze phenomenon.

Speaker: Felix P. G. Ziegler (The University of Edinburgh)

Talk_XQCD_2022_Ziegler.pdf

15 Hybrid stars with large strange quark cores

The possible existence of hybrid stars is studied using several multi-quark interaction channels. The hadronic phase consists of an EOS with presently accepted nuclear matter properties and the quark model constrained by the vacuum properties of several light mesons. The dependence of several NS properties on the different quark interactions is analyzed. We show that the present constraints from neutron stars observations allow for the existence of hybrid stars with a large strangeness content and large quark cores.

Speaker: Dr Márcio Ferreira (University of Coimbra)

XQCD2022_Ferreira.pdf

16 Best poster award ceremony

Speakers: Prof. Aleksi Vuorinen (University of Helsinki), Claudia Ratti, Dr Laura Tolos

12:30 Lunch Break

Session

Convener: Gergely Endrodi

17 Holographic approach to dense QCD and neutron star mergers

The gauge/gravity duality, combined with information from lattice QCD, nuclear theory, and perturbative QCD, can be used to constrain the equation of state of hot and dense QCD. I discuss an approach based on the V-QCD model, which predicts a strongly first order nuclear to quark matter phase transition with a critical endpoint. By using this model in state-of-the-art simulations of neutron star binaries with parameters consistent with GW170817, I study the formation of quark matter during the merger process.

Speaker: Matti Jarvinen

220728_jarvinen.pdf

18 All order resummed next-to-leading soft modes of cold and dense QCD pressure

By identifying {\em massive} renormalization group (RG) properties within the hard thermal loop (HTL)
formalism, we resum to all orders $\alpha_S^p, p\ge 3$ the leading and next-to-leading logarithmic
soft mode $m_E\sim \alpha_S^{1/2} \mu_B$ contributions to the cold and dense QCD pressure
at high baryon chemical potential $\mu_B$.
We obtain noticeably reduced residual scale dependence with respect to the state-of-the art results.
We will discuss applications to the NNLO Equation of State of cold and dense
quark matter, also including extension to massive quarks, relevant in particular for the phenomenology of neutron stars.

Mostly based on L. Fernandez and J.-L. Kneur, arXiv:2109.02410 and more recent work in progress.

Speaker: Loïc Fernandez (Laboratoire Charles Coulomb)

2_Loic_Fernandez_XQCD_2022.pdf

19 ​Effect of dark matter on observable neutron star's properties and it's discrimination from the strongly interacting matter equation of state

We study the impact of asymmetric fermionic and bosonic dark matter on neutron star properties,
including tidal deformability, maximum masses, radii, etc. The conditions at which dark matter
particles tend to condensate in the core of the star or create an extended halo are presented. We
show that dark matter condensed in a core leads to a decrease of the total gravitational mass
and tidal deformability compared to a pure baryonic star, which we will perceive as an effective
softening of the equation of state. On the other hand, the presence of a dark matter halo increases
those observable quantities. Thus, observational data on compact stars could be affected by an
accumulated dark matter and, consequently, constraints we put on strongly interacting matter at
high densities. We will discuss how the ongoing and future X-ray, radio and GW observations
could shed light on dark matter admixed compact stars and put multi-messenger constraints on
its effect

Speaker: Violetta Sagun (University of Coimbra)

Sagun_XQCD2022.pdf

15:30 Coffee break

Session

Convener: Matti Jarvinen (Utrecht University)

20 Gravitational wave signal for quark matter with realistic phase transition

At extremely high densities, QCD predicts the possible liberation of quark degrees of freedom and the formation of quark matter. The cores of neutron stars (NS) may accommodate such high-density matter. Whether the quark matter exists inside NSs is still an open question. If the quark matter exists, then there should be the imprint of the hadron-to-quark phase transition in the equation of state (EoS). We expect that gravitational waves from the binary NS merger can further constrain the EoS with the help of future third-generation detectors.
In this talk, we show that gravitational waves in the post-merger phase can distinguish the theory scenarios with and without a hadron-to-quark phase transition. Instead of adopting specific phenomenological EoS as studied previously, we compile reliable EoS constraints from the ab-initio QCD calculations. We demonstrate that early collapse to a black hole after the NS merger signifies softening of the EoS associated with quark matter even without a strong first-order transition. We also explain that the electromagnetic counterparts may further constrain the nature of the hadron-to-quark phase transition; we need substantial mass ejection to energize the observed luminosity of the associated kilonova.

Speaker: Yuki Fujimoto (University of Washington)

Fujimoto_XQCD.pdf

21 Early quark deconfinement in compact star astrophysics and heavy-ion collisions

We outline the role that an early deconfinement phase transition from normal nuclear matter to a color superconducting quark-gluon plasma phase plays for the phenomenology of supernova explosions and binary neutron star mergers. To this end we extend the compact star equation of state (EoS) from vanishing to moderately high temperatures that become accessible in the CBM experiment at FAIR. We study the connection of such hybrid EoS with the mass-radius relation of cold compact stars, including the intriguing possibility of additional families, as a consequence of the presence of an early and strong phase transition. Special emphasis is devoted to the simultaneous fulfillment of the new NICER mass and radius constraint from PSR J0740+6620 and the tidal deformability constraint from GW170817 which require the EoS to be soft at about twice saturation density and then to stiffen. Such a pattern is provided by an early and strong deconfinement transition. Dynamical scenarios are being considered, such as binary compact star mergers including the subsequent emission of gravitational waves and supernova explosions of massive supergiant stars where neutrinos play the role of messengers.

Speaker: Oleksii Ivanytskyi (University of Wroclaw)

DFA4qm.pdf

22 Holographic Baryons

The holographic method is one of the only ways of getting analytic insight into most strongly coupled systems. When numerical methods are not available, it is even the best technique we know of to tackle these problems. In particular, lattice QCD is unable to provide results at finite baryon density, which is why a lot of effort is put into the study of holographic QCD at high density. Densities beyond the nuclear density are not only a theoretical frontier, but are reached in real systems such as neutron star cores and future heavy ion collision experiments. In this talk I will discuss the most elementary aspect of baryonic physics in holography, that is the study of a single baryon state. I will start by reviewing the construction of baryons in the chiral effective field theory to highlight the common points and differences with the holographic approach. I will then discuss the qualitative features of holographic baryons, obtained already in models where the back-reaction of quarks on the glue sector of QCD is neglected and finally present our recent work in the framework of the V-QCD model, which is the most complete holographic model of QCD and the only one to implement flavor back-reaction.

Speaker: Edwan Préau

XQCD_28-07-22.pdf

XQCD_28-07-22.pptx

19:00 Workshop Dinner

Dinner

Friday 29 July

Session: Heavy-ion collisions

Convener: Rene Bellwied (University of Houston (US))

23 Heavy quarkonium dynamics at next-to-leading order in the binding energy over temperature

Using the potential non-relativistic quantum chromodynamics (pNRQCD) framework we derive a Lindblad equation for the evolution of the heavy-quarkonium reduced density matrix that is accurate to next-to-leading order (NLO) in the ratio of the binding energy of the state and the temperature. The resulting NLO Lindblad equation can be used to more reliably describe heavy-quarkonium evolution in the quark-gluon plasma at low temperatures when compared to the leading-order truncation. In order to apply this to phenomenology, we demonstrate how to numerically solve the resulting NLO Lindblad equation using a quantum trajectories algorithm. To achieve this, we map the solution of the three-dimensional Lindblad equation to the solution of an ensemble of one-dimensional Schr\"odinger evolutions with Monte-Carlo sampled quantum jumps. Upon averaging over the Monte-Carlo sampled quantum jumps, we obtain a solution to the NLO Lindblad equation which does not require truncation in the angular momentum quantum number of the states considered. We also investigate evolving the system using only the complex effective Hamiltonian without stochastic jumps and demonstrate that this provides a quite reliable approximation for the ground state survival probability at NLO. Finally, we make comparisons with our prior leading-order pNRQCD results and experimental data available from the ATLAS, ALICE, and CMS collaborations.

Speaker: Michael Strickland (Kent State University)

strickland.pdf

strickland.pptx

24 Three topics in extreme QCD

I discuss three topics in QCD at nonzero temperature and density. First, how confingurations with fractional topological charge $1/N$ in a $SU(N)$ gauge theory may contribute at low temperature. Second, how the Polyakov loop, which appears to be an artifact of Euclidean space-time, can be naturally introduced in the Hamiltonian form, by introducing new states with test charges. Lastly, I discuss the solution for the low energy excitations for a $SU(N_c)$ theory with $N_f$ light fermions in 1+1 dimensions, which is just a modified free boson for fermion number.

Speaker: Robert Pisarski (Brookhaven National Laboratory)

Pisarski_July29_Norway.pdf

10:30 Coffee break

Session

Convener: Dr Alexander Rothkopf (University of Stavanger)

25 Quarkonium transport in weakly and strongly coupled plasmas

Suppression of open heavy flavors and quarkonia in heavy-ion collisions is among the most informative probes of the quark-gluon plasma. Interpreting the full wealth of data obtained from the collision events requires a precise understanding of the evolution of heavy quarks and quarkonia as they propagate through the nearly thermal and strongly coupled plasma. In particular, a systematic theoretical calculation of the dissociation and recombination rates of quarkonia had been lacking until recently.

It has recently been shown in [1] that such a calculation requires the evaluation of a gauge-invariant correlator of chromoelectric fields dressed with Wilson lines, which is very similar to, but different from the correlator used to define the well-known heavy quark diffusion coefficient [2]. In this talk, we will show a complete next-to-leading order (NLO) calculation result of the chromoelectric field correlator for quarkonium at finite temperature [3], which only differs from that for open heavy quarks by a temperature-independent constant. We explain both their similarities and differences. Crucial insights are obtained by studying them in temporal axial gauge, where these correlators would naively be equal [4]. Finally, going beyond perturbation theory, we will explain how to perform an AdS/CFT calculation of the analogous correlator for quarkonium in $\mathcal{N}=4$ SYM and show some preliminary results in the strong coupling limit [5].

[1] X. Yao and T. Mehen, “Quarkonium Semiclassical Transport in Quark-Gluon Plasma: Factorization and Quantum Correction,” JHEP 02 (2021) 062
[2] J. Casalderrey-Solana and D. Teaney, “Heavy quark diffusion in strongly coupled N=4 Yang-Mills”
[3] T. Binder, K. Mukaida, B. Scheihing-Hitschfeld, X. Yao, “Non-Abelian Electric Field Correlator at NLO for Dark Matter Relic Abundance and Quakonium Transport,” JHEP 01 (2022) 137
[4] B. Scheihing-Hitschfeld, X. Yao, “Gauge Invariance of Non-Abelian Field Strength Correlators: the Axial Gauge Puzzle,” arXiv:2205.04477 [hep-ph]
[5] G. Nijs, B. Scheihing-Hitschfeld, X. Yao, in preparation

Speaker: Bruno Sebastian Scheihing Hitschfeld (Massachusetts Institute of Technology)

xQCDtalk-final.pdf

26 Anisotropic jet momentum broadening from effective kinetic theory

We study jet momentum broadening in heavy-ion collisions at anisotropic initial stages with focus on the jet quenching parameter $\hat q$, which we extract using effective kinetic theory. We obtain its non-equilibrium properties during the bottom-up thermalization scenario in a Bjorken-expanding plasma with non-thermal anisotropic initial conditions.
By studying the time-dependence of the momentum broadening we find that $\hat q$ along the beam axis is suppressed compared to the transverse plane during the overoccupied phase, and enhanced during the underoccupied phase.

Speaker: Florian Lindenbauer (TU Wien)

Lindenbauer_XQCD_2022.pdf

Lindenbauer_XQCD_2022.pdf

Lindenbauer_XQCD_2022.pptx

27 Quantum entanglement in relativistic particle collisions

I will discuss the potential connection between the entanglement entropy of the initial state and thermodynamic entropy of the final state in relativistic particle collisions, from proton-proton to heavy ion collisions. I will show that simple fragmentation models that ignore quantum mechanical effects break down in describing particle production and that evidence for the non-universality of heavy quark fragmentation in hadron collisions shows the impact of the initial entanglement. In heavy ion processes entanglement might also give rise to a seemingly thermal behavior and thus can be linked to the entropy and particle yields in the final state.

Speaker: Rene Bellwied (University of Houston (US))

bellwied-xqcd2022-final.pdf

28 3+1D observables in the dilute Glasma of relativistic heavy ion collisions

According to the Color Glass Condensate effective theory, the early stages of heavy ion collisions
are described by a quasi-classical state called Glasma, whose dynamics are governed by the Yang-
Mills (YM) equations. Extending prior work [1], we solve the YM equations analytically in the
weak-field limit, which reduces the field strength tensor components to three-dimensional inte-
grals. These integrals can then be evaluated numerically for different nuclear models. Specifically,
we allow the nuclei to have finite extent in the longitudinal direction, which gives access to the
non-trivial spacetime rapidity dependence of observables such as the energy-momentum tensor of the Glasma.

Speaker: Markus Leuthner

ML_XQCD_2022.pdf

29 Equilibration of quark-gluon plasma in heavy-ion collisions

Equilibration of quark-gluon plasma in heavy-ion collisions
Not scheduled
25m
Oral presentation Session
Speaker
Dr Xiaojian Du (Bielefeld University)
Description

Non-equilibrium systems are omnipresent in nature. QCD plasma out of equilibrium and its equilibration are of particular interest given that the relativistic heavy-ion collisions (HICs) produce the non-equilibrium quark-gluon plasma (QGP) which eventually emerges to thermal hydrodynamic states. We investigate the kinetic and chemical equilibration of weakly coupled QCD plasma at finite density with a numerical implementation of QCD　effective kinetic theory based on leading-order QCD, revealing the relevant equilibration pattern and turbulent nature of the QCD plasma far from equilibrium. We then show its equilibration in HICs as an attractor towards hydrodynamics. Based on that, some phenomenological applications such as pre-equilibrium dilepton production in HICs are discussed.

Speaker: Xiaojian Du (Bielefeld University)

XQCD-XiaojianDu.pdf

13:05 Lunch Break

Session

Convener: Michael Strickland (Kent State University)

30 An effective theory of medium induced radiation

We revisit the picture of jets propagating in the quark-gluon plasma. In addition to vacuum radiation, related to the high initial virtuality of a jet, jet particles scatter on the medium constituents resulting in induced emissions. Analytical approaches to resumming these interactions have traditionally dealt separately with multiple, soft [1,2], or rare, hard scatterings [3,4]. A full resummation has so far only been available using numerical methods [5,6,7]. Our goal is to achieve full analytical control of the relevant scales and map out the dominant physical processes in the full phase space. To this aim, we extend existing resummation schemes for the medium-induced spectrum [4,8,9] to the Bethe-Heitler regime, to cover the whole phase space from early to late times, and from hard splittings to emission below the thermal scale. Based on the separation of scales, a space-time picture naturally emerges: at early times, jets start to build from both vacuum and rare, hard scattering induced emissions. At a later stage, determined by a resolution criterion, these emissions initiate a turbulent cascade [10] that rapidly degrades their energy down to, and including the Bethe-Heitler regime. We quantify the impact of such an improved picture, compared to the current state of the art factorization that includes only soft scatterings [11], by both analytical and numerical methods for the jet fragmentation function. Our work serves to improve our understanding of jet quenching from small to large systems and for future upgrades of Monte Carlo generators.

[1] R. Baier, Y. L. Dokshitzer, A. H. Mueller, S. Peigne, and D. Schiff, Nucl. Phys. B 483 (1997) 291–320.
[2] B. Zakharov, JETP Lett. 63 (1996) 952–957.
[3] M. Gyulassy, P. Levai, and I. Vitev, Nucl. Phys. B 594 (2001) 371–419.
[4] U. A. Wiedemann, Nucl. Phys. B 588 (2000) 303–344.
[5] S. Caron-Huot and C. Gale, Phys. Rev. C82 (2010) 064902.
[6] C. Andres, L. Apolinario, and F. Dominguez, JHEP 07 (2020) 114.
[7] S. Schlichting and I. Soudi, arXiv:2111.13731.
[8] Y. Mehtar-Tani, JHEP 07 (2019) 057.
[9] J. a. Barata and Y. Mehtar-Tani, JHEP 10 (2020) 176.
[10] J.-P. Blaizot, E. Iancu, and Y. Mehtar-Tani, Phys. Rev. Lett. 111 (2013) 052001.
[11] P. Caucal, E. Iancu, A. H. Mueller, and G. Soyez, Phys. Rev. Lett. 120 (2018) 232001.

Speaker: Johannes Hamre Isaksen (University of Bergen)

isaksen_trondheim.pdf

31 Quarkonia phenomenology with ALICE

Heavy quarks are efficient probes of different physics aspects related to heavy-ion collisions (HIC), as they experience the full evolution of the system. Quarkonia notably provide a direct probe of the deconfinement of nuclear matter. Recently, the production of J/$\psi$ via (re)generation within the quark-gluon plasma (QGP) or at the phase boundary has been identified as an important ingredient for the interpretation of quarkonium production in Pb–Pb collisions at the LHC. In particular, it is found to be the dominant production mechanism at low $p_{\rm T}$ and in central collisions. Measurements of the quarkonium elliptic flow and polarization bring more insights on the properties of the QGP, while the non-prompt J/$\psi$, originating from beauty hadron decays, gives access to the interaction of b quarks with the QGP. Quarkonium measurements are also now contributing to the study of the initial state of the collision, via for instance polarization measurements in Pb–Pb as a function of the event plane, to probe the strong magnetic field generated by the fast motion of the charges of the nuclei as well as the large angular momentum of the medium in non-central events. J/$\psi$ measurements in p–Pb recently started to be included in the data pool used for nuclear Parton Distribution Function determination via global fits. Quarkonium studies in pp and p–Pb, besides serving as a reference for HIC, allow for investigating collective effects in small systems and shed light on multiparton interactions through measurements of the multiplicity-dependent production.

In this contribution, the latest ALICE results on quarkonia will be presented and their phenomenological implications will be discussed via comparisons with the latest theoretical developments describing quarkonium production and interaction with the QGP. Even in a high-multiplicity environment, the ALICE detector possesses excellent particle identificationm track and vertex reconstruction capabilities, offering unique opportunities to study the quarkonium production in small and large systems, at both mid- and forward rapidities, and down to zero $p_{\rm T}$. The measurements, carried out for different collision systems and energies, include the (multiplicity dependent) production and nuclear modification factor of (prompt and non-prompt) J/$\psi$, $\psi$(2S) and $\Upsilon$(nS), as well as the J/$\psi$ $v_2$ and polarization, with the latter measured also as a function of the event plane. A short overview of the expectations for the LHC Run 3 and 4 will conclude the talk.

Speaker: Guillaume Taillepied (GSI - Helmholtzzentrum fur Schwerionenforschung GmbH (DE))

xqcd22_quarkonia_alice.pdf

32 Stability of Classical Chromodynamic Fields

A system of gluon fields generated at the earliest phase of relativistic heavy-ion
collisions can be described in terms of classical fields. Numerical simulations show that the sys-
tem is unstable but a character of the instability is not well understood. With the intention to
systematically study the problem, we analyze a stability of classical chromomagnetic and chromo-
electric fields which are constant and uniform. We consider the Abelian configurations discussed
in the past where the fields are due to the single-color potentials linearly depending on coordinates.
However, we mostly focus on the nonAbelian configurations where the fields are generated by the
multi-color non-commuting constant uniform potentials. We derive a complete spectrum of small
fluctuations around the background fields which obey the linearized Yang-Mills equations. The
spectra of Abelian and nonAbelian configurations are similar but different and they both include
unstable modes. We briefly discuss the relevance of our results for fields which are uniform only
in a limited spatial domain.

Speaker: Sylwia Bazak (Jan Kochanowski University)

Bazak_xQCD.pdf

Closing remarks

Summary of the workshop and the announcement of the venue of the 19th edition of XQCD

Convener: Jens Oluf Andersen

farewell.pdf

16:00 Coffee break