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The 7th edition of the International Conference on the Initial Stages in High-Energy Nuclear Collisions will be held June 19-23, 2023 in Copenhagen. The conference is organized by the Niels Bohr Institute, University of Copenhagen. It is dedicated to new experimental results from LHC and RHIC, the developments in theoretical tools that predict and explain recent phenomena observed in experiments, as well as the plans for future experimental facilities including EIC and FAIR. The main topics of focus will be:
The conference format will be in-person.
Conference venue: The plenary sessions will be held at Lundbeckfond Auditoriet (Ole Maaløes Vej 5, 2100 Copenhagen), and the parallel session will take place at H.C. Ørsted Instituttet (Universitetsparken 5, 2100 Copenhagen). It takes about 5 minutes to walk from one to the other.
Travel: Copenhagen has good flight connections to all major cities worldwide. The conference venue, located at the University of Copenhagen, can be easily reached via public transportation within 40 minutes from the airport.
Accommodation: Participants are asked to arrange their accommodation themselves. As Copenhagen is a popular tourist city worldwide, and June is a busy season, we recommend booking your hotel as early as possible.
Financial support: We will provide supports for students and young postdocs who have limited financial support from their home institutes. Instructions for applications are available on the website.
The most updated information will be available on this website.
Contact information: IS2023-information@cern.ch, other detailed information see “contact information” page.
If you click the "Contribution list" below, you should find your poster listed below.
One of the main challenges in nuclear physics is the study of the structure of the atomic nucleus. Recently it has been shown that high-energy heavy-ion collisions, such as those at RHIC and the LHC, can serve as a complimentary study to the low-energy experiments. The heavy-ion collisions provide a snapshot of the nuclear distribution at the time of collisions, thus providing a unique, precise probe of the nuclear structure. We propose new, higher-order cumulants of the anisotropic flow, $v_n$, and the mean transverse momentum, $[p_\mathrm{T}]$, as a probe of the nuclear structure in deformed nuclei. Calculations of the cumulants in Xe-Xe collisions with A Multi-Phase Transport model (AMPT) reveal their significant impact on measurements of the triaxial structure of $^{129}$Xe. Additionally, we show their novel sensitivity to the $\alpha$-cluster structure in $^{16}$O. These new cumulants have high potential in future runs of deformed nuclei at the LHC and will be a crucial component in spanning the bridge between the fields of low-energy nuclear physics at the MeV energy scale and the high-energy heavy-ion physics at the TeV energy scale.
Atomic nuclei across the nuclide chart exhibit a wide range of collective degrees of freedom, such as quadruple, triaxial, and octupole deformations. Nuclear deformations enhance the fluctuation of harmonic flow and radial flow, increasing $v_2$, $v_3$, and mean transverse momentum $[p_{\rm T}]$. As demonstrated in recent model studies, the shape parameters can be constrained precisely from ratios of observables between collisions of nuclei with similar mass numbers, such as between $^{96}$Ru+$^{96}$Ru and $^{96}$Zr+$^{96}$Zr or between $^{197}$Au+$^{197}$Au and $^{238}$U+$^{238}$U collisions.
We present measurements of $v_n$, $[p_{\rm T}]$ fluctuations as well as $v_n$-$p_{\rm T}$ correlations in these collision systems from the STAR experiment. Significant differences are observed for mean, variance, and skewness of $[p_{\rm T}]$ fluctuations between $^{197}$Au+$^{197}$Au and $^{238}$U+$^{238}$U collisions, which can be quantitatively explained by the large prolate deformation of $^{238}$U, $\beta_{2,\rm U}\sim0.28$. Striking differences are also observed in isobar collisions of $^{96}$Ru and $^{96}$Zr, where ratios of many observables show significant deviations from unity and exhibit rich patterns as a function of centrality. A comparison with hydrodynamic model simulations suggests a large quadruple deformation in Ru nucleus $\beta_{2,\rm Ru}\sim0.16$ and a large octupole deformation in Zr nucleus $\beta_{3,\rm Zr}\sim0.2$. The non-monotonic dependence of ratios of multiplicity distribution, $v_2$, and $p_{\rm T}$ fluctuations in the mid-central collisions also require a difference in the surface diffuseness between Ru and Zr. By combining all these observables, we can simultaneously constrain the various nuclear deformations in isobar nuclei. Our results provide the first observation and quantitative extraction of the quadruple and octupole deformation in Ru and Zr nuclei using heavy ion collisions.
Low-energy nuclear experiments well constrain the structure of the nuclear ground state, but whether this picture is consistent at higher energies is an open question. Geometry-driven observables such as the anisotropic flow, $v_{\rm n}$, measured in high-energy heavy-ion collisions access the spatial positions of nucleons on an event-by-event basis and provide a snapshot of the many-body correlations within the colliding nuclei. These measurements, obtained with multiparticle correlations, are well suited to probe nuclear deformation in heavy-ion collisions.
In this talk, the nuclear structure of $^{129}$Xe nuclei, which have a quadrupole deformation, is studied by measuring various flow observables using multiparticle correlations in Xe--Xe collisions at $\sqrt{s_{NN}}$=5.44 TeV with ALICE. Comparisons to the measurements in the collisions between $^{208}$Pb nuclei, which are not deformed, will also be presented. The ratios of the measurements in Xe--Xe and Pb--Pb collisions could cancel out most of the final-state effects. Therefore, they are more sensitive to the initial conditions and thus provide information on the initial nuclear structure. Furthermore, the measurements are compared to the AMPT model and the IP-Glasma+MUSIC+UrQMD hydrodynamic model. It allows constraining the overall quadrupole deformation strength $\beta_2$ of $^{129}$Xe. Such comparisons reveal the potential of using flow observables as a probe of the nuclear structure of nuclei in ultrarelativistic heavy-ion collisions at the LHC energies.
Many atomic nuclei have a deformed intrinsic shape that affects the initial-state geometry of the related collider experiments, and hence the flow coefficients observed in the final states. We use collisions of deformed nuclei to study the connection between nuclear structure at low energy and the effective theory of high energy QCD. Along with 16O, the clustered and deformed structure of 20Ne is studied across different collision energies via JIMWLK evolution equations to search for QCD-induced modifications to the nuclear geometry and their observable signatures. Our range of $\sqrt{s_{\rm NN}}$ varies from top LHC energy at $7$ TeV, down 70 GeV, corresponding to the center-of-mass energy available in fixed-target experiments performed in the SMOG system of the LHCb detector. In addition, we discuss ratios of observables in isobar collisions involving $\rm{Ru}^{96}$ and $\rm{Zr}^{96}$, which differs from unity because of the structural difference of these isotopes, and examine their energy evolution from RHIC to LHC.
There is increasing interest in using high-energy collisions to study the structure of nuclei, in particular with the high-precision data made possible by collisions performed with pairs of isobaric species.
A systematic study requires a variation of parameters representing nuclear properties such as radius, skin thickness, angular deformation, and short-range correlations, to determine the sensitivity of the various observables on each of these properties.
In this work we propose a method for efficiently carrying out such study, based on the shifting of positions of nucleons in Monte-Carlo samples. We show that by using this method, statistical demands can be dramatically reduced --- potentially reducing the required number of simulated events by orders of magnitude --- paving the way for systematic study of nuclear structure in high-energy collisions,
The origin of the structure of azimuthal correlations in small collision systems, such as proton–proton and proton–nucleus collisions, is still not fully understood. In these systems, azimuthal correlations tend to extend far in rapidity. These so-called unequal rapidity correlations must therefore originate early in the collision. In the context of the Colour Glass Condensate effective field theory, correlations of particles separated by large rapidities can be studied using the stochastic Langevin picture of the Jalilian-Marian–Iancu–McLerran–Weigert–Leonidov–Kovner (JIMWLK) evolution equation. By separately evolving the Wilson lines in the direct and complex conjugate amplitudes, the formalism can be used to study two-particle production at large rapidity separations. In this talk, I will present results from a numerical implementation of the resulting bilocal Langevin JIMWLK equation. I will show the effects of the fully nonlinear evolution on the unequal rapidity correlations of the produced particles.
We argue that diffractive photo-production of jets in coherent nucleus-nucleus ultra-peripheral collisions (UPCs) at high energy is a golden channel to study gluon saturation. By ``coherent'' we mean elastic processes in which both nuclei emerge unbroken after the collision and the final state exhibits large rapidity gaps. We study such processes within the colour glass condensate effective theory, where the elastic photon-nucleus interactions are described as a colourless, multi-gluon, exchange, a.k.a. the Pomeron. We show that the dominant channel is the diffractive production of three jets in an asymmetric configuration. Two of the jets are hard and propagate at nearby pseudo-rapidities. The third jet is semi-hard, with transverse momentum comparable to the nuclear saturation momentum, and is well separated in pseudo-rapidity from the hard dijets. Such configurations allow for strong scattering and probe the unintegrated parton distributions of the Pomeron in the high gluon density regime. We show that gluon saturation controls the cross-section and leave its imprints on the structure of the final state, notably on the rapidity distribution of the three jets.
We study, to all orders in perturbative QCD, the universal asymptotic behavior of the saturation momentum $Q_s(L)$ controlling the transverse momentum distribution of a fast parton propagating through a dense QCD medium with large size $L$. Due to the double logarithmic nature of the quantum evolution of the saturation momentum, its large $L$ asymptotics is obtained by slightly departing from the double logarithmic limit of either next-to-leading log (NLL) BFKL or leading order DGLAP evolution equations. At fixed coupling, or in conformal N=4 SYM theory, we derive the large $L$ expansion of $Q_s(L)$ up to order $\alpha_s^{3/2}$. In QCD with massless quarks, where conformal symmetry is broken by the running of the strong coupling constant, the one-loop QCD $\beta$-function fully accounts for the universal terms in the $Q_s(L)$ expansion. Therefore, the universal coefficients of this series are known exactly to all orders in $\alpha_s$.
A double copy between 2 > N QCD amplitudes and Gravity amplitudes was first discovered by Lipatov in 1981. In published work with G. Dvali, we showed how a double copy between Black Holes and the CGC arises, with both systems saturating the Bekenstein bound. We discuss common features in the collision and subsequent evolution of shockwaves in the two systems.In particular, we show how this dual picture points to some missing features of the CGC, and conversely how the CGC can help compute gravitational radiation in the vicinity of a Black Hole.
We present a new 3+1D resolved model for the initial state of ultrarelativistic Heavy-Ion collisions, based on the $k_\perp$-factorized Color Glass Condensate hybrid approach [1-4]. This new model responds to the need for a rapidity-resolved initial-state Monte Carlo event generator which can deposit the relevant conserved charges (energy, charge and baryon densities) both in the midrapidity and forward/backward regions of the collision.
This event-by-event generator computes the gluon and (anti-) quark phase-space densities using the IP-Sat model, from where the relevant conserved charges can be computed directly. In the present work we have included the leading order contributions to the light flavor parton densities. As a feature, the model can be systematically improved in the future by adding next-to-leading order calculations (in the CGC hybrid framework), and extended to lower energies by including sub-eikonal corrections the channels included. We present relevant observables, such as the eccentricities and flow decorrelation, as tests of this new approach.
References:
[1] O. Garcia-Montero, H. Elfner and S. Schlichting. In preparation.
[2]T. Lappi and S. Schlichting, Phys. Rev. D 97, 034034 (2018), arXiv:1708.08625 [hep-ph].
[3] T. Lappi and H. Mäntysaari, Phys. Rev. D 88, 114020 (2013), arXiv:1309.6963 [hep-ph]
[4] H. Mäntysaari, Scattering off the Color Glass Condensate, Ph.D. thesis, Jyvaskyla U. (2015), arXiv:1506.07313 [hep-ph].
A promising area of development in QCD research is extending beyond the well-studied boost-invariant or (2+1)D picture of QGP evolution and into (3+1)D. Doing so offers a powerful means of further constraining the validity and the parameters of successful collision models and thus allows us to refine our knowledge of the properties of the QGP. However, the extra dimension also introduces challenging new degrees of freedom, particularly in the initial conditions, and exacts a high computational cost in hydrodynamics.
Seeking to facilitate (3+1)D analyses, we have developed TRENTo-3D, a fast, parametric model of 3D initial-state geometry. We have tested its validity through a large-scale calibration that we wish to report on in this contribution. For computational efficiency this validation implemented a (1+1)D linearized approximation of ideal hydrodynamics, which restricted the set of observables that can be computed with small uncertainties. We will present the results of this first calibration and preview a second, ongoing effort to perform a calibration featuring a full (3+1)D viscous hydrodynamic evolution and a much broader set of observables.
Measurements of different flow harmonics relative to the participant and spectator planes provide unique insight into the initial conditions and the space-time evolution of the quark-gluon plasma (QGP). Different particle-type dependencies of these flow harmonics allow to separate the effects of QGP evolution and initial state fluctuations. Elliptic flow coefficients ($v_{2}$) relative to the participant and spectator planes are measured by ALICE in Pb-Pb collisions for charged hadrons, pions, kaons, and protons at midrapidity as a function of transverse momentum and centrality. The spectator plane is reconstructed from the deflection of the neutrons using two Zero Degree Calorimeters. Comparison of the ratio of $v_{2}$ relative to the spectator and participant plane with the corresponding eccentricities predicted by the initial state models indicates a decorrelation between the neutron spectator and the reaction plane or an incomplete description of the spectator dynamics within these models. The particle-type dependence of the ratio between $v_{2}$ measured using two- and four-particle cumulants is compared with hydrodynamic model calculations coupled with quark coalescence and jet fragmentation, which suggests a significant influence of hadronic interactions on $v_{2}$ during the final stage of the collision evolution.
We present the first studies of net-charge fluctuations and charge-balance functions using the broad rapidity coverage of the CMS experiment. These types of event-by-event fluctuations are a powerful tool to characterize the thermodynamic properties of the quark-gluon plasma (QGP). The net-charge of the system, e.g., quantified by the $\nu_{dyn}$ observable, is a conserved quantity, meaning its fluctuations are sensitive to the QGP formation and phase transition, and hence providing complementary understanding of strong interactions from the charge balance function. Compared to previous measurements, the pseudorapidity range is extended up to $|\eta| < 2.4$. This larger phase space region is essential for studying the system time evolution. The width of the balance function, both in relative $|\eta|$ and relative azimuthal angle, is found to decrease with multiplicity for low particle transverse momentum ($p_{T} < 2$ GeV/c). The effect is observed for both collision systems, and it is consistent with a late hadronization scenario, where particles are produced at a later stage during the system evolution. The multiplicity dependence is weaker for higher $p_{T}$, which signifies that the balancing charge partners are strongly correlated compared to the low-$p_{T}$ region. Model comparisons cannot reproduce the multiplicity dependence of the width in $\Delta\eta$, albeit a model which incorporates collective effects can reproduce the narrowing of the width.
One of the main challenges in the theory of heavy ion collisions is understanding how an initial state of two highly Lorentz-contracted nuclei acquires the features of a hydrodynamic plasma in a characteristic time of 1 fm/c. Arguably, the most successful descriptions of this out-of-equilibrium stage have been established by seeking and finding so-called “attractor” solutions in the various (simplified) theories that attempt to capture the out-of-equilibrium dynamics of QCD. These attractors are characterized by a loss of sensitivity to the initial conditions, which is achieved because the system is dynamically driven to a preferred “attractor surface” in the phase space of the theory, often well before hydrodynamization.
In this context, the adiabatic hydrodynamization framework [1,2] presents itself as a promising candidate to describe and characterize attractors in a model-independent formulation. In principle, all that needs to be done is to establish the dominance of an effective ground state in the dynamics of the system. This was done analytically in [2] for the first stage of the bottom-up thermalization scenario [3], demonstrating a dynamically effected reduction in the number of active degrees of freedom much earlier than the hydrodynamic regime. A key observation made in [2] was the fact that such effective ground states may require a time-dependent change of coordinates for their dominance to be manifest. Ultimately, such a change of coordinates is one of the defining characteristics of the sought attractor solution, and therefore it becomes imperative to define a procedure to find the “optimal” coordinate transformation for a given theory. In this talk, we will lay out a candidate for such a procedure [4] and will consider the example of collision-driven dynamics in the kinetic theory of a dilute gluon gas to demonstrate the effectiveness of this method to describe attractor solutions. Understanding this simplified example will lay out the path to explore more realistic descriptions of the expanding QGP, including transverse expansion.
[1] J. Brewer, L. Yan, Y. Yin, Phys.Lett.B 816 (2021) 136189
[2] J. Brewer, B. Scheihing-Hitschfeld, Y. Yin, JHEP 05 (2022) 145
[3] R. Baier, A.H. Mueller, D. Schiff and D.T. Son, Phys. Lett. B 502 (2001) 51
[4] K. Rajagopal, B. Scheihing-Hitschfeld, R. Steinhorst, in preparation
High energy nuclear collisions produce far-from-equilibrium matter with a high density of gluons at early times. We identify for the first time two local order parameters for condensation, which can occur as a consequence of the large density of gluons. We demonstrate that an initial over-occupation of gluons can lead to the formation of a macroscopic zero mode towards low momenta that scales proportionally with the volume of the system—this defines a gauge invariant condensate. The formation of a condensate at early times has intriguing implications for early time dynamics in heavy ion collisions.
Heavy-ion collisions can be well described through relativistic viscous hydrodynamics, but questions still remain when hydrodynamics is applicable because the initial state may begin very far-from-equilibrium.
Thus, a pre-equilibrium evolution phase is used to bridge the gap between the initial state and hydrodynamics. K$\phi$MP$\phi$ST is one such pre-equilibrium model that propagates the energy-momentum tensor by decomposing it into the background and fluctuations around that background, whose evolution is captured by Green's functions.
We extend this formalism to include conserved charges and calculate the corresponding non-equilibrium Green's functions in the relaxation time approximation. The ICCING algorithm initializes conserved charges in the initial state by sampling $g \rightarrow q\bar{q}$ splitting probabilities and is, thus, perfectly positioned to implement Green's functions for charge propagation. We show that this method alters the initial state charge geometries and is applicable in central to mid-central collisions.
In relativistic heavy ion collisions, the charged ions produce an intense flux of equivalent photons. Thus, photon-induced processes are the dominant interaction mechanism when the colliding nuclei have a transverse separation 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 photo-nuclear hard-scattering processes. The precision of these measurements is augmented by studies of nuclear break-up effects, allowing for detailed comparisons with theoretical models in phase-space regions where significant nuclear PDF modifications are expected but not strongly constrained by existing data.
Gluons are found to become increasingly dominant constituents of nuclear matter when being probed at higher energies or smaller Bjorken-$x$ values. This has led to the question of the ultimate fate of nuclear gluonic structure and its interaction with external probes at extreme density regimes when approaching the limit allowed by unitarity. In ultraperipheral collisions (UPCs) of relativistic heavy ions, the coherent heavy-flavor vector meson production via photon-nuclear interactions is of particular interest, since its cross section is directly sensitive to the nuclear gluon density at leading order. However, in experimental measurements, because each of the two nuclei in symmetric UPCs can serve both as a photon-emitter projectile and a target, this two-way ambiguity has prevented us from disentangling contributions involving high- and low-energy photon-nucleus interactions, thus limiting our capability of probing the extremely small-$x$ regime, where nonlinear QCD effects are expected to emerge. In this talk, we will present a new measurement of coherent J/$\psi$ photoproduction, where the two-way ambiguity is solved by implementing for the first time a forward neutron tagging technique in UPC PbPb collisions in 2018 at 5.02 TeV. The coherent J/$\psi$ photoproduction cross section will be presented, for the first time, as a function of the photon-Pb center-of-mass energy in UPCs up to about 400 GeV, corresponding to an extremely low $x$ of ${\sim}5\times10^{-5}$. We will discuss the physics implications of this new result, as well as exciting opportunities in future LHC heavy ion runs.
Photon-induced reactions in ultra-peripheral collisions (UPCs) of heavy nuclei at the LHC have been studied using the ALICE detector for several years. The ALICE detector can measure the photoproduction cross section for vector mesons at various rapidities, centre-of-mass energies and collision systems. Beyond the recent ALICE studies of the rapidity and momentum transfer dependence of coherent $J/\psi$ photoproduction, new results on coherent $J/\psi$ measurements with forward neutron tagging will be presented for the first time. The coherent production of $J/\psi$ accompanied by electromagnetic nuclear dissociation (EMD) has been performed in a large range of the $J/\psi$ rapidity covering $0<|y|<4$, with the $J/\psi$ reconstructed through its decay channels into a lepton pair. Having cross sections in a given rapidity but with different configurations of EMD allows for the extraction of the energy dependence of this process in the range $17< W_{\rm\gamma Pb} < 920$ GeV, where $W_{\rm\gamma Pb}$ is the centre-of-mass energy per nucleon of the photon-lead system. This range corresponds to a Bjorken-$x$ interval of about three orders of magnitude $1.1\times 10^{-5}< x <3.3\times 10^{-2}$, covering an explored kinematic region of great interest.
In this talk we will discuss the impact of these measurements in testing our understanding of perturbative quantum chromodynamics at large energies by comparing the ALICE data to theoretical predictions of this process.
Quarkonium measurements in heavy-ion collisions provide insight into the mechanisms which cause the quarkonium bound states to dissociate in the Quark-Gluon Plasma (QGP). The $J/\psi$ suppression and $\Upsilon$ sequential melting provide information on the thermodynamic properties of the QGP, in particular the initial medium temperature. Quarkonium studies in $p$+$p$ and $p$+A collisions serve as the necessary baselines for heavy-ion collisions. They also help to understand the quarkonium production mechanism and the cold nuclear matter effects in $p$+$p$ and $p$+A collisions, respectively. In particular, the charged particle multiplicity dependent studies of quarkonium production in $p$+$p$ collisions could provide information on the interplay of hard vs. soft QCD processes during initial stages.
This contribution will focus on recent quarkonium studies from the STAR experiment at RHIC. We will present the $J/\psi$ and $\Upsilon$ measurements in $p$+$p$ collisions at $\sqrt{s} = $~200 and 500~GeV including production cross section, $J/\psi$ production in jets and with jet activity, and normalized quarkonium yield as a function of normalized charged particle multiplicity. Nuclear modification factors and elliptic flow of $J/\psi$ and $\Upsilon$ states in various collision systems ($p$+Au, Au+Au and isobar) at $\sqrt{s_{NN}} = $~200~GeV will be also shown. The presented measurements will be compared to different model predictions and physics implications will be discussed.
FoCal is a high-granularity forward calorimeter to be installed as an ALICE upgrade subsystem during the LHC Long Shutdown 3 and take data during the LHC Run 4. It consists of a compact silicon-tungsten sampling electromagnetic calorimeter (FoCal-E) with pad and pixel longitudinal and transverse segmented readout layers to achieve high spatial resolution for discriminating between isolated photons and decay photon pairs. Its hadronic component (FoCal-H) is constructed from copper capillary tubes filled with scintillator fibers and used for isolation energy measurement and jets.
The FoCal detector extends the ALICE physics programme with the capability, unique at the LHC, to investigate gluon Parton Distribution Functions (PDFs) down to Bjorken-x of ~10^-6 at a momentum transfer Q ~ 4GeV/c, where these are expected to behave non-linearly due to the high gluon densities, with direct photon measurements. Additionally, FoCal allows forward jet measurements in pp and p-Pb collisions, including gamma-jet and jet-jet correlations, but also photo-production of vector mesons such as the J/psi in proton-Pb and Pb-Pb ultra-peripheral collisions.
In this presentation we will discuss the small-x physics potential of the FoCal detector and present projected detector performance studies for its main physics observables.
The Forward Calorimeter (FoCal) in ALICE, which is planned to take data in Run 4, covers a pseudorapidity interval of 3.4 < η < 5.8 for probing non-linear QCD dynamics in an unexplored kinematic region at the LHC.
In its electromagnetic section, layers of high granularity Monolithic Si pixels are alternated to Si pads for sampling the longitudinal development of the electromagnetic showers, designed to allow for the reconstruction of neutral mesons with high efficiency.
Its hadronic section is made from constructing towers by grouping copper capillary tubes filled with scintillator fibers interface by SiPMs.
During 2021 and 2022, various ever-improving prototypes of the calorimeter were installed at the Test Beam facilities of CERN to evaluate their performance and compare to simulations.
In the talk, we report on the most recent results of these campaigns and outline the impact on the design of the detector.
The gluon distribution function grows rapidly with decreasing momentum fraction $x$. However, the total scattering cross section is bound by unitarity, which requires that the increase of gluon density be tamed. This is explained by gluon recombination under the color glass condensate (CGC) framework. A definitive discovery of nonlinear effects in QCD and the saturation regime would significantly improve our understanding of nucleon structure and nuclear interactions at high energy. Two-particle azimuthal correlation is one of the most direct and sensitive channels to access the underlying nonlinear gluon dynamics. In this talk, we will present recent results of forward di-hadron correlations measured at STAR, together with the signatures of gluon saturation predicted by CGC. In 2024, RHIC is planned to collect high statistics $p+$A data with STAR forward upgrade. New opportunities for measurements to study the nonlinear effects in QCD will also be discussed.
Neutron-rich matter exists naturally in neutron stars and neutron skins of heavy nuclei. It can also be created during mergers of neutron stars in space and heavy-ion collisions in terrestrial laboratories. The Equation of State (EOS) of such matter is still very poorly known but has broad impacts on many interesting issues in both astrophysics and nuclear physics. In particular, nuclear symmetry energy encoding the energy cost to make the nuclear matter more neutron-rich has been the most uncertain part of the EOS of dense neutron-rich nucleonic matter. It affects the masses, radii, tidal deformations, cooling rates, and frequencies of various oscillation modes of isolated neutron stars as well as the strain amplitude and frequencies of gravitational waves from neutron star mergers.
In this talk, we will discuss the interplay and effects of neutron skins and short-range nucleon-nucleon correlations in the initial state of colliding nuclei and the density dependence of nuclear symmetry energy on observables of heavy-ion reactions from FRIB to FAIR energies.
Measurements of muon pairs produced via two-photon scattering processes in hadronic (i.e. Non-UPC) Pb+Pb collisions are also presented. These non-UPC measurements provide a novel test of strong-field QED and may be a potentially sensitive electromagnetic probe of the quark-gluon plasma. These measurements include the dependence of the cross-section and angular correlation on the mean-$p_{\mathrm{T}}$ of the dimuon pair, the rapidity separation between the muons, and the pair angle relative to the second-order event plane, all measured differentially as a function of the Pb+Pb collision centrality. The presented results are compared with recent theory calculations.
In spring 2023, the sPHENIX detector at BNL’s Relativistic Heavy Ion Collider (RHIC) will begin measuring a suite of unique heavy flavor and quarkonia observables with unprecedented statistics and kinematic reach at the RHIC energies using combined electromagnetic and hadronic calorimeters and high precision tracking detectors. A Monolithic Active Pixel Sensor (MAPS)-based vertex detector upgrade to sPHENIX, the MVTX, will provide a precise determination of the impact parameter of tracks relative to the primary vertex in high multiplicity heavy-ion collisions and polarized proton-proton/proton-nuclei collisions. It will enable precision measurements of open heavy-flavor observables, covering an unexplored kinematic region at RHIC to explore the parton energy loss mechanism, parton transport coefficients in Quark Gluon Plasma (QGP) and the hadronization process under different medium conditions. The physics program, its potential impact, and the recent detector development will be discussed in this talk.
In vacuum, a highly virtual parton fragments into a collimated spray of hadrons—known as a jet. Jets are useful in studying both perturbative and non-perturbative regimes of QCD. In p+p collisions, jet substructure observables are used to study the QCD evolutions and hadronization. In addition, measurements of the event activity dependence of jet properties in p+A collisions can provide new insights to the initial states of these collisions.
In this talk, we present recent results on the jet substructure observables, such as jet mass, jet charge and their correlations, as well as the two-point energy-energy correlator, in p+p collisions at $\sqrt{s}$ = 200 GeV. We report the comparisons between data and different versions of PYTHIA and HERWIG event generators. We also report the correlations between event activities and hard scatterings in p+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 200 GeV. Dijet acoplanarity and transverse momentum imbalance measurements for different event activities are presented to study the cold nuclear-medium effect in p+Au collisions.
Charm and bottom quark production is an important experimental observable that sheds light on the heavy quark interaction with the nuclear medium. With high statistics datasets, tracking and PID at very low transverse momentum, and excellent vertexing capabilities, LHCb performs precision measurements of a rich set of heavy flavor hadrons. These capabilities allow for precise studies of strangeness enhancement, baryon enhancement, and charmonia suppression in various colliding systems from $pp$ to $p$Pb and PbPb. Furthermore, LHCb has unique access to the production of the exotic $\chi_{c1}$(3872) hadrons in $pp$ and $p$Pb collisions. In this contribution, we will present the most recent results, including $\Xi_c^+$ production in $p$Pb, along with comparisons to theoretical calculations.
Long-range azimuthal angle correlations of the produced particles have been observed in high multiplicity proton--lead (p--Pb) and proton--proton (pp) collisions, indicating the presence of collective effects in small systems. The origin of these effects is the subject of intense debate.
In this talk, we present the measurements of $v_{\rm n}$ of charged particle pairs and identified hadrons as a function of multiplicity and $p_{\rm T}$ in p--Pb and pp collisions at $\sqrt{s_{\rm NN}}=$ 5.02 TeV and $\sqrt{s}=$ 13 TeV, respectively. The non-flow contributions are significantly suppressed by the use of forward detectors, allowing a large pseudorapidity separation of the correlated particles up to $|\Delta \eta| \sim 8$. In addition, the template fit method is applied to further suppress the non-flow contamination in the measurements of $v_{\rm n}$. These results show the evolution of the anisotropic flow with the event multiplicity in small collision systems. It is observed that the splitting between $v_{\rm n}$ of baryons and mesons gradually decreases as the event centrality increases. Comparison with hydrodynamic model calculations indicates that the relative contributions from the quark-coalescence mechanism decrease towards lower multiplicity p--Pb collisions.
How collectivity originates and evolves in the collisions of small-size systems is a highly debated topic in the heavy-ion community. The evolution may be associated with both hydrodynamic and non-hydrodynamic modes. Furthermore, the uncertainties in the description of initial geometry and fluctuations at the sub-nucleonic scale will significantly degrade the predictive power of the available dynamical evolution models.
In this talk, we will present the measurements of flow harmonics ($v_2$, $v_3$) in p+Au, d+Au and $^3$He+Au collisions at 200 GeV in STAR. With the advantage of wide rapidity coverage of the Time Projection Chamber (-1$<\eta <$1), the flow coefficients are extracted via di-hadron correlations at mid-pseudorapidity with a relative pseudorapidity separation of $|\Delta\eta|>$ 1.0. The kinematics of our measurements can help avoid decorrelation and provide a benchmark to compare with boost-invariant models. Such measurements will also provide useful information to understand the effect of nucleonic vs. sub-nucleonic fluctuation on the initial geometry in the small collision systems.
Flow-like signals including the ridge structure observed in large collision systems have also been observed in small collision systems. This leads to questions about the onset of collectivity as well as the parton phase in nuclear collisions. Here [1], we use the string melting version of a multi-phase transport (AMPT) model to study multiparticle cumulants in p+p collisions at 13 TeV. The AMPT model already includes nonflow effects, fluctuating initial conditions, and a parton cascade that can address nonequilibrium evolutions; we also extend the model by implementing a sub-nucleon geometry for the incoming protons. We find that the model can produce negative $c_2\{4\}$ values at high multiplicities either with or without a proton substructure. In addition, the dependences of $c_2\{4\}$ and $c_2\{2\}$ on the parton cross section σ are strong and surprisingly non-monotonous, where only an intermediate range of σ values (between about 0.15 and 1.5 mb) leads to negative $c_2\{4\}$. Furthermore, the model with sub-nucleon geometry for the proton better describes the multiplicity dependence of $c_2\{4\}$; this indicates the importance of including the sub-nucleon geometry of the proton in studies of small collision systems.
[1] X.-L. Zhao, Z.-W. Lin, L. Zheng, and G.-L. Ma, Physics Letters B 839 (2023) 137799.
Small systems display large anisotropic flow coefficients that can potentially be interpreted as a hydrodynamic signal. At these moderate multiplicities anisotropic flow is however relatively sensitive to subtle effects. These include the precise experimental procedure, rapidity coverage and gaps as well as effects due to resonance decays. In this talk we quantify these effects for pPb, OO and PbPb collision using the Trajectum framework including systematic uncertainties, so that a reliable hydrodynamic baseline can be attained.
Particle production and collective flow of identified hadrons over the $p_{\rm T}$ range from 0 to 6 GeV in high multiplicity events of proton-proton and proton-lead collisions at the Large Hadron Collider (LHC) are investigated by Hydro-Coal-Frag model, which includes hydrodynamic models at low transverse momentum, quark recombination model at intermediate transverse momentum, and string fragmentation model at high transverse momentum. The Hydro-Coal-Frag model gives a good description of the transverse momentum ($p_{\rm T}$)-spectra and differential elliptic flow, $v_2(p_{\rm T})$, of pions kaons, and protons over the $p_{\rm T}$ range of 0 to 6 GeV. We also well reproduce the experimentally observed approximate NCQ scaling of hadrons $v_2$ at intermediate $p_{\rm T}$. We demonstrate that quark coalescence plays a crucial role in this process. Our results strongy hint the possible formation of quark-gluon plasma in high multiplicity p--p collision and p-Pb collisions at the LHC.
Hadronic collisions produce prompt photons which are characterized by a large transverse momentum and absence of event activity in their vicinity. Photons are a robust probe of cold nuclear matter effects in small and large collision systems because they do not interact strongly and are thus insensitive to medium-induced final-state effects. Prompt photon production is dominated by the Compton process ($g\text{q}\rightarrow \text{q}\gamma$), making it sensitive to the gluon Parton Distribution Function (PDF), and provides a test of high momentum pQCD calculations. Recent experimental results indicate the need for corrections beyond NLO to describe their production accurately. This talk presents the measurement of isolated prompt photon production in pp and p--Pb collisions, measured with the ALICE detector. The isolation method is applied to suppresses the background photons produced in the fragmentation process and electromagnetic decays. The production cross sections in both systems will be presented and compared with NLO calculations using recent (n)PDFs and fragmentation functions. In addition, the nuclear modification factor $R_{\rm pA}$ is measured, quantifying possible modifications of the parton distributions inside the nucleus. This is the first time the isolated prompt photon $R_{\rm pA}$ has been measured for low transverse momenta of $p_{\mathrm{T}} < 20 $ GeV/$c$ -- a regime in which a sizeable suppression is predicted by theoretical calculations.
$J/\psi$, a charmonium bound state made of a charm and an anti-charm quark, has been discovered in the 1970s and confirmed the quark model. Because the mass of charms quarks is significantly above the QCD scale, charmonia are considered as excellent probes to test pQCD calculations. Over the past decades, they have been studied extensively at different high-energy colliders. However, their production mechanisms, which involves in multiple scales, are still not very well understood. Recently, in high multiplicity $p+p$ collisions at RHIC and at the LHC, a significant enhancement of $J/\psi$ production yield has been observed, which suggests a strong contribution from the Multi-Parton Interaction (MPI). This is different from the traditional pQCD picture where charm quark pairs are produced from a single hard scattering between partons in $p + p$ collisions. In this talk, we will report the $J/\psi$ normalized production yield as a function of normalized charged particle multiplicity over a board range of rapidity and event multiplicity in the $J/\psi \to \mu^+ \mu^-$ channel with PHENIX Run 16 $p+p$ data at $\sqrt s =200$ GeV as a function of event multiplicity. The result is compared with PYTHIA 8 simulation with the MPI on and off. In addition, the status of this analysis using $p + Au$ and $Au + p$ collisions will be presented and discussed along with Color Glass Condensate (CGC) predictions. Finally, the status of multiplicity dependent $\psi(2S)/J/\psi$ ratio analysis in $p + p$ data will be presented along with a discussion on its sensitivity to charmonium bound state.
High-multiplicity measurements in pp and p--Pb collisions have revealed the presence of phenomena typically attributed to the creation of a quark-gluon plasma. Events with multiple parton-parton interactions (MPIs) have been proposed as one possible explanation of this observation. MPIs play a significant role in describing the soft component of the hadronic interactions, and at LHC energies also affect the production of heavy quarks. Quarkonium measurements in small systems, such as production and flow as a function of the event activity as well as double quarkonium production, offer a special insight into MPIs. In addition, other results integrated over multiplicity represent an important baseline. A wide variety of quarkonium measurements in pp collisions, including also less standard observables like polarization and production in association with jets, are crucial to clarify production mechanisms at play, as they are not yet fully understood. Furthermore, measurements in p--Pb collisions allow one to investigate cold nuclear matter effects, such as the modification of nuclear parton distribution functions.
In this contribution, new published multiplicity dependent $\psi$(2S) and $\Upsilon$($n$S) ($n$ = 1,2,3) measurements, carried out at forward rapidity in pp collisions at $\sqrt{s}$ = 13 TeV and p--Pb collisions at $\sqrt{s_{\rm NN}}$ = 8.16 TeV, along with quarkonium excited-to-ground state ratios, will be discussed. A wide collection of new measurements in pp collisions at 13 TeV, including $\Upsilon$(1S) polarization, J/$\psi$ $v_{2}$, as well as J/$\psi$ produced in pairs or associated with jets, with the latter based on the usage of TRD triggered data, will be shown. Moreover, new published results of the prompt and non-prompt J/$\psi$ nuclear modification at midrapidity in p-Pb collisions at $\sqrt{s}$ = 8.16 TeV will be presented. Results will be compared with available theoretical calculations.
With a unique geometry covering the forward rapidity region, the LHCb detector provides unprecedented kinematic coverage from high to very low Bjorken-$x$. The excellent momentum resolution, vertex reconstruction and particle identification allow precision measurements down to very low hadron transverse momentum. This contribution will include recent results on charged and neutral hadron production in pPb collisions and results on Z+charm jets, with discussions of how this data constrain models of partonic structure of nuclei and intrinsic charm in the proton.
The study of heavy-flavour particle production as a function of the event activity provides sensitivity to the interplay of hard and soft interaction processes occurring in hadronic collisions, and can shed light into the nucleon underlying partonic structure. Multiple partonic interaction, color-reconnection mechanisms, or modified heavy-quark hadronisation in high-density environments can affect the production of heavy-flavour particles in different event-multiplicity regimes of proton-proton and proton-nucleus collisions.
This contribution summarizes the most recent and relevant multiplicity-dependent measurements of heavy-flavour production in pp and p--Pb collisions performed by the ALICE Collaboration. Among these are the final results of non-prompt D-meson fraction as a function of multiplicity in pp collisions at $\sqrt{s}$ = 13 TeV, the final measurements of self-normalized-yields of heavy-flavour decay electrons in pp collisions at $\sqrt{s}$ = 13 TeV and p--Pb collisions at $\sqrt{s_{\rm NN}}$ = 8.16 TeV, as well as the final results on the enhancement of $\Lambda_{\rm c}^+/{\rm D}^{0}$ production yield ratios at various event multiplicities in pp collisions at $\sqrt{s}$ = 13 TeV. The measurement of D meson production as a function of the event spherocity in pp collisions at $\sqrt{s}$ = 13 TeV, probing the dependence of heavy-flavour production on different event topologies, is also reported.
All the presented results are compared to models including different mechanisms that can impact on the multiplicity dependence of heavy-flavour production.
Measurements of Deep Inelastic Scattering (DIS) provide a powerful tool to probe fundamental structure of protons and other nuclei. The DIS cross sections can be expressed in terms of structure functions which are conventionally constructed via parton distribution functions (PDFs) that obey the DGLAP evolution equations. However, it is also possible to formulate the DGLAP evolution directly in terms of measurable DIS structure functions thereby entirely sidestepping the need for introducing PDFs. We call this as the physical-basis approach. In a global analysis one would thereby directly parametrize the (observable) structure functions -- not the (unobservable) PDFs. Ideally, with data constraints at fixed $Q^2$, the initial condition for the evolution would be the same at each perturbative order (unlike for PDFs) and the approach thus provides a more clean test of the QCD dynamics.
We first study a physical basis consisting of the structure functions $F_2$ and $F_{\rm L}$ in the fixed-flavour number scheme to the leading non-zero order in $\alpha_s$. We show how to express the quark singlet and gluon PDFs in terms of $F_2$ and $F_{\rm L}$ directly in momentum space which then leads to the DGLAP evolution of the structure functions $F_2$ and $F_{\rm L}$. In the second step we expand the physical basis to include six independent structure functions which allows for a consistent global analysis. The steps towards the NLO accuracy and variable-flavour-number scheme are outlined. In the NLO accuracy (when scheme dependence of PDFs starts to play part), we can take advatage of physical basis and express e.g. the Drell-Yan cross sections at the LHC directly in terms of measurable DIS structure functions and thus without scheme dependence.
It has been postulated that nonperturbative quantum chromodynamics (QCD) evolution of a single parton in the vacuum can develop long-range collective effects of a multiparton system, reminiscent of those observed in high-energy nuclear interactions from the formation of a quark-gluon plasma. A search for such QCD collective effects is performed by the CMS experiment via two-particle correlation measurements of charged constituents inside jets produced in high-pileup pp collisions. The data set used at $\sqrt{s} = 13 $~TeV corresponds to an integrated luminosity of 138~fb$^{-1}$, where the effect of pileup is treated by dedicated per-particle algorithms. For charged constituents within a reconstructed jet cone radius of 0.8, two-particle correlations as functions of relative azimuthal angle ($\Delta\phi*$)) and pseudorapidity ($\Delta\eta*$) are performed in a novel ``jet frame'', where constituent kinematics are re-defined relative to the jet direction being the $z$ axis. The correlation functions are studied in classes of in-jet charged multiplicity up to nearly 100, and for different ranges of particle transverse momentum in the jet frame. Anisotropy Fourier harmonics are extracted from long-range azimuthal correlation functions for $|\Delta\eta*|>2$. The long-range elliptic anisotropy harmonic, $v^{j}_2$, in data is compared to Monte Carlo (MC) event generators such as PYTHIA 8 and Sherpa.
The evidence for correlations between particles significantly separated in pseudorapidity in both small and large colliding systems represents one of the most remarkable findings observed at the LHC and at RHIC. Long-range correlations in heavy-ion collisions are considered signatures for the creation of a strongly-coupled quark-gluon plasma, which converts the initial anisotropy of the fireball into final state hadron momentum correlations. In proton-proton and proton-nucleus collisions, where the formation of a QGP was initially not expected, their nature is not yet fully understood. Measurements involving heavy quarks, due to their early formation time and large mass, can provide unique constraints on the origin of these phenomena.
In this presentation, a selection of the latest heavy-flavour measurements performed by ALICE to constrain the collective properties of pp, p--Pb and Pb--Pb collisions will be presented. This will include the final results of the elliptic-flow coefficient of heavy-flavour decay muons at forward rapidity in high-multiplicity p--Pb collisions at $\sqrt{s_{\rm NN}}$ = 8.16 TeV, as well as elliptic-flow coefficient measurements of non-prompt D$^{0}$ mesons in Pb--Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV.
Measurements of two-particle correlations in $pp$ collisions show the presence of long-range correlations along $\Delta\eta$ that are similar to those seen in heavy-ion collisions. The similarity between the $pp$ and heavy-ion measurements raises the possibility that a tiny droplet of the QGP is produced even in $pp$ collisions. However, alternative models that attribute the correlations in $pp$ collisions to semi-hard processes can also qualitatively reproduce the measurements. Therefore, differentiating between particles produced from semi-hard processes, such as low-$p_{\mathrm{T}}$ jets, and those produced from soft interactions can help determine the origin of the correlations in $pp$ collisions. This talk presents measurements of two-particle correlations in $pp$ collisions at $\sqrt{s}=13$ TeV with two different particle-pair selections. In the first case, tracks associated with jets are excluded from the correlation analysis. This shows that excluding tracks associated with jets does not affect the measured correlations. In the second case, correlations are measured between tracks that are constituents of jets and tracks from the underlying event, which showed that jets do not exhibit any azimuthal correlations with the underlying event. These measurements provide a further understanding of the collective signatures observed in $pp$ collisions.
Viscous hydrodynamics serves as a successful mesoscopic description of the quark-gluon plasma produced in relativistic heavy-ion collisions. In order to investigate, how such an effective description emerges from the underlying microscopic dynamics we calculate the non-hydrodynamic and hydrodynamic modes of linear response in the sound channel from a first-principle calculation in kinetic theory. We do this with a new approach wherein we linearize and discretize the collision kernel to calculate eigenvalues directly. This allows us to study the Green's functions at any point in time or frequency space. Our study focuses on scalar theory with quartic interaction and we find that the analytic structure of Green's functions in the complex plane is far more complicated than just poles or cuts which is the first step towards an equivalent study in QCD kinetic theory.
We train a deep convolutional neural network to predict hydrodynamic results for flow coefficients, average transverse momenta and charged particle multiplicities in ultrarelativistic heavy-ion collisions from the initial energy density profiles [1]. We show that the neural network can be trained accurately enough so that it can reliably predict the hydrodynamic results for the flow coefficients and, remarkably, also their correlations like normalized symmetric cumulants, mixed harmonic cumulants and flow-transverse-momentum correlations. At the same time the required computational time decreases by several orders of magnitude. To demonstrate the advantage of the significantly reduced computation time, we generate 10M initial energy density profiles from which we predict the flow observables using the neural network, which is trained using 5k, and validated using 90k events per collision energy. We then show that increasing the number of collision events from 90k to 10M can have significant effects on certain statistics-expensive flow correlations, which should be taken into account when using these correlators as constraints in the determination of the QCD matter properties.
[1] H. Hirvonen, K. J. Eskola, H. Niemi, arXiv:2303.04517 [hep-ph]
In the view that the short wavelength response can be important in small colliding systems and at early-times of a heavy-ion collision, we investigate the response of the near-equilibrium quark-gluon plasma (QGP) to perturbation at non-hydrodynamic gradients. We propose a conceivable scenario under which sound mode continues to dominate the medium response in this regime. Such a scenario has been observed experimentally for various liquids and liquid metals. We further show this extended hydrodynamic regime (EHR) indeed exists for both the weakly-coupled kinetic equation in the relaxation time approximation (RTA) and the strongly-coupled N=4 supersymmetric Yang-Mills (SYM) theory. We construct a simple but nontrivial extension of Müeller-Isareal-Stewart (MIS) theory, namely MIS*, and demonstrate that it describes EHR response for both RTA and SYM theory. Finally, we discuss the possible connection between the extended hydrodynamic regime and observed collectivity in small colliding systems.
Ref: Weiyao Ke and Yi Yin, ArXiv: 2208.01046