High-energy Heavy-ion Collisions Research Articles

Review of Deep Learning in High-Energy Heavy-Ion Collisions

The hot deconfined matter called quark–gluon plasma (QGP) can be generated in relativistic heavy-ion collisions (HICs). Its properties under high temperatures have been widely studied. Since the short-lived QGP is not directly observable, data-driven methods, including deep learning, are often used to infer the initial-state properties from the final distributions of hadrons. This paper reviews various applications of machine learning in relativistic heavy-ion collisions, explains the fundamental concepts of deep learning, and discusses how the properties of HIC data can be interpreted using efficient machine learning models.

Λ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Lambda $$\\end{document} polarization in very high energy heavy ion collisions as a probe of the quark–gluon plasma formation and properties

We have studied the spin polarization of Λ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Lambda $$\\end{document} hyperons in heavy ion collisions at center-of-mass energies sNN=200\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sqrt{s_\ extrm{NN}}= 200$$\\end{document} GeV and sNN=5.02\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sqrt{s_\ extrm{NN}}= 5.02$$\\end{document} TeV carried out at RHIC and LHC colliders. We have calculated the mean spin vector at local thermodynamic equilibrium, including all known first-order terms in the gradients of the thermo-hydrodynamic fields, assuming that the hadronization hypersurface has a uniform temperature. We have also included the feed-down contributions to the polarization of Λ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Lambda $$\\end{document} stemming from the decays of polarized Σ∗\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Sigma ^*$$\\end{document} and Σ0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Sigma ^0$$\\end{document} hyperons. The obtained results are in good agreement with the data. In general, the component of the spin vector along the global angular momentum, orthogonal to the reaction plane, shows strong sensitivity to the initial longitudinal flow velocity. Furthermore, the longitudinal component of the spin vector turns out to be very sensitive to the bulk viscosity of the plasma at the highest LHC energy. Therefore, the azimuthal dependence of spin polarization can effectively constrain the initial hydrodynamic conditions and the transport coefficients of the quark gluon plasma.

Asymmetric jet shapes with two-dimensional jet tomography

Two-dimensional (2D) jet tomography is a promising tool to study jet medium modification in high-energy heavy-ion collisions. It combines gradient (transverse) and longitudinal jet tomography for selection of events with localized initial jet production positions. It exploits the transverse asymmetry and energy loss that depend, respectively, on the transverse gradient and jet path length inside the quark-gluon plasma (QGP). In this study, we employ 2D jet tomography to study medium modification of the jet shape of γ-triggered jets within the linear Boltzmann transport (LBT) model for jet propagation in heavy-ion collisions. Our results show that jets with small transverse asymmetry (ANn⃗) or small γ-jet asymmetry (xJγ=pTjet/pTγ) exhibit a broader jet shape than those with larger ANn⃗ or xJγ, since the former are produced at the center and go through longer path lengths while the later are off center and close to the surface of the QGP fireball. In events with finite values of ANn⃗, jet shapes are asymmetric with respect to the event plane. Hard partons at the core of the jet are deflected away from the denser region while soft partons from the medium response at large angles flow toward the denser part of QGP. Future experimental measurements of these asymmetric features of the jet shape can be used to study the transport properties of jets and medium responses. Published by the American Physical Society 2024

Centrality dependence of multistrange baryon production in high-energy heavy-ion collisions

We consider the experimental data on the production of strange \Lambda^{\prime}Λ′s, and multistrange baryons (\XiΞ, \OmegaΩ), and antibaryons, on nuclear targets, at the energy region from SPS up to LHC, in the framework of the Quark-Gluon String Model. One remarkable result of this analysis is the significant dependence on the centrality of the collision of the experimental \overline{\Xi}^+/\overline{\Lambda}Ξ¯+/Λ¯ and \overline{\Omega}^+/\overline{\Lambda}Ω¯+/Λ¯ ratios in heavy-ion collisions, at SPS energies.

Strangelets formation in high energy heavy-ion collisions

Strangelets formation in high energy heavy-ion collisions

Isospin Correlations in Isotope Yields at Intermediate and High Energy Heavy-Ion Collisions

Isospin Correlations in Isotope Yields at Intermediate and High Energy Heavy-Ion Collisions

Pre-equilibrium photons from the early stages of heavy-ion collisions

We use QCD kinetic theory to compute photon production in the chemically equilibrating Quark-Gluon Plasma created in the early stages of high-energy heavy-ion collisions. We do a detailed comparison of pre-equilibrium photon rates to the thermal photon production. We show that the photon spectrum radiated from a hydrodynamic attractor evolution satisfies a simple scaling form in terms of the specific shear viscosity η/s and entropy density dS/dζ ∼ (Tτ1/3)3/2∞. We confirm the analytical predictions with numerical kinetic theory simulations. We use the extracted scaling function to compute the pre-equilibrium photon contribution in sNN\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\sqrt{s_{NN}} $$\\end{document} = 2.76 TeV 0–20% PbPb collisions. We demonstrate that our matching procedure allows for a smooth switching from pre-equilibrium kinetic to thermal hydrodynamic photon production. Finally, our publicly available implementation can be straightforwardly added to existing heavy ion models.

Pion Interferometry with Lévy-Stable Sources in sNN = 200 GeV Au + Au Collisions at STAR

Measurements of femtoscopic correlations in high-energy heavy-ion collisions are used to unravel the space–time structure of the particle-emitting source (the quark–gluon plasma). Recent results indicate that the pion pair source exhibits a power law behavior and can be described well by a Lévy distribution. In this study, Lévy fits were applied to the measured one-dimensional two-pion correlation functions in Au + Au collisions at sNN = 200 GeV. The three extracted source parameters are the Lévy scale parameter, R, which relates to the size of the source; the correlation strength parameter, λ; and the Lévy exponent, α, which characterizes the power law tail of the source. In this paper, we report the current status of the analysis of the extracted Lévy source parameters and present their dependence on average transverse mass, mT, and on centrality.

Evaluation of a baseline for the study of azimuthal correlations of charmed mesons in heavy-ion collisions using PYTHIA 8 and Herwig++ models

Measurements of azimuthal correlations of charmed mesons in high-energy heavy-ion collisions can shed light on transport properties of the Quark-Gluon Plasma. The STAR experiment collected in 2014 and 2016 a large sample of Au+Au reactions at sNN\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sqrt{s_{NN}}$$\\end{document} = 200 GeV, making such a study possible. However, such a measurement in p+p collisions at the same energy is not feasible so far. To provide such a baseline, we report a model study of the azimuthal correlations between charmed mesons in p+p collisions at s\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sqrt{s}$$\\end{document} = 200 GeV, 500 GeV and 1.96 TeV. We used two Monte Carlo generators, PYTHIA 8 and Herwig++. We compare their predictions and validate them against available data from the STAR and CDF experiments to provide a reliable p+p baseline for the correlation studies in heavy-ion collisions at experiments at RHIC. We also discuss prospects for such measurements.

Unveiling the dynamics of little-bang nucleosynthesis

High-energy nuclear collisions provide a unique site for the synthesis of both nuclei and antinuclei at temperatures of kT ≈ 100 − 150 MeV. In these little bangs of transient collisions, a quark-gluon plasma (QGP) of nearly vanishing viscosity is created, which is believed to have existed in the early universe within the first few microseconds after the Big Bang. Analyses of identified particles produced in these little bangs based on the statistical hadronization model for the QGP have suggested that light (anti)nuclei are produced from the QGP as other hadrons and their abundances are little affected by later hadronic dynamics. Here, we find a strong reduction of the triton yield by about a factor of 1.8 in high-energy heavy-ion collisions based on a kinetic approach that includes the effects of hadronic re-scatterings, particularly that due to pion-catalyzed multi-body reactions. This finding is supported by the latest experimental measurements and thus unveils the important role of hadronic dynamics in the little-bang nucleosynthesis.

Probing the Short-Distance Structure of the Quark-Gluon Plasma with Energy Correlators.

Energy-energy correlators (EECs) are promising observables to study the dynamics of jet evolution in the quark-gluon plasma (QGP) through its imprint on angular scales in the energy flux of final-state particles. We carry out the first complete calculation of EECs using realistic simulations of high-energy heavy-ion collisions and dissect the different dynamics underlying the final distribution through analyses of jet propagation in a uniform medium. The EECs of γ-jets in heavy-ion collisions are found to be enhanced by the medium response from elastic scatterings instead of induced gluon radiation at large angles. In the meantime, EECs are suppressed at small angles due to energy loss and transverse momentum broadening of jet shower partons. These modifications are further shown to be sensitive to the angular scale of the in-medium interaction, as characterized by the Debye screening mass. Experimental verification and measurement of such modifications will shed light on this scale and the short-distance structure of the QGP in heavy-ion collisions.

Deep learning for flow observables in high energy heavyion collisions

We demonstrate how deep convolutional neural networks can be trained to predict 2+1 D hydrodynamic simulation results for flow coefficients, mean-pT and charged particle multiplicity from the initial energy density profile. We show that this method provides results that are accurate enough, so that one can use neural networks to reliably estimate multi-particle flow correlators. Additionally, we train networks that can take any model parameter as an additional input and demonstrate with a few examples that the accuracy remains good. The usage of neural networks can reduce the computation time needed in performing Bayesian analyses with multi-particle flow correlators by many orders of magnitude.

Resolving medium properties using high-pT jets with jet and in-jet correlations in PbPb collisions at 5.02 TeV with the CMS detector

Even though the quark-gluon plasma produced in high-energy heavy ion collisions has been studied in detail for years, there are still properties that are not known to a good precision. One example of such property is the medium resolution length, which is related to the smallest angle of emission where the medium can still resolve the daughter particles as individual particles. This can be probed for example through the energy-energy correlator. The medium resolution length has implications for parton energy loss, since two particles will emit energy more vigorously compared to one. Also other details of the parton energy loss, like the path-length dependency, could use precision measurements to provide better discriminating power between available theoretical models. Jets are a good observable to experimentally tackle these questions. In this talk, we present recent, high-precision CMS jet measurements in lead-lead collisions at √SNN = 5.02 TeV aimed to explore the properties of the quark-gluon plasma.

New theoretical developments on the early-time dynamics and approach to equilibrium in Heavy-Ion collisions

We discuss recent theoretical developments in understanding the early pre-equilibrium dynamics and onset of hydrodynamic behavior in high-energy heavy-ion collisions. We highlight possible experimental signatures of the pre-equilibrium phase, and present recent progress in developing a consistent theoretical description of collective flow in small systems.

Midrapidity average transverse momentum of identified charged particles in high-energy heavy-ion collisions

The average transverse momentum <inline-formula><tex-math id="M8">\begin{document}$\left\langle p_{\mathrm{T}} \right\rangle$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20240905_M8.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20240905_M8.png"/></alternatives></inline-formula> of final particles is an important observable in high-energy heavy-ion collision experiments. It reflects the properties of soft hadrons and thermonuclear matter, and it can also be used to deduce the information about the evolution of collision systems. By using the phenomenological linear and power-law functions, we study the dependence of the average transverse momentum <inline-formula><tex-math id="M9">\begin{document}$\langle p_{\mathrm{T}}\rangle$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20240905_M9.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20240905_M9.png"/></alternatives></inline-formula> at midrapidity in Au + Au and Pb + Pb collisions from the STAR, PHENIX and ALICE Collaborations on four normalized physical quantities, i.e. the collision centrality, the average number of binary collisions per participant pair <inline-formula><tex-math id="M10">\begin{document}$\dfrac{2N_{{\mathrm{coll}}}}{N_{{\mathrm{part}}}}$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20240905_M10.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20240905_M10.png"/></alternatives></inline-formula>, the average pseudorapidity density of charged particles per participant pair <inline-formula><tex-math id="M11">\begin{document}$\dfrac{2}{N_{{\mathrm{part}}}}\dfrac{{\mathrm{d}}N_{{\mathrm{ch}}}}{{\mathrm{d}}\eta}$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20240905_M11.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20240905_M11.png"/></alternatives></inline-formula> and the average pseudorapidity density of charged particles per binary collision <inline-formula><tex-math id="M12">\begin{document}$\dfrac{1}{N_{{\mathrm{coll}}}}\dfrac{{\mathrm{d}}N_{{\mathrm{ch}}}}{{\mathrm{d}}\eta} $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20240905_M12.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20240905_M12.png"/></alternatives></inline-formula>. The results show that the average transverse momentum <inline-formula><tex-math id="M13">\begin{document}$\langle p_{\mathrm{T}} \rangle$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20240905_M13.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20240905_M13.png"/></alternatives></inline-formula> of identified particles exhibits a good linear relationship with collision centrality, and it follows a nice power-law relationship with the average number of binary collisions per participant pair <inline-formula><tex-math id="M14">\begin{document}$\dfrac{2N_{{\mathrm{coll}}}}{N_{{\mathrm{part}}}}$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20240905_M14.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20240905_M14.png"/></alternatives></inline-formula>, the average pseudorapidity density of charged particles per participant pair <inline-formula><tex-math id="M15">\begin{document}$\dfrac{2}{N_{{\mathrm{part}}}}\dfrac{{\mathrm{d}}N_{{\mathrm{ch}}}}{{\mathrm{d}}\eta}$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20240905_M15.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20240905_M15.png"/></alternatives></inline-formula>, and the average pseudorapidity density of charged particles per binary collision <inline-formula><tex-math id="M16">\begin{document}$\dfrac{1}{N_{{\mathrm{coll}}}}\dfrac{{\mathrm{d}}N_{{\mathrm{ch}}}}{{\mathrm{d}}\eta}$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20240905_M16.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20240905_M16.png"/></alternatives></inline-formula>. It is also found that the fitting parameters in the proposed phenomenological functions for the average transverse momentum <inline-formula><tex-math id="M17">\begin{document}$\langle p_{\mathrm{T}} \rangle$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20240905_M17.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20240905_M17.png"/></alternatives></inline-formula> with collision centrality and the average number of binary collisions per participant pair follow a power-law function with collision energy, which endows the phenomenological approach with predictive ability. Therefore, the collision centrality and the average number of binary collisions per participant pair are good physical quantities for studying the average transverse momentum of identified particles in high-energy heavy-ion collisions. The results in this study can be used to predict the average transverse momentum of identified particles at other collision energy of which the experimental data are not available so far. The mass ordering of the average transverse momentum of identified particles, i.e. <inline-formula><tex-math id="M18">\begin{document}$\text{π}^{-},\;{\mathrm{K}}^{-} $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20240905_M18.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20240905_M18.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M20">\begin{document}$\bar{{\mathrm{p}}}$\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20240905_M20.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20240905_M20.png"/></alternatives></inline-formula>, is also discussed and explained by the particle production time related to energy conservation, at a given collision centrality and energy.

Jet-flow coupling in heavy-ion collisions and the jetinduced diffusion wake

The diffusion wake accompanying the jet-induced Mach cone serves as a distinctive tool for investigating the characteristics of quark-gluon plasma(QGP) in high-energy heavy-ion collisions. This phenomenon results in a reduction of soft hadrons opposite to the direction of the propagating jet. Our study explores the 3D structure of the diffusion wake induced by γ-triggered jets in Pb+Pb collisions at the LHC energy, utilizing the coupled linear Boltzmann transport and hydro model. We identify a valley structure caused by the diffusion wake, superimposed on the initial multiple parton interaction (MPI) ridge in both rapidity and azimuthal angle. This leads to a double-peak pattern in the rapidity distribution of soft hadrons opposite to the jets. In addition, when jet goes through the QGP medium, it will be affected by the flow velocity. So we take a new method to detect the effect of jet-flow coupling in heavy-ion collisions.

Measurement of in-medium modification of energy-space structure of jets via γ and π0 triggered hadrons in Au+Au collisions at RHIC

Since the discovery of the jet quenching at RHIC, the in-medium interaction of hard scattered partons with the nuclear medium created by highenergy heavy-ion collisions has been an excellent tool to understand not only the transport properties of the medium but also its time evolution towards hadronization. The multi-differential measurement of the high momentum twoparticle correlations can probe a particular space-time window as a function of energy transfer. Comparing the correlations with the prompt photon-triggered hadron spectra, one can extract the property of the medium from various aspects and contribute to distinct models. The PHENIX experiment at RHIC has collected its highest statistics of the γ and π0 triggered hadron events in Au+Au collisions at √SNN = 200 GeV in the RHIC Year-2014 run, and measured not only the inclusive spectra of the triggered hadrons but also the angle and energy dependent IAA and DAA. We will discuss the in-medium modification of the energy-space structure of the jets at the RHIC energies with the results obtained.

Charge-dependent anisotropic flow in relativistic resistive magneto-hydrodynamic expansion

We construct a dynamical model for high-energy heavy-ion collisions based on the relativistic resistive magneto-hydrodynamic framework. Using the code, we investigate the charge-dependent anisotropic flow in √SNN = 200 GeV Au+Au collisions and Cu+Au collisions at RHIC. We conclude that the charge-dependent anisotropic flow is a good probe to extract the electrical conductivity of QGP medium.

Pre-equilibrium Photon and Dilepton Production

We use QCD kinetic theory to compute photon and dilepton production in the chemically equilibrating out-of-equilibrium quark-gluon plasma created in the early stages of high-energy heavy-ion collisions. We derive universal scaling functions for the pre-equilibrium spectra of photons and dileptons. These scaling functions can be used to make realistic predictions for the pre-equilibrium emission and consequently establish the significance of the preequilibrium phase for the production of electromagnetic probes in heavy-ion collisions.

Influence of globally spin-aligned vector mesons to the measurements of the chiral magnetic effect in heavy-ion collisions

The chiral magnetic effect (CME) in high-energy heavy-ion collisions arises from the interplay between the chirality imbalance and the intense magnetic field and will cause a charge separation along the magnetic field direction. While the CME search is still ongoing in experiments, the non-CME contributions need to be excluded from the CME observables. In this work, we examine the influence of globally spin-aligned ρ mesons on the γ112 correlator, the RΨ2(ΔS) correlator, and the signed balance functions, via a toy model and a multiphase transport model (AMPT). We find that the CME observables are sensitive to the 00-component of the spin density matrix, ρ00: they receive positive (negative) contributions when ρ00 is larger (smaller) than 1/3.