Ultrarelativistic Nucleus-nucleus Collisions Research Articles

Probing collectivity in heavy-ion collisions with fluctuations of the pT spectrum

Event-by-event fluctuations in the initial stages of ultrarelativistic nucleus-nucleus collisions depend little on rapidity. The hydrodynamic expansion which occurs in later stages then gives rise to correlations among outgoing particles which depend weakly on their relative rapidity. Azimuthal correlations, through which anisotropic flow (vn(pT)) is defined, have been the most studied. Here we study a new observable introduced in 2020 by Schenke, Shen and Teaney and dubbed v0(pT), which quantifies the relative change in the pT spectrum induced by a fluctuation. We describe how it can be measured. Using hydrodynamic simulations, we make quantitative predictions for v0(pT) of charged and identified hadrons. We then discuss how v0(pT) relates to two phenomena which have been measured: The increase of the mean transverse momentum in ultracentral collisions, and the event-by-event fluctuations of the transverse momentum per particle [pT]. We show that v0(pT) determines the dependence of these quantities on the pT cuts implemented in the analysis. We quantitatively explain the rise of σpT observed by ATLAS as the upper pT-cut is increased from 2 to 5 GeV/c.

Search for baryon junctions in photonuclear processes and isobar collisions at RHIC

During the early development of quantum chromodynamics, it was proposed that baryon number could be carried by a non-perturbative Y-shaped topology of gluon fields, called the gluon junction, rather than by the valence quarks as in the QCD standard model. A puzzling feature of ultra-relativistic nucleus-nucleus collisions is the apparent substantial baryon excess in the mid-rapidity region that could not be adequately accounted for in most conventional models of quark and diquark transport. The transport of baryonic gluon junctions is predicted to lead to a characteristic exponential distribution of net-baryon density with rapidity and could resolve the puzzle. In this context we point out that the rapidity density of net-baryons near mid-rapidity indeed follows an exponential distribution with a slope of -0.61±0.03\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$-0.61\\pm 0.03$$\\end{document} as a function of beam rapidity in the existing global data from A+A collisions at AGS, SPS and RHIC energies. To further test if quarks or gluon junctions carry the baryon quantum number, we propose to study the absolute magnitude of the baryon vs. charge stopping in isobar collisions at RHIC. We also argue that semi-inclusive photon-induced processes (γ+p\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\gamma +p$$\\end{document}/A) at RHIC kinematics provide an opportunity to search for the signatures of the baryon junction and to shed light onto the mechanisms of observed baryon excess in the mid-rapidity region in ultra-relativistic nucleus-nucleus collisions. Such measurements can be further validated in A+A collisions at the LHC and e+p\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$e+p$$\\end{document}/A collisions at the EIC.

Jet modifications - Experimental overview

This proceeding summarizes a selection of experimental results on jet modifications in the quark-gluon plasma from the XXXth International Conference on Ultra-relativistic Nucleus-Nucleus Collisions, Quark Matter 2023.

Transverse mass scaling of dilepton radiation off a quark-gluon plasma

The spectrum of dileptons produced by the quark-gluon plasma in an ultrarelativistic nucleus-nucleus collision depends only, to a good approximation, on the transverse mass Mt of the dilepton. This scaling is exact as long as transverse flow is negligible, and the system is in local thermal equilibrium. We implement a state-of-the-art modelization of kinetic and chemical equilibration in the early stages of the evolution to study the modifications of the spectrum. Violations of Mt scaling resulting from these effects are evaluated as a function of the shear viscosity to entropy ratio (η/s) that controls the equilibration time. We determine the dependence of the spectrum on system size, centrality, rapidity, and collision energy. We show that the quark-gluon plasma produces more dileptons than the Drell-Yan process up to invariant masses of order M∼4 GeV at LHC energies. Due to different kinematics, for a given Mt, the dependence of the dilepton yield on M is opposite for the two processes, so that experiment alone can in principle determine which process dominates.

Bremsstrahlung photons from stopping in heavy-ion collisions

We examine the spectrum of bremsstrahlung photons that results from the stopping of the initial net charge distributions in ultrarelativistic nucleus-nucleus collisions at the CERN Large Hadron Collier (LHC). This effect has escaped detection so far since it becomes sizable only at very low transverse momentum and at sufficiently forward rapidity. We argue that it may be within reach of the next-generation LHC heavy-ion detector ALICE-3 that is currently under study, and we comment on the physics motivation for measuring it.

Intermediate mass dileptons as pre-equilibrium probes in heavy ion collisions

The production of dileptons with an invariant mass in the range 1GeV<M<5 GeV provides unique insight into the approach to thermal equilibrium in ultrarelativistic nucleus-nucleus collisions. In this mass range, they are produced through the annihilation of quark-antiquark pairs in the early stages of the collision. They are sensitive to the anisotropy of the quark momentum distribution, and also to the quark abundance, which is expected to be underpopulated relative to thermal equilibrium. We take into account both effects based on recent theoretical developments in QCD kinetic theory. We argue that the dilepton mass spectrum provides a measure of the shear viscosity to entropy ratio that controls the equilibration time. We evaluate the background from the Drell-Yan process and argue that future detector developments can suppress the additional background from semileptonic decays of heavy flavors.

Fluid velocity from transverse momentum spectra

We parametrize the transverse momentum distribution of outgoing hadrons in ultrarelativistic nucleus-nucleus collisions as a superposition of boosted thermal distributions. In this approach, which generalizes the conventional blast wave, the momentum distribution is determined by the distribution of the fluid velocity. We analyze the difference between this generalized blast-wave parametrization and a full hydrodynamic calculation. We then apply the generalized blast-wave fit to experimental data on Pb+Pb collisions at $\sqrt{s_{\rm NN}}=2.76\ \mathrm{TeV}$. The fit is reasonable up to $p_t\sim 6\ \mathrm{GeV}$, much beyond the range where hydrodynamics is usually applied, but not perfect. Based on the differences between the fit and the data, we argue that an ideal hydrodynamic calculation cannot fit simultaneously all identified particle spectra, irrespective of the specific implementation. In particular, data display a significant excess of pions at low $p_t$, whose physical interpretation is discussed. Data also show that the distribution of the fluid velocity becomes broader as the collision becomes less central. This broadening is explained by event-by-event hydrodynamic calculations, where it results from the centrality dependence of initial-state fluctuations.

Highlights from the CMS experiment

We review the recent CMS results reported at the XXVIIIth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions.

Pion femtoscopy in p+Au and Au+Au collisions at using transport approach

Correlation femtoscopy provides information about the space-time structure and evolution of the fireball created in ultra-relativistic nucleus-nucleus collisions. The dependence of the femtoscopic radii on transverse pair momentum and charged particle multiplicity of an event reflects the mechanism of collective behavior. In this work, the femtoscopic radii of identical charged pions were calculated in the UrQMD model for Au+Au and p+Au collisions at and compared to each other at the same multiplicities. The physics implications of this comparison will be discussed.

Kurtosis of elliptic flow fluctuations

Elliptic flow ($v_2$) in ultrarelativistic nucleus-nucleus collisions fluctuates event to event, both in magnitude and in orientation with respect to the reaction plane. Even though the reaction plane is not known event to event in experiment, we show that the statistical properties of $v_2$ fluctuations in the reaction plane can be precisely extracted from experimental data. Previous studies have shown how to measure the mean, variance and skewness using the first three cumulants $v_2\{2\}$, $v_2\{4\}$ and $v_2\{6\}$. We complement these studies by providing a formula for the kurtosis, which requires an accurate determination of the next cumulant $v_2\{8\}$. Using existing data, we show that the kurtosis is positive for most centralities, in contrast with the kurtosis of triangular flow fluctuations, which is negative. We argue that these features are robust predictions of fluid-dynamical models.

Initial conditions for nuclear collisions: theory overview

We overview the current status and recent developments on initial conditions in ultra-relativistic nucleus-nucleus collisions. Specifically, we look at the progress in understanding the role of sub-nucleonic fluctuations in large and small collision systems. Next, we review the current ideas of going beyond boost invariant initial conditions and introducing physically motivated rapidity fluctuations at RHIC and LHC energies. Finally, we discuss the time evolution of initial stages and the matching between different descriptions of the QGP.

Resolving discrepancies in the estimation of heavy quark transport coefficients in relativistic heavy-ion collisions

Heavy flavor observables provide valuable information on the properties of the hot and dense Quark-Gluon Plasma (QGP) created in ultra-relativistic nucleus-nucleus collisions. Various microscopic models have successfully described many of the observables associated with its formation. Their transport coefficients differ, however, due to different assumptions about the underlying interaction of the heavy quarks with the plasma constituents, different initial geometries and formation times, different hadronization processes and a different time evolution of the QGP. In this study we present the transport coefficients of all these models and investigate systematically how some of these assumptions influence the heavy quark properties at the end of the QGP expansion. For this purpose we impose on these models the same initial condition and the same model for the QGP expansion and show that both have considerable influence on $R_{AA}$ and $v_2$.

Photon and dilepton observables: An experimental overview

Dilepton and direct photon measurements in heavy ion collisions provide access to the early stages of the collision. Dilepton continuum measurements at SPS and RHIC show an excess consistent with broadening of the rho meson spectral function as a result of hadronic interactions. Direct photon measurements at RHIC and the LHC show an excess of low pT photons over the expected yield. This excess has a large azimuthal anisotropy with respect to the collision plane. Understanding the direct photon excess and its anisotropy has become an outstanding question in the field of heavy ion physics. This proceeding consists of a summary of new dilepton and direct photon experimental results shown at the 2017 International Conference on Ultrarelativistic Nucleus-Nucleus Collisions and presents them in the context of our current understanding of these electromagentic probes.

Hot Quarks 2016: Workshop for young scientists on the physics of ultrarelativistic nucleus-nucleus collisions

The 7th edition of the Workshop for Young Scientists on the Physics of Ultra-relativistic Nucleus-Nucleus Collisions (Hot Quarks 2016) was held on South Padre Island, Texas, United States from September 12-17, 2016. Following the traditions of the conference, the meeting gathered almost 70 participants in the first years of their scientific careers. The present issue contains the proceedings of this workshop.As in the past, the Hot Quarks workshop offered a unique atmosphere for lively discussions and interpretation of the current measurements from high-energy nuclear collisions. Dedicated time at the end of each session for questions, including anonymous questions from the “box”, are crucial for this workshop. Recent results and upgrades at CERN's Large Hadron Collider (LHC) and Brookhaven's Relativistic Heavy Ion Collider (RHIC) were presented. Recent theoretical developments were also extensively discussed as well as the perspectives for future facilities such as the Facility for Antiproton and Ion Research (FAIR) at Darmstadt and the Electron-Ion Collider at Brookhaven. The conference's goal to provide a platform for young researchers to learn and foster their interactions was successfully met.We wish to thank the sponsors of the Hot Quarks 2016 Conference, who supported the authors of this volume: European Laboratory for Particle Physics CERN (Switzerland), Cyclotron Institute at Texas A&M University (USA), ExtreMe Matter Institute EMMI (Germany), Helmholtz Association and GSI under grant VH-NG-822 (Germany), Helmholtz International Center for FAIR (Germany), National Science Foundation (USA), Netherlands Organization for Scientific Research (Netherlands), Nuclear Physics Institute of the CAS (Czech Republic), the Ministry of Education, Youth and Sport (Czech Republic) and 3 sponsors who wish to remain anonymous.Javier López Albacete, Universidad de Granada (Spain)Jana Bielcikova, Nuclear Physics Inst. of the Czech Academy of Sciences (Czech Republic)Rainer J. Fries, Texas A&M University (USA)Jiangyong Jia, Stony Brook University and Brookhaven National Laboratory (USA)André Mischke, Utrecht University and Nikhef Amsterdam (The Netherlands)Hannah Petersen, Goethe University, FIAS and GSI (Germany)Lijuan Ruan, Brookhaven National Laboratory (USA)Sevil Salur, Rutgers University, (USA)

Heavy-quark dynamics in heavy-ion collisions

The dynamics of partons and hadrons in ultra-relativistic nucleus-nucleus collisions are analyzed within the Parton-Hadron-String Dynamics (PHSD) transport approach, which is based on a dynamical quasiparticle model (DQPM) for the partonic phase including a dynamical hadronization scheme while reproducing lattice QCD results in thermodynamic equilibrium for the equation-of-state as well as transport coefficients like shear and bulk viscosities, the electric conductivity or the charm diffusion coefficient of the hot QCD medium. In this contribution, we report on recent results on the charm dynamics and elliptic flow in Au+Au collisions at RHIC and Pb+Pb collisions at the LHC.

Direct photon production and jet energy-loss in small systems: The XXVth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions

Two types of penetrating probes, direct photon and QCD jets, are investigated in the background of a small and rapidly expanding droplet of quark-gluon plasma. The additional thermal electromagnetic radiation results in a ∼50% enhancement of the direct photons. In high multiplicity p+Pb collisions, jets can lose a sizeable fraction of their initial energy, leading to a charged hadron RpA of ∼0.8 at a transverse momentum around 10GeV. Those two proposed measurements can help understand the apparent collective behaviour observed in small collision systems.

Measurements of Quarkonium Polarization and Production versus Charged-Particle Multiplicity in p + p Collisions at [formula omitted] in the STAR Experiment: The XXVth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions

We report on the J/ψ production and polarization as a function of pT and xT, measured via the dielectron decay channel at mid-rapidity in p+p collisions at s=500 GeV in the STAR experiment. The measured J/ψpT spectrum is in agreement with NRQCD and CGC+NRQCD predictions, while the J/ψ polarization shows a trend towards longitudinal polarization with increasing pT. In addition, first measurements of the J/ψ production as a function of charged-particle multiplicity are presented at s=500 GeV. A strong correlation between the J/ψ yield and charged-particle multiplicity is observed with a hint of J/ψpT dependence.

In-medium jet evolution: interplay between broadening and decoherence effects: The XXVth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions

The description of the modifications of the coherence pattern in a parton shower, in the presence of a QGP, has been actively addressed in recent studies. Among the several achievements, finite energy corrections, transverse momentum broadening due to medium interactions and interference effects between successive emissions have been extensively improved as they seem to be essential features for a correct description of the results obtained in heavy-ion collisions. In this work, based on the insights of our previous work [L. Apolinário, N. Armesto, J. G. Milhano, C. A. Salgado, Medium-induced gluon radiation and colour decoherence beyond the soft approximation, JHEP 1502 (2015) 119. arXiv:1407.0599], we explore the physical interplay between broadening and decoherence, by generalising previous studies of medium-modifications of the antenna spectrum [Y. Mehtar-Tani, C. A. Salgado, K. Tywoniuk, Antiangular Ordering of Gluon Radiation in QCD Media, Phys. Rev. Lett. 106 (2011) 122002. arXiv:1009.2965, J. Casalderrey-Solana, E. Iancu, Interference effects in medium-induced gluon radiation, JHEP 08 (2011) 015. arXiv:1105.1760, Y. Mehtar-Tani, C. A. Salgado, K. Tywoniuk, The Radiation pattern of a QCD antenna in a dense medium, JHEP 10 (2012) 197. arXiv:1205.5739] – so far restricted to the case where transverse motion is neglected. The result allow us to identify two quantities controlling the decoherence of a medium modified shower that can be used as building blocks for a successful future generation of jet quenching Monte Carlo simulators: a generalisation of the Δmed parameter of the works of [Y. Mehtar-Tani, C. A. Salgado, K. Tywoniuk, Antiangular Ordering of Gluon Radiation in QCD Media, Phys. Rev. Lett. 106 (2011) 122002. arXiv:1009.2965, Y. Mehtar-Tani, C. A. Salgado, K. Tywoniuk, The Radiation pattern of a QCD antenna in a dense medium, JHEP 10 (2012) 197. arXiv:1205.5739] – that controls the interplay between the transverse scale of the hard probe and the transverse resolution of the medium – and of the Δcoh in [L. Apolinário, N. Armesto, J. G. Milhano, C. A. Salgado, Medium-induced gluon radiation and colour decoherence beyond the soft approximation, JHEP 1502 (2015) 119. arXiv:1407.0599] – that dictates the interferences between two emitters as a function of the transverse momentum broadening acquired by multiple scatterings with the medium.

Electromagnetic probes of a pure-glue initial state in nucleus-nucleus collisions at energies available at the CERN Large Hadron Collider

Partonic matter produced in the early stage of ultrarelativistic nucleus-nucleus collisions is assumed to be composed mainly of gluons, and quarks and antiquarks are produced at later times. To study the implications of such a scenario, the dynamical evolution of a chemically nonequilibrated system is described by the ideal (2+1)-dimensional hydrodynamics with a time dependent (anti)quark fugacity. The equation of state interpolates linearly between the lattice data for the pure gluonic matter and the lattice data for the chemically equilibrated quark-gluon plasma. The spectra and elliptic flows of thermal dileptons and photons are calculated for central Pb+Pb collisions at the CERN Large Hadron Collider energy of $\sqrt{s_{_{\rm NN}}} = 2.76$ TeV. We test the sensitivity of the results to the choice of equilibration times, including also the case where the complete chemical equilibrium of partons is reached already at the initial stage. It is shown that a suppression of quarks at early times leads to a significant reduction of the yield of the thermal dileptons, but only to a rather modest suppression of the $p_T$-distribution of direct photons. It is demonstrated that an enhancement of photon and dilepton elliptic flows might serve as a promising signature of the pure-glue initial state.

Tomography of the QGP by heavy quarks

The dynamics of partons and hadrons in ultra-relativistic nucleus-nucleus collisions is analyzed within the Parton-Hadron-String Dynamics (PHSD) transport approach, which is based on a dynamical quasiparticle model for the partonic phase (DQPM) including a dynamical hadronization scheme while reproducing lattice QCD results in thermodynamic equilibrium for the equation-of-state as well as transport coefficients like shear and bulk viscosities, the electric conductivity or the charm diffusion coefficient of the hot QCD medium. In this contribution we report on the recent results on the charm dynamics and elliptic flow in Au+Au collisions at RHIC and Pb+Pb reactions at the LHC as well as on the single electron spectra from D— and B—meson semileptonic decays in Au+Au collisions at √sNN =200 and 62.4 GeV. We find that the PHSD approach well describes the Raa and elliptic flow v2 of open charm mesons in Au+Au collisions at √sNN = 200 GeV (from STAR) and 2.76 TeV (from ALICE) as well as the elliptic flow of single electrons at √sNN = 200 and 62.4 GeV (from PHENIX), however, the large Raa at √sNN = 62.4 GeV is not reproduced at all which might indicate a new ’PHENIX puzzle’.