Ultrarelativistic Heavy-ion Collisions Research Articles

Heavy flavor jets in ALICE

Heavy-flavor quarks are considered to be effective probes of the Quark–Gluon Plasma (QGP) produced in ultra-relativistic heavy-ion collisions. Since quarks have a large mass, their production takes place mostly in initial hard processes, and it is calculable using perturbative QCD. Thus, heavy flavor quarks can be considered as ideal early-generated penetrating probes of the created medium and utilized to investigate mass-dependent properties of in-medium parton energy loss. Moreover, the measurement of heavy-flavor jet production in pp, besides being a natural reference for Pb–Pb studies, allows testing pQCD calculations and models of charm fragmentation in vacuum. In addition, similar measurements in p–Pb collisions allow assessing the importance of cold nuclear matter effects. The ALICE experiment at the LHC has excellent particle tracking capabilities, that allow for a precise jet reconstruction and for the identification of [Formula: see text]-meson and beauty hadron decay vertices, displaced hundreds of micrometers from the primary interaction vertex. In this proceedings, we will report the latest heavy-flavor jet measurements performed in p–Pb and pp collisions.

Masses and decay widths of charmonium states in the presence of strong magnetic fields

The masses and decay widths of charmonium states are studied in the presence of strong magnetic fields. The mixing between the pseudoscalar and vector charmonium states at rest is observed to lead to appreciable negative (positive) shifts in the masses of the pseudoscalar (longitudinal component of the vector) charmonium states in vacuum/hadronic medium in the presence of high magnetic fields. The pseudoscalar and vector charmonium masses in the hadronic medium, calculated in an effective chiral model from the medium changes of a scalar dilaton field, have additional significant modifications due to the mixing effects. The masses of the $D$ and $\bar D$ mesons in the magnetized hadronic matter are calculated within the chiral effective model. The partial decay widths of the vector charmonium state to $D\bar D$ are computed using a field theoretical model for composite hadrons with quark/antiquark constituents, and are compared to the decay widths calculated using an effective hadronic Lagrangian. The effects of the mixing are observed to lead to significant contributions to the masses of the pseusoscalar and vector charmonium states, and an appreciable increase in the decay width $\psi(3770) \rightarrow D\bar D$ at large values of the magnetic fields. These studies of the charmonium states in strong magnetic fields should have observable consequences on the dilepton spectra, as well as on the production of the open charm mesons and the charmonium states in ultra relativistic heavy ion collision experiments.

Hydrodynamic Attractors in Phase Space.

Hydrodynamic attractors have recently gained prominence in the context of early stages of ultrarelativistic heavy-ion collisions at the RHIC and LHC. We critically examine the existing ideas on this subject from a phase space point of view. In this picture the hydrodynamic attractor can be seen as a special case of the more general phenomenon of dynamical dimensionality reduction of phase space regions. We quantify this using principal component analysis. Furthermore, we adapt the well known slow-roll approximation to this setting. These techniques generalize easily to higher dimensional phase spaces, which we illustrate by a preliminary analysis of a dataset describing the evolution of a five-dimensional manifold of initial conditions immersed in a 16-dimensional representation of the phase space of the Boltzmann kinetic equation in the relaxation time approximation.

Effects of local event-by-event conservation laws in ultrarelativistic heavy-ion collisions at particlization

Many simulations of relativistic heavy ion collisions involve the switching from relativistic hydrodynamics to kinetic particle transport. This switching entails the sampling of particles from the distribution of energy, momentum and conserved currents provided by hydrodynamics. Usually this sampling ensures the conservation of these quantities only on the average, i.e. the conserved quantities may actually fluctuate among the sampled particle configurations and only their averages over many such configurations agree with their values from hydrodynamics. Here we apply a recently invented method [Oliinychenko, Koch; PRL 123, 18, 182302 (2019)] to ensure conservation laws for each sampled configuration in spatially compact regions (patches) and study their effects: from the well-known (micro-)canonical suppression of means and variances to little studied (micro-)canonical correlations and higher order fluctuations. Most of these effects are sensitive to the patch size. Many of them do not disappear even in the thermodynamic limit, when the patch size goes to infinity. The developed method is essential for particlization of stochastic hydrodynamics. It is useful for studying the chiral magnetic effect, small systems, and in general for fluctuation and correlation observables.

Measurement of D0 -meson + hadron two-dimensional angular correlations in Au+Au collisions at sNN=200 GeV

Open heavy flavor hadrons provide unique probes of the medium produced in ultra-relativistic heavy-ion collisions. Due to their increased mass relative to light-flavor hadrons, long lifetime, and early production in hard-scattering interactions, they provide access to the full evolution of the partonic medium formed in heavy-ion collisions. This paper reports two-dimensional (2D) angular correlations between neutral $D$-mesons and unidentified charged particles produced in minimum-bias Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 200 GeV. $D^0$ and $\bar{D}^0$ mesons are reconstructed via their weak decay to $K^{\mp} \pi^{\pm}$ using the Heavy Flavor Tracker (HFT) in the Solenoidal Tracker at RHIC (STAR) experiment. Correlations on relative pseudorapidity and azimuth $(\Delta\eta,\Delta\phi)$ are presented for peripheral, mid-central and central collisions with $D^0$ transverse momentum from 2 to 10 GeV/$c$. Attention is focused on the 2D peaked correlation structure near the triggered $D^0$-meson, the {\em near-side} (NS) peak, which serves as a proxy for a charm-quark containing jet. The correlated NS yield of charged particles per $D^0$-meson and the 2D widths of the NS peak increase significantly from peripheral to central collisions. These results are compared with similar correlations using unidentified charged particles, consisting primarily of light-flavor hadrons, at similar trigger particle momenta. Similar per-trigger yields and widths of the NS correlation peak are observed. The present results provide additional evidence that $D^0$-mesons undergo significant interactions with the medium formed in heavy-ion collision and show, for the first time, significant centrality evolution of the NS 2D peak in the correlations of particles associated with a heavy-flavor hadron produced in these collisions.

Multiple-scattering production of lepton pairs in ultraperipheral heavy-ion collisions

Ultra-relativistic peripheral collisions of heavy ions at LHC can produce copies of numbers of lepton pairs via the two-photon process. Since the energy of the heavy ions is so high, multi-pair production cross-sections of the light leptons especially electrons are quite large so that it is possible to measure them experimentally. To calculate the multi-pair production probabilities, first, we should have an impact parameter dependence cross-section. We have obtained a well-behaved impact parameter dependence cross-section and by using the Monte-Carlo methods, we have calculated multi-pair production cross-sections of electrons and muons in Pb-Pb heavy-ion collisions at LHC energies. We have also used some experimental restrictions in our calculation to compare our findings with the experimental results.

Dilepton measurements with the ALICE experiment at the LHC

The production of low-mass dielectrons and dimuons is a powerful tool for the understanding of the properties of the quark–gluon plasma (QGP) created in ultra-relativistic heavy-ion collisions. Since such pairs do not interact strongly and are emitted during all stages of the collisions, they provide information about the full time evolution and dynamics of the medium created. Measurements in pp and p–Pb collisions are the necessary reference for heavyion studies. In addition, they can be used to extract charm and beauty cross sections.In this contribution, we present the latest measurements of e + e − and µ + µ − pair production in pp, p–Pb and Pb–Pb collisions performed by ALICE at different energies.

Summary of charm results from ALICE

An overview of the most recent charm measurements from the ALICE Collaboration is presented in this document. The study of heavy-flavour production is important to understand the properties of the hot and dense medium formed in ultra-relativistic heavyion collisions. In this document the journey of charm in the medium, from the production to the hadronisation into hadrons, is described. Both open charm and quarkonium measurements are presented, starting from D-meson and J/ψ production cross sections in pp collisions and their comparison with theoretical calculations. Recent measurements of relative abundances of different particle species are discussed, in particular the Ds/D0 and ratios, that are sensitive to the hadronisation mechanisms and to the strangeness and to the baryon-to-meson enhancement. The energy loss mechanisms and the collective motion in the medium are investigated through the D-meson and J/ψ nuclear modification factor and the elliptic flow measurements. Further investigations on possible medium-like effects observed in high-multiplicity pp and p-Pb collisions are also addressed.

Production of hadronic resonances measured with ALICE at the LHC

Measurements of short-lived hadronic resonances are used to probe the properties of the hadronic phase in ultra-relativistic heavy-ion collisions. Since these resonances have lifetimes comparable to that of the produced fireball, they are sensitive to the competitive rescattering and regeneration effects in the hadronic gas, which modify the observed particle momentum distributions and yields after hadronization. Having different masses, quantum numbers, and quark content, hadronic resonances can provide insight into processes that determine the shapes of particle momentum spectra, strangeness production, and the possible onset of collective effects in small systems.We here present the latest results on ρ(770)0, K*(892), φ(1020), Σ(1385)±, Λ(1520), Ξ(1530)0 and Ξ(1820) production in pp, p–Pb, Pb–Pb and Xe–Xe collisions at different LHC energies. Results include system-size and collision-energy evolution of transverse momentum spectra, integrated yields, mean transverse momenta, particle ratios, and nuclear modification factors.

Phase diagram of the Polyakov–Nambu–Jona-Lasinio approach for finite chemical potentials

We extend the SU(3) (Polyakov) Nambu Jona-Lasinio in two ways: We introduce the next to leading order contribution (in $N_c$) in the partition function. This contribution contains explicit mesonic terms. We introduce a coupling between the gluon field and the quark degrees of freedom which goes beyond a simple rescaling of the critical temperature. With both these improvements we can reproduce, for vanishing chemical potentials, the lattice results for the thermal properties of a strongly interacting system like pressure, energy density, entropy density, interaction measure and the speed of sound. Also the expansion parameter towards small but finite chemical potentials agrees with the lattice results. Extending the calculations to finite chemical potentials (what does not require any new parameter) we find a first order phase transition up to a critical end point of $T_{CEP}= 110\ MeV$ and $\mu_q = 320\ MeV$. For very large chemical potentials, we find agreement with pQCD calculations. We calculate the mass of mesons and baryons as a function of temperature and chemical potential and the transition between the hadronic and the chirally restored phase. These calculations provide an equation of state in the whole $T,\mu$ plane an essential ingredient for dynamical calculations of ultra-relativistic heavy ion collisions but also for the physics of neutron stars and neutron star collisions.

Jet-hadron correlations measured relative to the second order event plane in Pb-Pb collisions at sNN=2.76TeV

The Quark Gluon Plasma (QGP) produced in ultra relativistic heavy-ion collisions at the Large Hadron Collider (LHC) can be studied by measuring the modifications of jets formed by hard scattered partons which interact with the medium. We studied these modifications via angular correlations of jets with charged hadrons for jets with momenta 20 < $p_{\rm{T}}^{\rm{jet}}$ < 40 GeV/$c$ as a function of the associated particle momentum. The reaction plane fit (RPF) method is used in this analysis to remove the flow modulated background. The analysis of angular correlations for different orientations of the jet relative to the second order event plane allows for the study of the path length dependence of medium modifications to jets. We present the dependence of azimuthal angular correlations of charged hadrons with respect to the angle of the axis of a reconstructed jet relative to the event plane in Pb-Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 2.76 TeV. The dependence of particle yields associated with jets on the angle of the jet with respect to the event plane is presented. Correlations at different angles relative to the event plane are compared through ratios and differences of the yield. No dependence of the results on the angle of the jet with respect to the event plane is observed within uncertainties, which is consistent with no significant path length dependence of the medium modifications for this observable.

Charmed hadron production via equal-velocity quark combination in ultrarelativistic heavy ion collisions

Recent data on the production of $D$ mesons and $\Lambda_c^+$ baryons in heavy ion collisions at the Relativistic Heavy Ion Collider and the Large Hadron Collider exhibit a number of striking characteristics such as enhanced yield ratios $D_s^+/D^0$, $\Lambda_c^+/D^0$ and their transverse momentum dependences. In this paper, we derive the momentum dependence of open charm mesons and singly charmed baryons produced in ultra-relativistic heavy ion collisions via the equal-velocity quark combination. We present analytic expressions and numerical results of yield ratios and compare them with the experimental data available. We make predictions for other charmed hadrons.

New Solutions of Viscous Relativistic Hydrodynamics

Relativistic hydrodynamics represents a powerful tool to investigate the time evolution of the strongly interacting quark gluon plasma created in ultrarelativistic heavy ion collisions. The equations are solved often numerically, and numerous analytic solutions also exist. However, the inclusion of viscous effects in exact, analytic solutions has received less attention. Here we utilize Hubble flow to investigate the role of bulk viscosity, and present different classes of exact, analytic solutions valid also in the presence of dissipative effects.

Mechanisms of Forward–Backward Correlations in the Multiplicity of Particles in Ultrarelativistic Heavy-Ion Collisions

The contributions of various effects to correlations of the average multiplicity of particles emitted in the forward hemisphere of heavy-ion collisions with the multiplicity of particles emitted in the backward hemisphere (forward–backward correlations) have been comparatively analyzed. It has been shown that the experimentally observed dependence of the strength of such correlations on the centrality of lead–lead ion collisions at the Large Hadron Collider energy $$\sqrt {{{s}_{{NN}}}} $$ = 2.76 TeV can be reproduced by the HYDJET++ model if the hydrodynamic particle flow, decay of resonances, and production and suppression of high-energy quarks and gluons in hot matter are taken into account. The dominant mechanism of forward–backward correlations for the most central collisions involves the production and fragmentation of quark–gluon jets.

Three Particle Lévy HBT from PHENIX

Bose-Einstein correlations of identical bosons reveal information about the space-time structure of particle emission from the sQGP formed in ultra-relativistic heavy-ion collisions. Previous measurements of two particle correlations have shown that the source can best be described by a symmetric Levy distribution. Here we report on the measurement of three-particle correlations in 0-30 % centrality Au+Au collisions at $\sqrt{s_{_\text{NN}}} = 200$ GeV, and describe them with a Levy type source. This measurement may shed light on hadron creation mechanisms beyond chaotic emission. We measure three particle correlation strength ($\lambda_3$) as a function of pair transverse momentum. This parameter, combined with two-particle correlation strength $\lambda_2$ may reveal the level of chaoticity and coherence in particle production.

Quasinormal modes for heavy vector mesons in a finite density plasma

There are strong indications that ultra-relativistic heavy ion collisions, produced in accelerators, lead to the formation of a new state of matter: the quark gluon plasma (QGP). This deconfined QCD matter is expected to exist just for very short times after the collision. All the information one can get about the plasma is obtained from the particles that reach the detectors. Among them, heavy vector mesons are particularly important. The abundance of cc¯ and bb¯ states produced in a heavy ion collision is a source of information about the plasma. In contrast to the light mesons, that completely dissociate when the plasma is formed, heavy mesons presumably undergo a partial thermal dissociation. The dissociation degree depends on the temperature and also on the presence of magnetic fields and on the density (chemical potential). So, in order to get information about the plasma out of the quarkonium abundance data, one needs to resort to models that provide the dependence of the dissociation degree on these factors. Holographic phenomenological models provide a nice description for charmonium and bottomonium quasi-states in a plasma. In particular, quasi-normal modes associated with quarkonia states have been studied recently for a plasma with magnetic fields. Here we extend this analysis of quasinormal models to the case when charmonium and bottomonium are inside a plasma with finite density. The complex frequencies obtained are then compared with a Breit Wigner approximation for the peaks of the corresponding thermal spectral functions, in order to investigate the quantitative agreement of the different descriptions of quarkonium quasi-states.

Modification of the spectra of correlated charm-anticharm quark pairs in the quark-gluon plasma * * This work was supported by Chinese government scholarship (CSC) for foreigner graduate students and by NSFC (11675079)

Heavy quarks play an important role in probing the properties of strongly interacting quark-gluon plasma (QGP) created in ultra-relativistic heavy-ion collisions. We study the interactions of single heavy (charm) quarks and correlated charm and anticharm ( ) quark pairs with the medium constituents of QGP by performing fireball+Langevin simulations of the pertinent Brownian motion with elastic collisions. Besides studying the traditional observables, the nuclear modification factor and the elliptic flow of single heavy quarks in QGP for different thermal relaxation rates, we also study the broadening of the azimuthal correlations of charm and anticharm quark pairs in the QGP medium for different relaxation rates and transverse momenta classes. We quantified the smearing of pair azimuthal correlations with an increasing thermal relaxation rate: while the (nearly) back-to-back correlations among pairs are almost completely washed out at low transverse momentum (pT), these correlations at high pT largely survive the pair diffusion. This provides a novel observable for diagnosing the properties of QGP.

Hadronization and Charm-Hadron Ratios in Heavy-Ion Collisions.

Understanding the hadronization of the quark-gluon plasma (QGP) remains a challenging problem in the study of strong-interaction matter as produced in ultrarelativistic heavy-ion collisions (URHICs). The large mass of heavy quarks renders them excellent tracers of the color neutralization process of the QGP when they convert into various heavy-flavor (HF) hadrons. We develop a 4-momentum conserving recombination model for HF mesons and baryons that recovers the thermal and chemical equilibrium limits and accounts for space-momentum correlations (SMCs) of heavy quarks with partons of the hydrodynamically expanding QGP, thereby resolving a long-standing problem in quark coalescence models. The SMCs enhance the recombination of fast-moving heavy quarks with high-flow thermal quarks in the outer regions of the fireball. We also improve the hadrochemistry with "missing" charm-baryon states, previously found to describe the large Λ_{c}/D^{0} ratio observed in proton-proton collisions. Both SMCs and hadrochemistry, as part of our HF hydro-Langevin-recombination model for the strongly coupled QGP, importantly figure in the description of recent data for the Λ_{c}/D^{0} ratio and D-meson elliptic flow in URHICs.

Comparison Between Data from Ultrarelativistic Heavy-ion Collisions and a Geometrical Model Based on Local Energy and Momentum Conservation

Comparison Between Data from Ultrarelativistic Heavy-ion Collisions and a Geometrical Model Based on Local Energy and Momentum Conservation

Measurements of heavy-flavor jets with ALICE at the LHC

Heavy quarks created in ultra-relativistic heavy-ion collisions are mostly produced in hard QCD processes during the early stages of the reaction. They interact with the hot and cold nuclear matter throughout the evolution of the medium via semi-hard and soft processes such as energy loss via gluon radiations and collisions. Nuclear modification of heavy flavors in p-A systems provides insight into cold nuclear matter effects such as (anti)shadowing and kT- broadening, and serves as a baseline for A-A studies. In addition to that the fully reconstructed heavy-flavor jets provide additional information on the flavor (or mass) dependence of fragmentation, color charge effects as well as insight into the contribution of late gluon splitting. In this contribution, we present the measurements of b-jet production in p–Pb collisions at sNN=5.02 TeV and c-jet production in pp, p–Pb and Pb–Pb collisions measured by the ALICE experiment at the LHC. The measurements of the nuclear modification factors for c-jet in p–Pb and Pb–Pb collisions are also presented. The experimental measurements are compared with the results from Monte Carlo event generators (PYTHIA 6, PYTHIA 8 and Herwig 7) and the NLO pQCD calculations (POWHEG+PYTHIA6). We find good agreement of the measurements with the results from Monte Carlo event generators and from NLO pQCD calculations.