Phase Transition In Heavy-ion Collisions Research Articles

Enhanced dilepton emission from a phase transition in dense matter

It is demonstrated that the presence of a phase transition in heavy-ion collisions, at beam energies that probe dense quantum chromodynamics (QCD) matter, leads to a significant enhancement of the dilepton yield at low invariant mass and requires the embedding of low beam energies per produced pion due to the extended emission time. In addition, the temperature of low-mass dileptons shows a modest decrease due to the mixed phase. The emission of dileptons in the SIS18–SIS100 beam energy range is studied by augmenting the ultra-relativistic quantum molecular dynamics (UrQMD) transport model with a realistic density-dependent equation of state, as well as two different phase transitions. This is achieved by extending the molecular dynamics interaction part of the UrQMD model to a density-dependent interaction potential with a high-density minimum, leading to a phase transition and metastable coexisting high-density states. Together with a high-precision measurement, these simulations will be able to constrain the existence of a phase transition in QCD up to densities of several times nuclear saturation density.

Testing the phase transition parameters inside neutron stars with the production of protons and lambdas in relativistic heavy-ion collisions

We demonstrate the consistency of the quark deconfinement phase transition parameters in the beta-stable neutron star matter and in the nearly symmetric nuclear matter formed in heavy-ion collisions (HICs). We investigate the proton and $\mathrm{\ensuremath{\Lambda}}$ flow in $\mathrm{Au}+\mathrm{Au}$ collisions at 3 and $4.5\text{ }\text{ }\mathrm{GeV}/\mathrm{nucleon}$ incident beam energies with the pure hadron cascade version of a multiphase transport model. The phase transition in HICs and neutron stars is described based on a class of hybrid equations of state from the quark mean-field model for the hadronic phase and a constant-speed-of-sound parametrization for the high-density quark phase. The measurements of the anisotropic proton flow at $3\text{ }\text{ }\mathrm{GeV}/\mathrm{nucleon}$ by the STAR Collaboration favor a relatively low phase transition density lower than $\ensuremath{\sim}2.5$ times saturation density indicated by the gravitational wave and electromagnetic observations of neutron stars. And the proton flow data at the higher energy of $4.5\text{ }\text{ }\mathrm{GeV}/\mathrm{nucleon}$ can be used to effectively constrain the softness of high-density quark matter equations of state. Finally, compared to the proton flow, the $\mathrm{\ensuremath{\Lambda}}$ flow is found to be less sensitive and not constraining to the equations of state.

Enhanced pion-to-proton ratio at the onset of the QCD phase transition

The pion-to-proton ratio is identified as a potential signal for a non-equilibrium first-order chiral phase transition in heavy-ion collisions, as the pion multiplicity is directly related to entropy production. To showcase this effect, a non-equilibrium Bjorken expansion starting from realistic initial conditions along a Taub adiabat is used to simulate the entropy production. Different dynamical criteria to determine the final entropy-per-baryon number are investigated and matched to a hadron resonance gas model along the chemical freeze out curve to obtain the final pion and proton numbers. We detect a strong enhancement of their multiplicity ratio at the energies where the system experiences a strong phase transition as compared to a smooth crossover which shows almost no enhancement.

Light Nuclei Production in Au+Au Collisions at $\sqrt{s_{\mathrm{NN}}}$ = 3 GeV from the STAR experiment

Light nuclei production is expected to be sensitive to baryon density fluctuations and can be used to probe the signatures of QCD critical point and/or a first-order phase transition in heavy-ion collisions. In this proceedings, we present the spectra and yields of protons (\bm{p}𝐩) and light nuclei (\bm{d}𝐝, \bm{t}𝐭, \bm{^{3}\mathrm{He}}3He, \bm{^{4}\mathrm{He}}4He) in Au+Au collisions at \bm{\sqrt{s_{\mathrm{NN}}}}𝐬NN = 3 GeV by the STAR experiment. Finally, it is found that the kinetic freeze-out dynamics (temperature \bm{T_{kin}}𝐓𝐤𝐢𝐧\bm{vs.}𝐯𝐬. average radial flow velocity \langle\beta_{T}\rangle⟨βT⟩) at \bm{\sqrt{s_{\mathrm{NN}}}}𝐬NN = 3 GeV extracted with the blast-wave model deviate from the trends at high energies (\bm{\sqrt{s_{\mathrm{NN}}}}𝐬NN = 7.7 - 200 GeV), indicating a different medium equation of state.

Efficiency corrections for mutually inclusive variables and particle identification effect for mixed-cumulants in heavy-ion collisions * *Supported by the National Key Research and Development Program of China (2020YFE0202002, 2018YFE0205201), the National Natural Science Foundation of China (12122505, 11890711, 11861131009) and Ito Science Foundation and JSPS KAKENHI (25105504, 19H05598)

Mix-cumulants of conserved charge distributions are sensitive observables for probing properties of the QCD medium and phase transition in heavy-ion collisions. To perform precise measurements, efficiency correction is one of the most important step. In this study, using the binomial efficiency model, we derive efficiency correction formulas for mutually exclusive and inclusive variables. The UrQMD model is applied to verify the validity of these formulas for different types of correlations. Furthermore, we investigate the effect of the multiplicity loss and contamination emerging from the particle identifications. This study provides important steps toward future measurements of mixed-cumulants in relativistic heavy-ion collisions.

Effects of QCD critical point on light nuclei production

Using the nucleon coalescence model, which can naturally take into account the correlations in the nucleon density distribution, we study the effects of QCD critical point on light nuclei production in relativistic heavy-ion collisions. We find that the yield ratio NtNp/Nd2 of proton (p), deuteron (d) and triton (t) increases monotonically with the nucleon density correlation length, which is expected to increase significantly near the critical point in the QCD phase diagram. Our study thus demonstrates that the yield ratio NtNp/Nd2 can be used as a sensitive probe of the QCD critical phenomenon. We further discuss the relation between the QCD phase transitions in heavy-ion collisions and the possible non-monotonic behavior of NtNp/Nd2 in its collision energy dependence.

Isoscalar-vector interaction and its influence on the hadron-quark phase transition

The hadron-quark phase transition is studied with the newly constructed Hadron–Poyakov-Nambu– Jona-Lasinio(PNJL) model. Particularly,in the description of quark matter,we include the isoscalar-vector interaction. With the constraints of neutron star observations,our calculation shows the isoscalar-vector interaction between quarks is indispensable if massive hybrids star exist in the universe. Its strength determines the onset density of quark matter,and the mass-radius relations of hybrid stars. Also,as a connection with heavy-ion-collision experiments,we discuss the strength of isoscalar-vector interaction and its effect on the signals of hadron-quark phase transition in heavy-ion collisions,such as NICA at JINR-Dubna and FAIR at GSI-Darmstadt.

Viscous effects on the dynamical evolution of QCD matter during the first-order confinement phase transition in heavy-ion collisions

We investigate viscous effects on the dynamical evolution of QCD matter during the first-order phase transition, which may happen in heavy-ion collisions. We first obtain the first-order phase transition line in the QCD phase diagram under the Gibbs condition by using the MIT bag model and the hadron resonance gas model for the equation of state of partons and hadrons. The viscous pressure, which corresponds to the friction in the energy balance, is then derived from the energy and net baryon number conservation during the phase transition. We find that the viscous pressure relates to the thermodynamic change of the two-phase state and thus affects the timescale of the phase transition. Numerical results are presented for demonstrations.

Effects of mean-field and softening of equation of state on elliptic flow in Au+Au collisions at from the JAM model**Supported by the MoST of China 973-Project (2015CB856901), NSFC (11575069, 11221504). Y. N. is supported by the Grants-in-Aid for Scientific Research from JSPS (15K05079, 15K05098)

We perform a systematic study of elliptic flow (v2) in Au+Au collisions at by using a microscopic transport model, JAM. The centrality, pseudorapidity, transverse momentum and beam energy dependence of v2 for charged as well as identified hadrons are studied. We investigate the effects of both the hadronic mean-field and the softening of equation of state (EoS) on elliptic flow. The softening of the EoS is realized by imposing attractive orbits in two body scattering, which can reduce the pressure of the system. We found that the softening of the EoS leads to the enhancement of v2, while the hadronic mean-field suppresses v2 relative to the cascade mode. It indicates that elliptic flow at high baryon density regions is highly sensitive to the EoS and the enhancement of v2 may probe the signature of a first-order phase transition in heavy-ion collisions at beam energies of a strong baryon stopping region.

Measurement of the cumulants of net-proton multiplicity distribution in Au+Au collisions at [formula omitted] – 200 GeV from the STAR experiment

One of the main goals of the RHIC Beam Energy Scan program is to search for the QCD Critical Point (CP) and phase transition in heavy-ion collisions. Fluctuations of conserved quantities are highly sensitive to the correlation length, and are directly connected to the susceptibilities in the Lattice QCD. Therefore, they are ideal observables for finding the CP and phase transition signatures. We present measurements of the cumulants of net-proton distributions from Au+Au collisions at sNN=7.7,11.5,14.5,19.6,27,39,62.4 (up to the fourth-order) and 200 GeV (up to the sixth-order) measured by the STAR experiment at RHIC. We also present the multiparticle correlation functions extracted from the proton and anti-proton cumulants. It is observed that the four-particle correlations are positive and show non-monotonic energy dependence in the fourth-order fluctuations. Finally, we discuss the corresponding physics implications.

Overview of recent results from the STAR experiment

The Solenoidal Tracker at RHIC (STAR) experiment utilizes its excellent mid-rapidity tracking and particle identification capabilities to study the emergent properties of Quantum Chromodynamics (QCD). The STAR heavy-ion program at vanishingly small baryon density is aimed to address questions about the quantitative properties of the strongly-interacting Quark Gluon Plasma (QGP) matter created in high energy collisions (η/s, qˆ, chirality, transport parameters, heavy quark diffusion coefficients…). At finite baryon density, the questions concern the phases of nuclear matter (the QCD phase diagram) and the nature of the phase transition, namely: what is the onset collision energy for the formation of QGP? What is the nature of phase transition in heavy-ion collisions? Are there two phase transition regions? If yes, where is the critical point situated? At Quark Matter 2015, the STAR collaboration has presented a wealth of new experimental results which address these questions. In these proceedings I highlight a few of those results.

Overview of fluctuations at the QCD phase transition in heavy-ion collisions

I will summarize our current understanding of the QCD phase diagram with special emphasis on what we can expect to learn from heavy-ion collision experiments. With the upcoming Beam Energy Scan Phase II at RHIC and the experimental facilities FAIR, GSI, and NICA, Dubna, several challenges need to be met both experimentally and theoretically. I will discuss in which aspects theoretical advances are necessary in order to be able to interpret the experimental results.

Dynamics of the Chiral Phase Transition

The intention of this study is the search for signatures of the chiral phase transition in heavy-ion collisions. To investigate the impact of fluctuations, e.g., of the baryon number, at the transition or at a critical point, the linear sigma model is treated in a dynamical (3+1)-dimensional numerical simulation. Chiral fields are approximated as classical mean fields, and quarks are described as quasi particles in a Vlasov equation. Additional dynamics is implemented by quark-quark and quark-sigma-field interactions. For a consistent description of field-particle interactions, a new Monte-Carlo-Langevin-like formalism has been developed and is discussed.

Isoscalar-vector interaction and hybrid quark core in massive neutron stars

The hadron-quark phase transition in the core of massive neutron stars is studied with a newly constructed two-phase model. For nuclear matter, a nonlinear Walecka type model with general nucleon-meson and meson-meson couplings, recently calibrated by Steiner, Hemper and Fischer, is taken. For quark matter, a modified Polyakov-Nambu--Jona-Lasinio (mPNJL) model, which gives consistent results with lattice QCD data, is used. Most importantly, we introduce an isoscalar-vector interaction in the description of quark matter, and we study its influence on the hadron-quark phase transition in the interior of massive neutron stars. With the constraints of neutron star observations, our calculation shows that the isoscalar-vector interaction between quarks is indispensable if massive hybrids star exist in the universe, and its strength determines the onset density of quark matter, as well as the mass-radius relations of hybrid stars. Furthermore, as a connection with heavy-ion-collision experiments we give some discussions about the strength of isoscalar-vector interaction and its effect on the signals of hadron-quark phase transition in heavy-ion collisions, in the energy range of the NICA at JINR-Dubna and FAIR at GSI-Darmstadt facilities.

The impact of dissipation and noise on fluctuations in chiral fluid dynamics

We investigate the nonequilibrium evolution of the sigma field coupled to a fluid dynamic expansion of a hot fireball to model the chiral phase transition in heavy-ion collisions. The dissipative processes and fluctuations are allowed under the assumption that the total energy of the coupled system is conserved. We use the linear sigma model with constituent quarks to investigate the effects of the chiral phase transition on the equilibration and excitation of the sigma modes. The quark fluid acts as a heat bath in local thermal equilibrium and the sigma field evolves according to a semiclassical stochastic Langevin equation of motion. The effects of supercooling and reheating of the fluid in a first order phase transition are observed via the delayed relaxation of the sigma field to a new equilibrium state. At the first order phase transition the nonequilibrium fluctuations are strongly enhanced.Communicated by Steffen Bass

Viscosity and thermal conductivity effects at first-order phase transitions in heavy-ion collisions

Effects of viscosity and thermal conductivity on the dynamics of first-order phase transitions are studied. The nuclear gas-liquid and hadron-quark transitions in heavy-ion collisions are considered. We demonstrate that at non-zero thermal conductivity, $\kappa \neq 0$, onset of spinodal instabilities occurs on an isothermal spinodal line, whereas for $\kappa =0$ instabilities take place at lower temperatures, on an adiabatic spinodal.

Charge fluctuations in chiral models and the QCD phase transition

We consider the Polyakov loop-extended two-flavor chiral quark–meson model and discuss critical phenomena related to the spontaneous breaking of the chiral symmetry. The model is explored beyond the mean-field approximation in the framework of the functional renormalization group. We discuss properties of the net-quark number density fluctuations as well as their higher cumulants. We show that with the increasing net-quark number density, the higher order cumulants exhibit a strong sensitivity to the chiral crossover transition. We discuss their role as probes of the chiral phase transition in heavy-ion collisions at RHIC and LHC.

Hydrodynamic modeling of the deconfinement phase transition in heavy-ion collisions in the NICA–FAIR energy domain

We use (3 $+$ 1) dimensional ideal hydrodynamics to describe the space-time evolution of strongly interacting matter created in Au $+$ Au and Pb $+$ Pb collisions. The model is applied for the domain of bombarding energies 1--160 GeV/nucleon which includes future NICA (Dubna) and FAIR (Darmstadt) experiments. Two equations of state are used, the first one corresponding to resonance hadron gas and the second one including the deconfinement phase transition. The initial state is represented by two Lorentz-boosted nuclei. Dynamic trajectories of matter in the central box of the system are analyzed. They can be well represented by a fast shock-wave compression followed by a relatively slow isentropic expansion. The parameters of collective flows and hadronic spectra are calculated under assumption of the isochronous freeze-out. It is shown that the deconfinement phase transition leads to broadening of proton rapidity distributions, increase of elliptic flows, and formation of the directed antiflow in the central rapidity region. These effects are most pronounced at bombarding energies around 10 GeV/nucleon, when the system spends the longest time in the mixed phase. From the comparison with three-fluid calculations we conclude that the transparency effects are not so important in central collisions at NICA--FAIR energies (below 30 GeV/nucleon).

Hydrodynamical description of a hadron-quark first-order phase transition

Solutions of hydrodynamical equations are presented for the equation of state of the Van der Waals type allowing for a first-order phase transition. As an example we consider the hadron-quark phase transition in heavy-ion collisions. It is shown that fluctuations dissolve and grow as if the fluid is effectively very viscous. In the vicinity of the critical point even in spinodal region seeds are growing slowly due to viscosity, surface tension and critical slowing down. These non-equilibrium effects prevent enhancement of fluctuations in the near-critical region, which in thermodynamical approach is frequently considered as a signal of the critical endpoint in heavy-ion collisions.

Experimental results for fluctuations and correlations as a signature of QCD phase transitions in heavy-ion collisions

The search for signatures of QCD phase transitions in heavy-ion collisions can be enhanced by using experimental signatures based on event-by-event fluctuations and correlations. We introduce several experimental observables and study the incident energy, system size and centrality dependence of these quantities. In this paper, we will present experimental results for K/π fluctuations, balance functions, net charge fluctuations and pt correlations at energies ranging from to 200 GeV. We will also comment on the upcoming RHIC energy scan and new SPS experimental program.