Baryon-rich Quark-gluon Plasma Research Articles

Dynamical versus equilibrium properties of the QCD phase transition: A holographic perspective

We employ an Einstein-Maxwell-Dilaton (EMD) holographic model, which is known to be in good agreement with lattice results for the QCD equation of state with $(2+1)$ flavors and physical quark masses, to investigate the temperature and baryon chemical potential dependence of the susceptibilities, conductivities, and diffusion coefficients associated with baryon, electric, and strangeness conserved charges. We also determine how the bulk and shear viscosities of the plasma vary in the plane of temperature and baryon chemical potential. The diffusion of conserved charges and the hydrodynamic viscosities in a baryon rich quark-gluon plasma are found to be suppressed with respect to the zero net baryon case. The transition temperatures associated with equilibrium and non-equilibrium quantities are determined as a function of the baryon chemical potential for the first time. Because of the crossover nature of the QCD phase transition even at moderately large values of the chemical potential, we find that the transition temperatures associated with different quantities are spread in the interval between $130-200$ MeV and they all decrease with increasing baryon chemical potential.

Spinodal instabilities of baryon-rich quark-gluon plasma in the Polyakov–Nambu–Jona-Lasinio model

Using the Polyakov-Nambu-Jona-Lasinia (PNJL) model, we study the spinodal instability of a baryon-rich quark-gluon plasma in the linear response theory. We find that the spinodal unstable region in the temperature and density plane shrinks with increasing wave number of the unstable mode and is also reduced if the effect of Polyakov loop is not included. In the small wave number or long wavelength limit, the spinodal boundaries in both cases of with and without the Polyakov loop coincide with those determined from the isothermal spinodal instability in the thermodynamic approach. Also, the vector interactions among quarks is found to suppress unstable modes of all wave numbers. Moreover, the growth rate of unstable modes initially increases with the wave number but is reduced when the wave number becomes large. Including the collisional effect from quark scattering via the linearized Boltzmann equation, we further find that it decreases the growth rate of unstable modes of all wave numbers. Relevance of these results to relativistic heavy ion collisions is discussed.

Mean-field effects on matter and antimatter elliptic flow

We report our recent work on mean-field potential effects on the elliptic flows of matters and antimatters in heavy ion collisions leading to the production of a baryon-rich matter. Within the framework of a multiphase transport (AMPT) model that includes both initial partonic and final hadronic interactions, we have found that including mean-field potentials in the hadronic phase leads to a splitting of the elliptic flows of particles and their antiparticles, providing thus a plausible explanation of the different elliptic flows between p and anti-p, K+ and K-, and pi(+) and pi(-) observed by the STAR Collaboration in the Beam Energy Scan (BES) program at the Relativistic Heavy Ion Collider (RHIC). Using a partonic transport model based on the Nambu-Jona-Lasinio (NJL) model, we have also studied the effect of scalar and vector mean fields on the elliptic flows of quarks and antiquarks in these collisions. Converting quarks and antiquarks at hadronization to hadrons via the quark coalescence model, we have found that the elliptic flow differences between particles and antiparticles also depend on the strength of the quark vector coupling in baryon-rich quark-gluon plasma, providing thus the possibility of extracting information on the latter's properties from the BES program at RHIC.

Nuclear suppression at low energy in relativistic heavy ion collisions

The effects of non-zero baryonic chemical potential on the drag and diffusion coefficients of heavy quarks propagating through a baryon rich quark gluon plasma have been studied. The nuclear suppression factor, $R_{\mathrm AA}$ for non-photonic single electron spectra resulting from the semileptonic decays of hadrons containing heavy flavours have been evaluated for low energy collisions. The role of non-zero baryonic chemical potential on $R_{\mathrm AA}$ has been highlighted.

Photon production from quark gluon plasma at finite baryon density

The photon yield from a baryon-rich quark gluon plasma (QGP) at SPS energy has been estimated. In the QGP phase, rate of photon production is evaluated up to two-loop level. In the hadron phase, dominant contribution from π,ρ, ω mesons has been considered. The evolution of the plasma has been studied with appropriate equation of state in both QGP and hadron phase for a baryon-rich system. At SPS energy, the total photon yield is found to increase marginally in the presence of baryon density.

Baryon-Rich Quark–Gluon Plasma in Nuclear Collisions

The maximum achievable temperature (energy density) and minimum kinetic energy required for the formation of a baryon-rich quark-gluon plasma formed at central rapidity in small impact parameter nuclear collisions is estimated. A possible mechanism leading to the pile-up of matter is introduced. Plasma formation is expected to appear at about 15 GeV/Nucleon uranium beam energy on a stationary target or 2.7 GeV/Nucleon in colliding beams.

Dense, strongly interacting matter: strangeness in heavy-ion reactions 1-10 A GeV

This talk reviews the physics of dense, strongly interacting matter. The newly acquired experimental data on strangeness production in heavy-ion reactions from 1-10 A GeV is used to probe the physics of in-medium properties of matter, multi-body collisions, and the search for the baryon-rich quark-gluon plasma.

Kaon production in Au+Au collisions at11.6A GeV/c

A systematic study of the spectra and yields of K{sup +} and K{sup {minus}} is reported by experiment E866 as a function of centrality in Au+Au collisions at 11.6A GeV/c. The invariant transverse spectra for both kaon species are well described by exponentials in m{sub t}, with inverse slope parameters that are largest at midrapidity and which increase with centrality. The inverse slopes of the K{sup +} spectra are slightly larger than the inverse slopes of the K{sup {minus}} spectra. The kaon rapidity density peaks at midrapidity with the K{sup +} distribution wider in rapidity than K{sup {minus}}. The integrated total yields of K{sup +} and K{sup {minus}} increase nonlinearly and steadily with the number of projectile participants. The yield per participant for kaons is two to three times larger than the yield from N-N collisions. This enhancement suggests that the majority of kaons in central Au+Au reactions are produced in secondary hadronic collisions. There is no evidence for an onset of additional kaon production from a possible small volume of baryon-rich quark-gluon plasma. The differences between K{sup +} and K{sup {minus}} rapidity distributions and transverse spectra are consistent with a lower phase space for K{sup {minus}} production due tomore » a higher energy threshold. These differences also exclude simple thermal models that assume emission from a common equilibrated system. {copyright} {ital 1998} {ital The American Physical Society}« less

Photon production from baryon rich quark-gluon plasma

We study high energy photon production from a quark-gluon plasma at finite baryon density. We find that the photon production spectrum from the quark-gluon plasma maintained at constant temperature is only mildly dependent on the quark chemical potential.

Hydrodynamical-model analysis of the (3+1)-dimensional cylindrically symmetric baryon-rich quark-gluon plasma with phase transition.

We analyze numerically the space-time evolution of a baryon-rich quark-gluon plasma fluid by means of a hydrodynamical model which takes account of a net baryon-number conservation law. We first introduce a simple effective equation of state that gives QCD phase-transition-like behavior of thermodynamical quantities on the $T\ensuremath{-}\ensuremath{\mu}$ plane (here $T$ is temperature and $\ensuremath{\mu}$ is the chemical potential for the net baryon number.) This equation of state is based on a semiphenomenological quantum Langevin equation. Numerically solving the hydrodynamical equation coupled with the net baryon-number current density conservation law using the model equation of state, we discuss in detail the space-time evolution of the thermodynamical quantities and the local four-velocity of a baryon-rich quark-gluon plasma fluid. Finally we apply our hydrodynamical model to the recent experimental results from the CERN NA35 S+S 200 GeV/A collisions and discuss the possible occurrence of a quark-gluon plasma phase.

Energy distribution of muon pairs: A signal for the creation of a baryon-rich quark-gluon plasma in relativistic heavy-ion collisions.

A study of the energy distribution of muon pairs and photon pairs produced in ultrarelativistic heavy-ion collisions shows that muon pairs may be better signatures than photon pairs for the detection of a baryon-rich quark-gluon plasma (QGP). The value of the transverse mass of the muon pairs at which the {rho}-meson peak disappears from their invariant-mass spectrum is sensitive to the equation of state of the QGP. Hence dimuons are useful not only for the detection of a QGP but also for a study of its equation of state.

Distillation and survival of strange-quark-matter droplets in ultrarelativistic heavy-ion collisions.

Recently it has been suggested that rather cold droplets of absolutely stable or metastable strange-quark matter may be distilled in heavy-ion collisions during the phase transition from a baryon-rich quark-gluon plasma (QGP) to hadron matter. Here we present a model describing the hadronization of the QGP through particle emission, which is based solely on thermodynamical arguments. Pions and ${\mathit{K}}^{+}$'s and ${\mathit{K}}^{0}$'s carry away entropy and antistrangeness from the system, thus facilitating the cooling process and the strangelet formation. Our results are supported by revised more sophisticated rate calculations. Two rather unexpected results are obtained when this model is applied to the investigation of strangelet production. The strangeness separation mechanism and the formation process works well even for higher initial entropies per baryon, tantamount to higher bombarding energies. The surviving strangelets have a rather high strangeness content, ${\mathit{f}}_{\mathit{s}}$\ensuremath{\sim}1.2--2 [i.e., Z/A\ensuremath{\sim}(-0.1)--(-0.5)]. Hence droplets of strange-quark matter with a baryon number of \ensuremath{\sim}10--30 and with a negative charge may be produced. They may serve as a unique signature for the transient formation of a quark-gluon plasma in heavy-ion collisions.

Influence of massive quasi-particle excitations on a baryon-rich quark-gluon plasma

We present an equation of state for a baryon-rich quark-gluon plasma with a cut-off for colour charges with low momenta to simulate the influence of massive quasi-particle excitations. The deconfinement transition is studied and compared with that in the bag model. Physical consequences of the new model for lattice QCD simulations at finite chemical potential and experiments on heavy-ion collisions are discussed.

Dilepton as a possible signature for the baryon-rich quark-gluon plasma.

In an expanding fireball model, we study dilepton production from high-energy heavy-ion collisions at the stopping regime. We find that the pion-pion annihilation gives the dominant contribution to the dilepton yield at invariant masses between 2${m}_{\ensuremath{\pi}}$ and 1 GeV. The total dilepton yield in this invariant mass region increases with the incident energy of the collision, but a saturation is seen once a baryon-rich quark-gluon plasma is formed in the initial stage.

Hadronization from supercooled baryon-rich quark-gluon plasma.

A simple phenomenological model for the nuclear equation of state is introduced and the dependence of the phase boundaries on the parameters is discussed. The equations for relativistic shocks are then applied to study the compression of hadronic matter and the hadronization of supercooled baryon-rich plasma. The results show little sensitivity to the different parametrizations of the hadronic equation of state. However, they may be sensitive to dynamic properties such as the hadronization rate.

Dilepton suppression as a signature for the baryon-rich quark-gluon plasma.

The dilepton yield at invariant masses between 2${m}_{\ensuremath{\pi}}$ and 1 GeV is shown to be reduced substantially if a baryon-rich quark-gluon plasma is formed in high-energy heavy-ion collisions. We suggest therefore to measure the total dilepton yield in this region of invariant masses as a function of the incident energy. The onset of a baryon-rich quark-gluon plasma is then signaled by a decrease in this excitation function.

Creation of strange-quark-matter droplets as a unique signature for quark-gluon plasma formation in relativistic heavy-ion collisions.

We demonstrate that strangeness separates in the Gibbs-phase coexistence between a baryon-rich quark-gluon plasma and hadron matter, even at T=0. For finite temperatures this is due to the associated production of kaons (containing s\ifmmode\bar\else\textasciimacron\fi{} quarks) in the hadron phase while s quarks remain in the deconfined phase. The s-s\ifmmode\bar\else\textasciimacron\fi{} separation results in a strong enhancement of the s-quark abundance in the quark phase. This mechanism is further supported by cooling and net strangeness enrichment due to the prefreezeout evaporation of pions and ${K}^{+}$, ${K}^{0}$, which carry away entropy and anti- strangeness from the system. Metastable droplets (i.e., stable as far as weak interactions are not regarded) of strange-quark matter (``strangelets'') can thus be formed during the phase transition. Such cool, compact, long-lived clusters could be experimentally observed by their unusually small Z/A ratio (\ensuremath{\le}0.1--0.3). Even if the strange-quark-matter phase is not stable under strong interactions, it should be observable by the delayed correlated emission of several hyperons. This would serve as a unique signature for the transient formation of a quark-gluon plasma.

Muon pairs and photon pairs as signals for the hadronization phase transition at finite baryon density.

In an isentropic expansion of a baryon-rich quark-gluon plasma (QGP) formed in heavy-ion collisions and undergoing a first-order phase transition into a hadron-resonance gas, it is found that an analysis of the production cross sections of dimuons in the region where the invariant mass M is greater than 2.25 GeV is enough for the detection of a QGP, whereas an analysis in the region with M\ensuremath{\gtrsim}7.5 GeV is required for photon pairs. While the production cross section of dimuons is found to be almost independent of the equation of state used for the QGP, that for photon pairs depends on the equation of state of the QGP. We conclude that for the detection of the QGP muon pairs are better signals than photon pairs, whereas for a study of the equation of state of the QGP photon pairs seem to be more suitable than muon pairs.

ANTIBARYON-BARYON RATIOS AS A SIGNAL FOR QUARK GLUON PLASMA FORMATION

In a previously developed non-equilibrium model for hadronization of a quark gluon plasma, we study hadronic signals for plasma formation. We find that fragmentation of gluons into quark-antiquark pairs can lead to a strong enhancement of various antibaryon to baryon ratios if a baryon-rich quark gluon plasma is formed.

K+ and K- slope parameters as a signature for deconfinement at finite baryon density.

We propose to study the slope parameters of K/sup +/ and K/sup -/ energy spectra from relativistic heavy-ion collisions in the regime of complete nuclear stopping, and suggest a characteristic change of these slopes as a signal for transition into a baryon-rich quark-gluon plasma.