Baryon-rich Matter Research Articles

Emergent conformality in high-density matter

This contribution discusses baryon-rich matter in the context of recent astrophysical observations of neutron stars (NSs) and the structure of the equation of state that arises from these observations. We show that, generating an ensemble of equations of state that fulfill multimessenger constraints, the speed of sound and trace anomaly can reach their conformal values at the center of maximally massive NSs. The local peak of the speed of sound appears at values of the energy and particle densities being consistent with deconfinement and percolation conditions of QCD phase transition. Some implications of advocating a pseudo-conformal symmetry in dense matter are presented.

NICA heavy-ion collider at JINR (Dubna). Status of accelerator complex and first physics at NICA.

The NICA facility is under active realization at the Joint Institute for Nuclear Research (JINR, Dubna). Its main goal includes experimental studies of hot and dense baryon rich matter forming in heavy ion collisions to search for possible manifestations of phase transitions and critical phenomena, as well as investigation of nucleon spin structure with polarized proton and deuteron beams. The NICA general design, construction status, and prospects for physics program are presented.

Topological Aspects of Dense Matter: Lattice Studies

Topological fluctuations change their nature in the different phases of strong interactions, and the interrelation of topology, chiral symmetry and confinement at high temperature has been investigated in many lattice studies. This review is devoted to the much less explored subject of topology in dense matter. After a short overview of the status at zero density, which will serve as a baseline for the discussion, we will present lattice results for baryon rich matter, which, due to technical difficulties, has been mostly studied in two-color QCD, and for matter with isospin and chiral imbalances. In some cases, a coherent pattern emerges, and in particular the topological susceptibility seems suppressed at high temperature for baryon and isospin rich matter. However, at low temperatures the topological aspects of dense matter remain not completely clear and call for further studies.

Baseline predictions of elliptic flow and fluctuations for the RHIC Beam Energy Scan using response coefficients

Currently the RHIC Beam Energy Scan is exploring a new region of the Quantum Chromodynamic phase diagram at large baryon densities that approaches nuclear astrophysics regimes. This provides an opportunity to study relativistic hydrodynamics in a regime where the net conserved charges of baryon number, strangeness, and electric charge play a role, which will significantly change the theoretical approach to simulating the baryon-dense Quark-Gluon Plasma. Here we detail many of the important changes needed to adapt both initial conditions and the medium to baryon-rich matter. Then, we make baseline predictions for the elliptical flow and fluctuations based on extrapolating the physics at LHC and top RHIC energies to support future analyses of where and how the new baryon-dense physics causes these extrapolations to break down. First we compare eccentricities across beam energies, exploring their underlying assumptions; we find the the extrapolated initial state is predicted to be nearly identical to that at AuAu $\sqrt{s_{NN}}=200$ GeV. Then the final flow harmonic predictions are based on linear+cubic response. We discuss preliminary STAR results in order to determine the implications that they have for linear+cubic response coefficients at the lowest beam energy of AuAu $\sqrt{s_{NN}}=7$ GeV.

Early thermalization and shear viscosity to entropy ratio in heavy-ion collisions at energies of BES, FAIR and NICA

Equilibration of highly excited baryon-rich matter is studied within the microscopic model calculations in A+A collisions at energies of BES, FAIR and NICA. It is shown that the system evolution from the very beginning of the collision can be approximated by relativistic hydrodynamics, although the hot and dense nuclear matter is not in local equilibrium yet. During the evolution of the fireball the extracted values of energy density, net baryon and net strangeness densities are used as an input to Statistical Model (SM) in order to calculate temperature T, chemical potentials μB and μS, and entropy density s of the system. Also, they are used as an input for the box with periodic boundary conditions to investigate the momentum correlators in the infinite nuclear matter. Shear viscosity η is calculated according to the Green-Kubo formalism. At all energies, shear viscosity to entropy density ratio shows minimum at time corresponding to maximum baryon density. The ratio dependence on T,μB,μS is investigated for both in- and out of equilibrium cases.

High baryon and energy densities achievable in heavy-ion collisions at sNN=39 GeV

Baryon and energy densities, which are reached in central Au+Au collisions at collision energy of $\sqrt{s_{NN}}= 39$ GeV, are estimated within the model of three-fluid dynamics. It is shown that the initial thermalized mean proper baryon and energy densities in a sizable central region approximately are $n_B/n_0 \approx$ 10 and $\varepsilon\approx$ 40 GeV/fm$^3$, respectively. The study indicates that the deconfinement transition at the stage of interpenetration of colliding nuclei makes the system quite opaque. The final fragmentation regions in these collisions are formed not only by primordial fragmentation fireballs, i.e. the baryon-rich matter passed through the interaction region (containing approximately 30\% of the total baryon charge), but also by the baryon-rich regions of the central fireball pushed out to peripheral rapidities by the subsequent almost one-dimensional expansion of the central fireball along the beam direction.

Vorticity and Λ polarization in baryon rich matter

The polarization of Λ hyperons due to axial chiral vortical effect is discussed. The effect is proportional to (strange) chemical potential and is pronounced at lower energies in baryon-rich matter. The polarization of ¯ has the same sihn and larger magnitude. The emergence of vortical structures is observed in kinetic QGSM models. The hydrodynamical helicity separation receives the contribution of longitudinal velocity and vorticity implying the quadrupole structure of the latter. The transition from the quark vortical effects to baryons in confined phase may be achieved by exploring the axial charge. At the hadronic level the polarization corresponds to the cores of quantized vortices in pionic superfluid. The chiral vortical effects may be also studied in the frmework of Wigner function establishing the relation to the thermodynamical approach to polarization.

Spinodal instabilities in baryon-rich quark matter

For quark matter at finite baryon chemical potential, its density develops large fluctuations when it undergoes a first-order phase transition. Based on the Nambu–Jona–Lasinio (NJL) model, we have used the linear response theory to study the growth rate of density fluctuations and its dependence on the wavelength of unstable modes. Using the transport equation derived from the NJL model, we have also studied the time evolution of the unstable modes and the density fluctuations in a baryon-rich quark matter that is confined in a finite volume. Allowing the expansion of the quark matter, we have further studied the survivability of the density fluctuations as the density and temperature of the quark matter decrease. Possible experimental signatures of the density fluctuations have been suggested.

Density fluctuations in baryon-rich quark matter

At finite baryon chemical potential, the density of a quark matter develops large fluctuations when it undergoes a first-order phase transition. Based on the transport equation derived from the Nambu–Jona-Lasinio (NJL) model, we have studied the density fluctuations in a baryon-rich quark matter that is confined in a finite volume. Allowing the expansion of the quark matter using initial conditions from either a blast wave model or a multiphase transport (AMPT) model, we have further studied the survivability of the density fluctuations as the density and temperature of the quark matter decrease. Possible experimental signatures of the density fluctuations are suggested.

Strangeness as a probe to baryon-rich QCD matter at NICA

We elucidate a prospect of strangeness fluctuation measurements in the heavy-ion collision at NICA energies. The strangeness fluctuation is sensitive to quark deconfinement. At the same time strangeness has a strong correlation with the baryon number under the condition of vanishing net strangeness, which leads to an enhancement of \(\Lambda^{0}\), \(\Xi^{0}\), \(\Xi^{-}\), and K+ at high baryon density. The baryon density is maximized around the NICA energies, and strangeness should be an ideal probe to investigate quark deconfinement phenomena of baryon-rich QCD matter created at NICA. We also utilize the hadron resonance gas model to estimate a mixed fluctuation of strangeness and baryon number.

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.

Hydrodynamic modeling of deconfinement phase transition in nuclear collision

The (3+1)-dimensional ideal hydrodynamics is used to simulate collisions of gold nuclei with bombarding energies from 1 to 160 GeV per nucleon. The initial state is represented by two cold Lorentz-boosted nuclei. Two equations of state: with and without the deconfinement phase transition are used. We have investigated dynamical trajectories of compressed baryon-rich matter as functions of various thermodynamical variables. The parameters of collective flow and hadronic spectra are calculated. It is shown that presence of the deconfinement phase transition leads to increase of the elliptic flow and to flattening of proton rapidity distributions.

Thermodynamics of dense hadronic matter in a parity doublet model

We study thermodynamics of nuclear matter in a two-flavored parity doublet model within the mean field approximation. Parameters of the model are chosen to reproduce correctly the properties of the nuclear ground state. The model predicts two phase transitions in nuclear matter, a liquid-gas phase transition at normal nuclear density and a chiral transition at higher density. At finite temperature the pion decay constant exhibits a considerable reduction at intermediate values of chemical potential, which is traced back to the presence of the liquid-gas transition, and approaches zero at higher chemical potential associated with the chiral symmetry restoration. A "transition" from meson-rich to baryon-rich matter is also discussed.

Phase transition dynamics for baryon-dense matter

We construct a simple two-phase equation of state intended to resemble that of compressed baryon-rich matter and then introduce a gradient term in the compressional energy density to take account of finite-range effects in nonuniform configurations. With this model we study the interface between the two coexisting phases and obtain estimates for the associated interface tension. Subsequently, we incorporate the finite-range equation of state into ideal or viscous fluid dynamics and derive the collective dispersion relation for the mechanically unstable modes of bulk matter in the spinodal region of the thermodynamic phase diagram. Combining these results with time scales extracted from existing dynamical transport simulations, we discuss the prospects for spinodal phase separation to occur in nuclear collisions. We argue that these can be optimized by a careful tuning of the collision energy to maximize the time spent by the bulk of the system inside the mechanically unstable spinodal region of the phase diagram. Our specific numerical estimates suggest cautious optimism that this phenomenon may in fact occur, though a full dynamical simulation is needed for a detailed assessment.

ELECTROMAGNETIC RADIATION FROM BROKEN SYMMETRIES IN RELATIVISTIC NUCLEAR COLLISIONS

A new channel of direct photon production from a quark gluon plasma (QGP) is explored in the framework of high-temperature QCD. This process appears at next-to-leading order, in the presence of a charge asymmetry in the excited matter. The photon production rate from this new mechanism is suppressed compared to the QCD annihilation and Compton scattering at low baryon density but assumes importance in baryon-rich matter.

Photon production from charge-asymmetric hot and dense matter

A new channel of direct photon production from a quark-gluon plasma (QGP) is explored. This process appears at next-to-leading order in the presence of a charge asymmetry in the heated matter and may be effectively described as the bremsstrahlung of a real photon from a thermal gluon. The photon production from this new mechanism is calculated in the effective theory of QCD at high temperature. The results show that the photon production rate may not be as high as that of the annihilation and Compton scattering processes at low baryon density, but could become important in baryon-rich matter.

Catastrophic rearrangement of a compact star due to the quark core formation

We study properties of compact stars with the deconfinement phase transition in their interiors. The equation of state of cold baryon-rich matter is constructed by combining a relativistic mean-field model for the hadronic phase and the MIT Bag model for the deconfined phase. In a narrow parameter range two sequences of compact stars (twin stars), which differ by the size of the quark core, have been found. We demonstrate the possibility of a rapid transition between the twin stars with the energy release of about 1052 ergs. This transition should be accompanied by the prompt neutrino burst and the delayed gamma-ray burst.

Constraints on possible phase transitions above the nuclear saturation density

We compare different models for hadronic and quark phases of cold baryon-rich matter in an attempt to find a deconfinement phase transition between them. For the hadronic phase we consider Walecka-type mean-field models, which describe well the nuclear saturation properties. We also use the variational chain model, which takes into account correlation effects. For the quark phase we consider the MIT bag model, the Nambu--Jona-Lasinio, and the massive quasiparticle models. By comparing pressure as a function of baryon chemical potential we find that crossings of hadronic and quark branches are possible only in some exceptional cases while for most realistic parameter sets these branches do not cross at all. Moreover, the chiral phase transition, often discussed within the framework of QCD motivated models, lies in the region where the quark phases are unstable with respect to the hadronic phase. We discuss possible physical consequences of these findings.

Interaction length and spontaneous decay of a cosmic ray “Centauro” fireball

We consider in detail the experimental fluxes of the Centauro cosmic ray events and Centauro-type fireballs before they have decayed and compare them to theoretical estimations. Our results relax the very stringent limit on the fireball's interaction length, imposed by recent analysis of Centauro data. In addition, we consider the production and separation of stangeness in the baryon-rich quark matter Centauro fireball before its spontaneous explosive decay. We find that its initially quark matter state may transform into a partially strange quark matter one, within its lifetime. This may have as a result the formation of a long-lived fireball, giving reasonable expectations for Centauro events and low mass strangelets in the future accelerator experiments at RHIC and LHC.

Coherent effects, parton distributions, and baryon rich matter in central high energy heavy ion collisions.

We argue that the parton distributions measured in heavy ion collisions depend on the trigger for the centrality of the collisions as a result of coherent effects specific for the collisions of energetic composite particles. Percolation phase transitions in central heavy ion collisions are predicted and methods to form and to investigate such baryon rich matter are suggested.