Nonzero Baryon Density Research Articles

Dense QCD2 with matrix product states

We study one-flavor SU(2) and SU(3) lattice QCD in (1 + 1) dimensions at zero temperature and finite density using matrix product states and the density matrix renormalization group. We compute physical observables such as the equation of state, chiral condensate, and quark distribution function as functions of the baryon number density. As a physical implication, we discuss the inhomogeneous phase at nonzero baryon density, where the chiral condensate is inhomogeneous, and baryons form a crystal. We also discuss how the dynamical degrees of freedom change from hadrons to quarks through the formation of quark Fermi seas.

Chiral condensate and the equation of state at nonzero baryon density from the hadron resonance gas model with a repulsive mean field

We study the QCD equation of state and the chiral condensate using the hadron resonance gas model with repulsive mean-field interactions. We find that the repulsive interactions improve the agreement with the lattice results on the derivatives of the pressure with respect to the baryon chemical potential up to eighth order. From the temperature dependence of the chiral condensate we estimate the crossover temperature as a function of baryon chemical potential, Tpc(μB). We find that the chiral crossover line starts to deviate significantly from the chemical freeze-out line already for μB>400MeV. Furthermore, we find that the chiral pseudocritical line can be parametrized as Tpc(μB)/Tpc(0)=1−κ2[μB/Tpc(0)]2−κ4[μB/Tpc(0)]4 with κ2=0.0150(2) and κ4=3.1(6)×10−5, which are in agreement with lattice QCD results for small values of μB. For the first time we find a tiny but nonzero value of κ4 in our study. Published by the American Physical Society 2024

QCD equation of state at nonzero baryon density in an external magnetic field

QCD equation of state at nonzero baryon density in an external magnetic field

Can rooted staggered fermions describe nonzero baryon density at low temperatures?

Can rooted staggered fermions describe nonzero baryon density at low temperatures?

Thermal dilepton production in heavy-ion collisions at beam-energy-scan (BES) energies

The lepton pair production rate at finite temperature and at nextto-leading-order (NLO) is calculated for quark-gluon plasma at non-zero net baryon density. Yields are obtained using a (3+1)D multicomponent simulation capable of reproducing hadronic observables measured in the RHIC Beam Energy Scan (BES). Spectra of intermediate invariant mass dileptons are compared with measurements from the STAR collaboration. The result of a study where the temperature information obtained from the dilepton spectrum from heavyion collisions performed at different energies and centralities is presented.

Comprehensive simulation of heavy-ion collisions at nonzero baryon chemical potential

We present results of hydrodynamic modelling of Au-Au collisions from $\sqrt{s_{NN}}$ = 7.7 to 200 GeV. Our simulations have three novel components. Firstly, we use a Linear EXtrapolation of Ultrarelativistic nucleon-nucleon Scattering to nucleus-nucleus collisions (LEXUS) inspired Monte-Carlo initial state model. Secondly, we use a crossover equation of state at finite baryon densities without a critical point. Finally, we use departure functions derived from the quasiparticle theory of transport coefficients for hadronic matter at non-zero baryon densities.

Lepton pair production from a hot and dense QCD medium in the presence of an arbitrary magnetic field

In this article, we have explored the very important quantity of lepton pair production from a hot and dense QCD medium in presence of an arbitrary external magnetic field for simultaneous nonzero values of both the parallel (along the direction of the external field) and perpendicular (lying on the transverse plane to the external field) components of the dilepton momentum. As opposed to the zero magnetic field case (the so-called Born rate) or the lowest Landau level approximated rate, where only the annihilation process contributes, here we observe contributions also arising out of the quark and antiquark decay processes. We found the encouraging result of considerable enhancement of lepton pair production in presence of an arbitrary magnetic field. We decompose the total rate into different physical processes and discuss their behaviors for both zero and nonzero baryon density. The whole analysis is then subjected to an effective model treatment, where we have incorporated the magnetic field induced novel effects of magnetic catalysis (MC) and inverse MC (IMC) through a medium dependent scalar coupling, which leads to some further interesting observations.

Symmetry, pattern formation, and finite-density QCD

A longstanding issue in the study of quantum chromodynamics (QCD) is its behavior at nonzero baryon density, which has implications for many areas of physics. The path integral has a complex integrand when the quark chemical potential is nonzero and therefore has a sign problem, but it also has a generalized symmetry. We review some new approaches to -symmetric field theories, including both analytical techniques and methods for lattice simulation. We show that -symmetric field theories with more than one field generally have a much richer phase structure than their Hermitian counterparts, including stable phases with patterning behavior. The case of a -symmetric extension of a φ4 model is explained in detail. The relevance of these results to finite density QCD is explained, and we show that a simple model of finite density QCD exhibits a patterned phase in its critical region.

Polarization tensor of magnetized quark-gluon plasma at nonzero baryon density

We derive a general expression for the absorptive part of the one-loop photon polarization tensor in a strongly magnetized quark-gluon plasma at nonzero baryon chemical potential. To demonstrate the application of the main result in the context of heavy-ion collisions, we study the effect of a nonzero baryon chemical potential on the photon emission rate. The rate and the ellipticity of photon emission are studied numerically as a function the transverse momentum (energy) for several values of temperature and chemical potential. When the chemical potential is small compared to the temperature, the rates of the quark and antiquark splitting processes (i.e., qrightarrow q +gamma and {bar{q}}rightarrow {bar{q}} +gamma , respectively) are approximately the same. However, the quark splitting gradually becomes the dominant process with increasing the chemical potential. We also find that increasing the chemical potential leads to a growing total photon production rate but has only a small effect on the ellipticity of photon emission. The quark-antiquark annihilation (q+{bar{q}}rightarrow gamma ) also contributes to the photon production, but its contribution remains relatively small for a wide range of temperatures and chemical potentials investigated.

New developments in lattice QCD on equilibrium physics and phase diagram

I review recent new lattice QCD results on a few selected topics which are relevant to the heavy ion physics community. Special emphasis is put on the QCD phase diagram towards the chiral limit and at nonzero baryon density as well as the fate of quarkonia and heavy quark drag coefficients.

Lattice study of thermodynamic properties of dense QC2D

In this paper we study thermodynamic properties of dense cold $SU(2)$ QCD within lattice simulation with dynamical rooted staggered quarks which in the continuum limit correspond to ${N}_{f}=2$ quark flavours. We calculate baryon density, renormalized chiral and diquark condensates for various baryon chemical potentials in the region $\ensuremath{\mu}\ensuremath{\in}(0,2000)\text{ }\text{ }\mathrm{MeV}$. It is found that in the region $\ensuremath{\mu}\ensuremath{\in}(0,540)\text{ }\text{ }\mathrm{MeV}$ the system is well described by the ChPT predictions. In the region $\ensuremath{\mu}>540\text{ }\text{ }\mathrm{MeV}$ the system becomes sufficiently dense and ChPT is no longer applicable to describe lattice data. For chemical potentials $\ensuremath{\mu}>900\text{ }\text{ }\mathrm{MeV}$ we observe formation of the Fermi sphere, and the system is similar to the one described by the Bardeen-Cooper-Schrieffer theory where the diquarks play a role of Cooper pairs. In order to study how nonzero baryon density influences the gluon background we calculate chromoelectric and chromomagnetic fields, as well as the topological susceptibility. We find that the chromoelectric field and the topological susceptibility decrease, whereas the chromomagnetic field increases with rising of baryon chemical potential. Finally we study the equation of state of dense two-color quark matter.

(3+1)-dimensional dissipative relativistic fluid dynamics at non-zero net baryon density

Heavy-ion collisions at center-of-mass energies between 1 and 100 GeV/nucleon are essential to understand the phase diagram of QCD and search for its critical point. At these energies the net baryon density of the system can be high, and simulating its evolution becomes an indispensable part of theoretical modeling. We here present the (3+1)-dimensional diffusive relativistic hydrodynamic code BEShydro which solves the equations of motion of second-order Denicol–Niemi–Molnar–Rischke (DNMR) theory, including bulk and shear viscous currents and baryon diffusion currents. BEShydro features a modular structure that allows to easily turn on and off baryon evolution and different dissipative effects and thus to study their physical effects on the dynamical evolution individually. An extensive set of test protocols for the code, including several novel tests of the precision of baryon transport that can also be used to test other such codes, is documented here and supplied as a permanent part of the code package. Program summaryProgram Title:BEShydroProgram Files doi:http://dx.doi.org/10.17632/xwywvb2psm.1Licensing provisions: GPLv3Programming language: C++External routines/libraries: GNU Scientific Library (GSL)Nature of problem: (3+1)-dimensional dynamical evolution of hot and dense matter created in relativistic heavy-ion collisions using second-order dissipative relativistic fluid dynamics, including evolution of net baryon number and its dissipative diffusion current.Solution method: Runge–Kutta Kurganov–Tadmor algorithm.

Dense quark\u2013gluon plasma in strong magnetic fields

A non-perturbative (np) method of Field correlators (FCM) was applied to study QCD at temperatures above the deconfinement transition (1<T/T_c<3,~T_csim 0.16~mathrm{GeV}) and nonzero baryon densities (baryon chemical potential mu _B<0.5~mathrm{GeV}) in an external uniform magnetic field (eB<0.5~mathrm{GeV}^2). Within FCM, the np high-temperature dynamics is embodied in the Polyakov loop and in the Debye mass due to the Color-Magnetic confinement. Analytic expressions for quark pressure and magnetic susceptibility were obtained. The expressions were represented as series and in integral form. Magnetic susceptibility was found to increase rapidly with temperature and slowly with density. The results at the zero density limit are in agreement with lattice data.

Charged pion condensation and duality in dense and hot chirally and isospin asymmetric quark matter in the framework of the NJL2 model

In this paper we investigate in the large-$N_c$ limit ($N_c$ is the number of colored quarks) the phase structure of a massless (1+1)-dimensional quark model with four-quark interaction and in the presence of baryon ($\mu_B$), isospin ($\mu_I$) and chiral isospin ($\mu_{I5}$) chemical potentials as well as at nonzero temperature. It is established that chiral isospin chemical potential leads to the generation of charged pion condensation (PC) in dense (nonzero baryon density) and chiral asymmetric quark matter for a wide range of isospin densities. It is shown that there exists a duality correspondence between the chiral symmetry breaking and the charged PC phenomena at any values of temperature even for very hot quark gluon plasma. Moreover, it is shown that charged PC phase with nonzero baryon density can be induced in the model at comparatively high temperatures. This opens up new possible physical systems, where it can be of importance, such as heavy ion collisions, just born neutron stars (proto-neutron stars), supernovas as well as neutron star mergers.

The QCD crossover at zero and non-zero baryon densities from Lattice QCD

We map out the QCD crossover line Tc(μB)Tc(0)=1−κ2(μBTc(0))2−κ4(μBTc(0))+O(μB6) for the first time up to O(μB4) for a strangeness neutral system by performing a Taylor expansion of chiral observables in temperature T and chemical potentials μ. At vanishing chemical potential, we report a crossover temperature Tc(0)=(156.5±1.5)MeV defined by the average of several second-order chiral susceptibilities. For a system with thermal conditions appropriate for a heavy-ion collision, we determined a curvature from the subtracted condensate as κ2=0.0120(20) and from the disconnected susceptibility as κ2=0.0123(30). The next order κ4 is significantly smaller. We also report the crossover temperature as a function of the chemical potentials for: baryon-number, electric charge, strangeness and isospin. Additionally, we find that Tc(μB) is in agreement with lines of constant energy density and constant entropy density. Along this crossover line, we study net baryon-number fluctuations and show that their increase is substantially smaller compared to that obtained in HRG model calculations. Similarly, we analyze chiral susceptibility fluctuations along the crossover line and show that these are constant. We conclude that no signs for a narrowing of the crossover region can be found for baryon chemical potential μB<250MeV.

Duality and Charged Pion Condensation in Chirally Asymmetric Dense Quark Matter in the Framework of an NJL2 Model

In this talk we present investigation of the phase structure of a (1+1)-dimensional quark model with four-quark interaction and in the presence of baryon ([Formula: see text]), isospin ([Formula: see text]) and chiral isospin ([Formula: see text]) chemical potentials. Spatially homogeneous and inhomogeneous (chiral density wave (for chiral condensate) and single wave (for charged pion condensate)) condensates are considered. It is established that in the large-[Formula: see text] limit ([Formula: see text] is the number of colored quarks) there exists a duality correspondence between the chiral symmetry breaking phase and the charged pion condensation (PC) one. The primary conclusion of this investigation is the fact that chiral isospin chemical potential generates charged pion condensation with non-zero baryon density in dense quark matter. Moreover, it is shown that inhomogeneous charged PC phase with nonzero baryon density is induced in the model by arbitrary small values of the chemical potential [Formula: see text] (for a rather large region of [Formula: see text] and [Formula: see text]).

Affinity of NJL2 and NJL4 model results on duality and pion condensation in chiral asymmetric dense quark matter

In this paper we investigate the phase structure of a (1+1) and (3+1)-dimensional quark model with four-quark interaction and in the presence of baryon (μB), isospin (μI) and chiral isospin (μI5) chemical potentials. It is shown that the chemical potential μI5 promotes the appearance of the charged PC phase with nonzero baryon density. Results of both models are qualitatively the same, this fact enhances one's confidence in the obtained predictions. It is established that in the large-Nc limit (Nc is the number of colored quarks) there exists a duality correspondence between the chiral symmetry breaking phase and the charged pion condensation one.

Inhomogeneous charged pion condensation in chiral asymmetric dense quark matter in the framework of a NJL2 model

In this paper we investigate the phase structure of a ($1+1$)-dimensional quark model with four-quark interaction and in the presence of baryon (${\ensuremath{\mu}}_{B}$), isospin (${\ensuremath{\mu}}_{I}$), and chiral isospin (${\ensuremath{\mu}}_{I5}$) chemical potentials. Spatially inhomogeneous chiral density wave (for chiral condensate) and single wave (for charged pion condensate) approaches are used. It is established that in the large-${N}_{c}$ limit (${N}_{c}$ is the number of colored quarks) there exists a duality correspondence between the chiral symmetry breaking phase and the charged pion condensation (PC) one. Moreover, it is shown that inhomogeneous charged PC phase with nonzero baryon density is induced in the model by arbitrary small values of the chemical potential ${\ensuremath{\mu}}_{I5}$ (for a rather large region of ${\ensuremath{\mu}}_{B}$ and ${\ensuremath{\mu}}_{I}$).

Phenomenological consequences of enhanced bulk viscosity near the QCD critical point

In the proximity of the QCD critical point the bulk viscosity of quark-gluon matter is expected to be proportional to nearly the third power of the critical correlation length, and become significantly enhanced. This work is the first attempt to study the phenomenological consequences of enhanced bulk viscosity near the QCD critical point. For this purpose, we implement the expected critical behavior of the bulk viscosity within a non-boost-invariant, longitudinally expanding $1+1$ dimensional causal relativistic hydrodynamical evolution at non-zero baryon density. We demonstrate that the critically-enhanced bulk viscosity induces a substantial non-equilibrium pressure, effectively softening the equation of state, and leads to sizable effects in the flow velocity and single particle distributions at the freeze-out. The observable effects that may arise due to the enhanced bulk viscosity in the vicinity of the QCD critical point can be used as complimentary information to facilitate searches for the QCD critical point.

Complex Langevin simulations of a finite density matrix model for QCD

We study a random matrix model for QCD at finite density via complex Langevin dynamics. This model has a phase transition to a phase with nonzero baryon density. We study the convergence of the algorithm as a function of the quark mass and the chemical potential and focus on two main observables: the baryon density and the chiral condensate. For simulations close to the chiral limit, the algorithm has wrong convergence properties when the quark mass is in the spectral domain of the Dirac operator. A possible solution of this problem is discussed.