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A. S. Khvorostukhin

Publications and source records attributed to A. S. Khvorostukhin.

15 recordsLinked to original sources
arXiv2021

Calculation of the one loop box integral at Finite Temperature and Density

Calculation of hadronization, decay or scattering processes at non-zero temperatures and densities within the Nambu-Jona-Lasinio-like models requires some techniques for computation of Feynmann diagrams. Decomposition of Feynman diagrams at the one loop level leads to the appearance of elementary integrals with one, two, three, and four fermion lines. For example, evaluation of the $ππ$ scattering amplitude requires calculating a box diagram with four fermion lines. In this work, the real and imaginary parts of the box integral at the one loop level are provided in the form suitable for numerical evaluation. The obtained expressions are applicable to any value of temperature, particle mass, and chemical potential. We pay special attention to the conditions for the existence of the appearing improper integrals and correct the results \cite{Klevansky} for the three fermion lines. As a result, we have obtained constraints on possible values of particle momenta. Among the expressions for the box integral, the general formulas for the integral with an arbitrary number of lines are derived for the case with zero or collinear fermion momenta.

hep-ph↗
arXiv2021

Hybrid model with viscous relativistic hydrodynamics: a role of constraints on the shear-stress tensor

We present the hybrid hadron string dynamic (HydHSD) model connecting the parton-hadron-string dynamic model (PHSD) and a hydrodynamic model taking into account shear viscosity within the Israel-Stewart approach. The performance of the code is tested on the pion and proton rapidity and transverse mass distributions calculated for Au+Au and Pb+Pb collision at AGS--SPS energies. The influence of the switch time from transport to hydro models, the viscous parameter, and freeze-out time are discussed. Since the applicability of the Israel-Stewart hydrodynamics assumes the perturbative character of the viscous stress tensor, $π^{μν}$, which should not exceed the ideal energy-momentum tensor, $T_{\rm id}^{μν}$, hydrodynamical codes usually rescale the shear stress tensor if the inequality $\|π^{μν}\|\ll \|T_{\rm id}^{μν}\|$ is not fulfilled in some sense. We show that the form of the corresponding condition plays an important role in the sensitivity of hydrodynamic calculations to the viscous parameter -- a ratio of the shear viscosity to the entropy density, $η/s$. It is shown that the constraints used in the vHLLE and MUSIC models give the same results for the observables. With these constraints, the rapidity distributions and transverse momentum spectra are most sensitive to a change of the $η/s$ ratio. As an alternative, a strict condition is used. We performed global fits the rapidity and transverse mass distribution of pion and protons. It was also found that $η/s$ as a function of the collision energy monotonically increases from $E_{\rm lab}=6A$GeV up to $E_{\rm lab}=40A$GeV and saturates for higher SPS energies. We observe that it is difficult to reproduce simultaneously pion and proton rapidity distribution within our model with the present choice of the equation of state without a phase transition.

nucl-th↗
arXiv2020

Constraints on shear stress tensor in viscous relativistic hydrodynamics

We extend our hybrid model HydHSD by taking into account shear viscosity within the Israel-Stewart hydrodynamics. The influence of different forms of $π^{μν}$ constraints on observables is analyzed. We show that the form of the corresponding condition plays an important role for the sensitivity of viscous hydrodynamics to the ratio of shear viscosity to the entropy density, $η/s$. It is shown that the constraint used in the vHLLE model, results in most sensitivity of rapidity distributions and transverse momentum spectra to a change of the $η/s$ ratio; however, their applicability for large values of $η/s$ is doubtful. On the contrary, the strict constraints from \cite{MNR2010} are very strong but most established. We also found that $η/s$ as a function of the collision energy probably has an extremum at $E_{\rm lab}=10.7$ AGeV. However, we obtain that any considered condition does not allow to reproduce simultaneously pion and proton experimental data within our model.

nucl-th↗
arXiv2016

Hadron rapidity spectra within a hybrid model

A 2-stage hybrid model is proposed that joins the fast initial state of interaction, described by the hadron string dynamics (HSD) model, to subsequent evolution of the expanding system at the second stage, treated within ideal hydrodynamics. The developed hybrid model is assigned to describe heavy-ion collisions in the energy range of the NICA collider under construction in Dubna. Generally, the model is in reasonable agreement with the available data on proton rapidity spectra. However, reproducing proton rapidity spectra, our hybrid model cannot describe the rapidity distributions of pions. The model should be improved by taking into consideration viscosity effects at the hydrodynamical stage of system evolution.

nucl-th↗
arXiv2013

Remarks concerning bulk viscosity of hadron matter in relaxation time ansatz

The bulk viscosity is calculated for hadron matter produced in heavy-ion collisions, being described in the relaxation time approximation withi n the relativistic mean- field-based model with scaled hadron masses and couplings. W e show how different approximations used in the literature affect the result. Nume rical evaluations of the bulk viscosity with three considered models deviate not much from each other confirming earlier results.

nucl-th↗
arXiv2012

Fractional exclusion statistics applied to relativistic nuclear matter

The effect of statistics of the quasiparticles in the nuclear matter at extreme conditions of density and temperature is evaluated in the relativistic mean-field model generalized to the framework of the fractional exclusion statistics (FES). In the model, the nucleons are described as quasiparticles obeying FES and the model parameters were chosen to reproduce the ground state properties of the isospin-symmetric nuclear matter. In this case, the statistics of the quasiparticles is related to the strengths of the nucleon-nucleon interaction mediated by the neutral scalar and vector meson fields. The relevant thermodynamic quantities were calculated as functions of the nucleons density, temperature and fractional exclusion statistics parameter $α$. It has been shown that at high temperatures and densities the thermodynamics of the system has a strong dependence on the statistics of the particles. The scenario in which the nucleon-nucleon interaction strength is independent of the statistics of particles was also calculated, but it leads in general to unstable thermodynamics.

hep-ph↗
arXiv2011

Relaxation time ansatz and shear and bulk viscosities of gluon matter

Shear and bulk viscosity-to-entropy density ratios are calculated for the pure gluon matter in a non-equilibrium mean-field quasiparticle approach within the relaxation time approximation. We study how different approximations used in the literature affect the results for the shear and bulk viscosities. Though the results for the shear viscosity turned out to be quite robust, all evaluations of the shear and bulk viscosities obtained in the framework of the relaxation time approximation can be considered only as rough estimations.

nucl-th↗
arXiv2011

Shear and bulk viscosities for pure glue matter

Shear $η$ and bulk $ζ$ viscosities are calculated in a quasiparticle model within a relaxation time approximation for pure gluon matter. Below $T_c$ the confined sector is described within a quasiparticle glueball model. Particular attention is paid to behavior of the shear and bulk viscosities near $T_c$. The constructed equation of state reproduces the first-order phase transition for the glue matter. It is shown that with this equation of state it is possible to describe the temperature dependence of the shear viscosity to entropy ratio $η/s$ and the bulk viscosity to entropy ratio $ζ/s$ in reasonable agreement with available lattice data but absolute values of the $ζ/s$ ratio underestimate the upper limits of this ratio in the lattice measurements typically by an order of magnitude.

nucl-th↗
arXiv2010

Viscosity coefficients for hadron and quark-gluon phases

The shear ($η$) and bulk ($ζ$) viscosities are calculated in a quasiparticle relaxation time approximation. The hadron phase is described within the relativistic mean field based model with scaled hadron masses and couplings. The quark phase is treated in terms of the heavy quark bag model fitted to the lattice data. A two-phase model allowing for the first order phase transition from the hadron phase to the strongly coupled quark gluon plasma is constructed by means of the Gibbs conditions. Temperature and baryon density dependence of the calculated viscosity-to-entropy ratios ($η/s$, $ζ/s$) are analyzed and compared with those obtained in other models. Special attention is paid to the behavior of viscosity coefficients near the critical temperature, from both hadron and quark-gluon side. Effects of resonance widths on viscosities and viscosity-to-entropy ratios are estimated.

nucl-th↗
arXiv2009

Viscosity of hadron matter within relativistic mean-field based model with scaled hadron masses and couplings

The shear ($η$) and bulk ($ζ$) viscosities are calculated in a quasiparticle relaxation time approximation for a hadron matter described within the relativistic mean-field based model with scaled hadron masses and couplings. Comparison with results of other models is presented. We demonstrate that a small value of the shear viscosity to entropy density ratio required for explaining a large elliptic flow observed at RHIC may be reached in the hadron phase. Large values of the bulk viscosity are noted in case of the baryon enriched matter.

nucl-th↗
arXiv2008

Relativistic Mean-Field Model with Scaled Hadron Masses and Couplings

Here we continue to elaborate properties of the relativistic mean-field based model (SHMC) proposed in ref. [6] where hadron masses and coupling constants depend on the $σ$-meson field. The validity of approximations used in [6] is discussed. We additionally incorporate contribution of meson excitations to the equations of motion. We also estimate the effects of the particle width. It is demonstrated that the inclusion of the baryon-baryon hole and baryon-antibaryon loop terms, if performed perturbatively, destroys the consistency of the model.

nucl-th↗
arXiv2007

Equation of State for Hot and Dense Matter: $σ$-$ω$-$ρ$ Model with Scaled Hadron Masses and Couplings

The proposed earlier relativistic mean-field model with hadron masses and coupling constants depending on the $σ$-meson field is generalized to finite temperatures. Within this approach we simulate the in-medium behavior of the hadron masses motivated by the Brown-Rho scaling. The high-lying baryon resonances and boson excitations as well as excitations of the $σ$, $ω$ and $ρ$ fields interacting via mean fields are incorporated into this scheme. Thermodynamic properties of hot and dense hadronic matter are elaborated with the constructed equation of state. Even at zero baryon density, effective masses of $σ$-$ω$-$ρ$-$N$ excitations abruptly drop down for $T\gsim 170$ MeV and reach zero at a critical temperature $T=T_{cσ}\sim 210$ MeV. Below $T_{cσ}$ (at $T\sim 190$ MeV) the specific heat gets a peak like at crossover. We demonstrate that our EoS can be matched with that computed on the lattice for high temperatures provided the baryon resonance couplings with nucleon are partially suppressed. In this case the quark liquid would masquerade as the hadron one. The model is applied to description of heavy ion collisions in a broad collision energy range. It might be especially helpful for studying phase diagram in the region near possible phase transitions.

nucl-th↗
arXiv2006

Lattice QCD Constraints on the Nuclear Equation of State

Based on the quasi-particle description of the QCD medium at finite temperature and density we formulate the phenomenological model for the equation of state that exhibits crossover or the first order deconfinement phase transition. The models are constructed in such a way to be thermodynamically consistent and to satisfy the properties of the ground state nuclear matter comply with constraints from intermediate heavy--ion collision data. Our equations of states show quite reasonable agreement with the recent lattice findings on temperature and baryon chemical potential dependence of relevant thermodynamical quantities in the parameter range covering both the hadronic and quark--gluon sectors. The model predictions on the isentropic trajectories in the phase diagram are shown to be consistent with the recent lattice results. Our nuclear equations of states are to be considered as an input to the dynamical models describing the production and the time evolution of a thermalized medium created in heavy ion collisions in a broad energy range from SIS up to LHC.

nucl-th↗
arXiv2005

Lattice QCD Constraints on Hybrid and Quark Stars

A QCD-motivated dynamical-quasiparticle model with parameters adjusted to reproduce the lattice-QCD equation of state is extrapolated from region of high temperatures and moderate baryonic densities to the domain of high baryonic densities and zero temperature. The resulting equation of state matched with realistic hadronic equations of state predicts a phase transition into the quark phase at higher densities than those reachable in neutron star interiors. This excludes the possibility of the existence of hybrid (hadron-quark) stars. Pure quark stars are possible and have low masses, small radii and very high central densities. Similar results are obtained for a simple bag model with massive quarks, fitted to reproduce the same lattice results. Self-bound quark matter is also excluded within these models. Uncertainties in the present extrapolation re discussed. Comparison with standard bag models is made.

astro-ph↗