Analytical formulas, general properties and calculation of transport coefficients in the hadron gas: shear and bulk viscosities

TL;DR

This paper presents detailed calculations of shear and bulk viscosities in the hadron gas using improved cross sections and resonance treatments, providing analytical formulas and insights into their temperature dependence near the QCD transition.

Contribution

It introduces a comprehensive method for calculating transport coefficients in the hadron gas, including analytical formulas and improved cross section analysis, enhancing hydrodynamical modeling accuracy.

Findings

Bulk viscosity peaks around 150-190 MeV temperature range.

Bulk viscosity in hadron gas is significantly larger than in pion gas.

Shear viscosity is less sensitive to hadronic mass spectrum.

Abstract

Elaborated calculations of the shear and the bulk viscosities in the hadron gas, using the ultrarelativistic quantum molecular dynamics (UrQMD) model cross sections, are made. These cross sections are analyzed and improved. A special treatment of the resonances is implemented additionally. All this allows for better hydrodynamical description of the experimental data. The previously considered approximation of one constant cross section for all hadrons is justified. It's found that the bulk viscosity of the hadron gas is much larger than the bulk viscosity of the pion gas while the shear viscosity is found to be less sensitive to the hadronic mass spectrum. The maximum of the bulk viscosity of the hadron gas is expected to be approximately in the temperature range ${T=150 190 MeV}$ with zero chemical potentials. This range covers the critical temperature values found from lattice calculations. We comment on some important aspects of calculations of the bulk viscosity, which were not taken into account or were not analyzed well previously. Doing this, a generalized Chapman-Enskog procedure, taking into account deviations from the chemical equilibrium, is outlined. Some general properties, features, the physical meaning of the bulk viscosity and some other comments on the deviations from the chemical equilibrium supplement this discussion. Analytical closed-form expressions for the transport coefficients and some related quantities within a quite large class of cross sections can be obtained. Some examples are explicitly considered. Comparisons with some previous calculations of the viscosities in the hadron gas and the pion gas are done.