Tuesday, January 29, 2013

1301.6670 (Oleg N. Moroz)

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

Oleg N. Moroz
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 corrected, and the consequences of the corrections for the freeze-out temperatures are discussed. A special treatment of the resonances is implemented additionally. 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. In qualitative calculations the maximum of the bulk viscosity due to the hadron gas mass spectrum is found at $T=190 MeV$ with zero chemical potentials. It's expected that this value can be used as an estimation of the critical temperature. A misleading viewpoint, resulting in mistakes in the calculations of the bulk viscosity in some papers, is commented on. 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 accomplish 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.
View original: http://arxiv.org/abs/1301.6670

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