Viscosity in the excluded volume hadron gas model

Abstract

The shear viscosity $\hat{I}\cdot$ in the van der Waals excluded volume hadron-resonance gas model is considered. For the shear viscosity the result of the non-relativistic gas of hard-core particles is extended to the mixture of particles with different masses, but equal values of hard-core radius r. The relativistic corrections to hadron average momenta in thermal equilibrium are also taken into account. The ratio of the viscosity $\hat{I}\cdot$ to the entropy density s is studied. It monotonously decreases along the chemical freeze-out line in nucleus-nucleus collisions with increasing collision energy. As a function of hard-core radius r, a broad minimum of the ratio $\hat{I}\cdot /s\approx 0.3$ near $r\approx 0.5$ fm is found at high collision energies. For the charge-neutral system at $T=T_c=180$ MeV, a minimum of the ratio $\hat{I}\cdot /s\cong 0.24$ is reached for $r\cong 0.53$ fm. To justify a hydrodynamic approach to nucleus-nucleus collisions within the hadron phase the restriction from below, $r\ge 0.2$ fm, on the hard-core hadron radius should be fulfilled in the excluded volume hadron-resonance gas.

12 pages, 3 figures