Towards precise calculation of transport coefficients in the hadron gas.   The shear and the bulk viscosities
  The shear and the bulk viscosities of the hadron gas at low temperatures are studied in the model with constant elastic cross sections being relativistic generalization of the hard spheres model. One effective radius ${r=0.4 fm}$ is chosen for all elastic collisions. Only elastic collisions are considered which are supposed to be dominant at temperatures ${T\leq 120-140 MeV}$. The calculations are done in the framework of the Boltzmann equation with the Boltzmann statistics distribution functions and the ideal gas equation of state. The applicability of these approximations is discussed. 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 mass spectrum of hadrons. The constant cross sections and the Boltzmann statistics approximation allows one not only to conduct precise numerical calculations of transport coefficients in the hadron gas but also to obtain some relatively simple relativistic analytical closed-form expressions. Namely, the correct single-component first-order shear viscosity coefficient is found. The single-component first-order nonequilibrium distribution function, some analytical results for the binary mixture and expressions for mean collision rates, mean free paths and times are presented. Comparison with some previous calculations for the hadron gas and the pion gas is done too. This paper is the first step towards calculations with inelastic processes included. 
Oleg N. Moroz
hep-ph physics.flu-dyn