Shear and bulk viscosities of the hadron gas within relaxation time approximation and its test

Abstract

We concentrate on calculation of the shear and bulk viscosities of the hadron gas. They define its dissipative dynamics and influence its experimentally measurable elliptic flow. Due to difficulty of this calculation the relaxation time approximation (RTA) was used in previous works. As those results have approached the realistic ones, there is a need to find out how accurate the RTA is. For this sake we calculate the viscosities in the RTA using cross sections extracted from the ultrarelativistic quantum molecular dynamics (UrQMD) model and compare them with the same ones calculated without the RTA. This allows us to find the estimates of errors due to the application of RTA in the calculations of the viscosities, which are valid also for other similar models. For instance, in the temperature region $100 MeV \lesssim T \lesssim 160 MeV$ at zero chemical potentials the shear viscosity becomes smaller up to $1.57$ times, or up to $1.45$ times if the averaged relaxation time is used. This has important consequences for interpretation of the previously made calculations of the viscosities and some other related calculations. Within the RTA, we also find estimation of the enhancement of the bulk viscosity of the hadron gas because of nonconservation of particle numbers. This is a little more extended version in compare to the published paper, where we were limited in paper's size and time.