Bulk viscosity for pion and nucleon thermal fluctuation in the hadron resonance gas model

摘要

We have calculated microscopically bulk viscosity of hadronic matter, whereequilibrium thermodynamics for all hadrons in medium are described by HadronResonance Gas (HRG) model. Considering pions and nucleons as abundant mediumconstituents, we have calculated their thermal widths, which inversely controlthe strength of bulk viscosities for respective components and represent theirin-medium scattering probabilities with other mesonic and baryonic resonances,present in the medium. Our calculations show that bulk viscosity increases withboth temperature and baryon chemical potential, whereas viscosity to entropydensity ratio decreases with temperature and with baryon chemical potential,the ratio increases first and then decreases. The decreasing nature of theratio with temperature is observed in most of the earlier investigations withfew exceptions. We find that the temperature dependence of bulk viscositycrucially depends on the structure of the relaxation time. Along the chemicalfreeze-out line in nucleus-nucleus collisions with increasing collision energy,bulk viscosity as well as the bulk viscosity to entropy density ratiodecreases, which also agrees with earlier references. Our results indicate thepicture of a strongly coupled hadronic medium.