Dissipative properties of hot and dense hadronic matter in an excluded-volume hadron resonance gas model

We estimate dissipative properties, viz., shear and bulk viscosities of hadronic matter using relativistic Boltzmann equation in relaxation time approximation within the framework of excluded-volume hadron resonance gas (EHRG) model. We find that at zero baryon chemical potential the shear viscosity to entropy ratio (eta/s) decreases with temperature while at finite baryon chemical potential this ratio shows the same behavior as a function of temperature but reaches close to the Kovtun-Son-Starinets (KSS) bound. Further along the chemical freezeout curve, ratio eta/s is almost constant apart from small initial monotonic rise. This observation may have some relevance to the experimental finding that the differential elliptic flow of charged hadrons does not change considerably at lower center-of-mass energy. We further find that bulk viscosity to entropy density (zeta/s) decreases with temperature while this ratio has higher value at finite baryon chemical potential at higher temperature. Along the freezeout curve zeta/s decreases monotonically at lower center-of-mass energy and then saturates.