An estimate of the bulk viscosity of the hadronic medium

TL;DR

This paper estimates the bulk viscosity of the hadronic medium using the Hadron Resonance Gas model, analyzing its dependence on spectrum, conformal symmetry breaking, and conditions relevant to heavy-ion collisions.

Contribution

It provides a novel estimation of the bulk viscosity within the HRG model including Hagedorn states, linking it to conformal symmetry breaking and freezeout conditions.

Findings

Bulk viscosity correlates with conformal symmetry breaking measure.

Higher $e/s$ at FAIR compared to LHC energies.

Bulk viscosity varies along freezeout curves in the $T-$ plane.

Abstract

The bulk viscosity ($\zeta$) of the hadronic medium has been estimated within the ambit of the Hadron Resonance Gas (HRG) model including the Hagedorn density of states. The HRG thermodynamics within a grand canonical ensemble provides the mean hadron number as well as its fluctuation. The fluctuation in the chemical composition of the hadronic medium in the grand canonical ensemble can result in non-zero divergence of the hadronic fluid flow velocity, allowing us to estimate the $\zeta$ of the hadronic matter upto a relaxation time. We study the influence of the hadronic spectrum on $\zeta$ and find its correlation with the conformal symmetry breaking (CSB) measure, $\epsilon-3P$. We estimate $\zeta$ along the contours with constant, $S/N_B$ (total entropy/net baryon number) in the $T-\mu$ plane (temperature-baryonic chemical potential) for $S/N_B=30, 45$ and 300. We also assess the value of $\zeta$ on the chemical freezeout curve for various centre of mass energy ($\sqrt{s_{NN}}$) and find that the bulk viscosity to entropy density ratio, $\zeta/s$ is larger at FAIR than LHC energies.