Bulk viscosity of quark matter across the QCD phase transitions

TL;DR

This paper investigates the bulk viscosity of quark matter across various QCD phase transitions using kinetic theory and the PNJL model, revealing how viscosity behaves near critical points and phase boundaries.

Contribution

It provides a detailed analysis of bulk viscosity and its ratio to shear viscosity across different QCD phase transitions, including the Mott, chiral crossover, and first-order transition, using a kinetic theory approach.

Findings

Bulk viscosity is very small at high temperatures, indicating near-conformal behavior.

Both $/s$ and $/$ increase near the chiral phase transition.

A peak in bulk viscosity appears beyond the chiral boundary, linked to strange quark crossover.

Abstract

Based on the kinetic theory with relaxation time approximation, we investigate the bulk viscosity ($\zeta$) and its ratio to shear viscosity ($\zeta/\eta$) of quark matter at finite temperature and chemical potential with the in-medium particle masses derived in the 2+1 flavor Polyakov-loop improved Nambu--Jona-Lasinio (PNJL) model. We explore the behaviors of specific bulk viscosity ($\zeta/s$) and $\zeta/\eta$ across different QCD phase transitions, including the Mott phase transition, the chiral crossover, and the first-order transition with the associated metastable phase. The calculation shows that both $\zeta/s$ and $\zeta/\eta$ are extremely small at high temperatures, approaching the nature of a conformal theory. Larger $\zeta/s$ and $\zeta/\eta$ are derived near the chiral phase transition at finite temperature. Along the chiral crossover line, $\zeta/s$ and $\zeta/\eta$ generally increase with decreasing temperature, though $\zeta/\eta$ exhibits a slight decline near the critical endpoint (CEP). On the boundary of the first-order transition, $\zeta/s$ shows a non-monotonic variation with temperature. Furthermore, an additional peak structure emerges beyond the chiral phase boundary for both $\zeta/s$ and $\zeta/\eta$, with magnitudes even exceeding those near the chiral crossover of $u, d$ quarks. Our analysis indicates this peak originates from the chiral crossover transformation of strange quark.