Speed of sound in QCD matter

TL;DR

This paper investigates how the speed of sound in QCD matter varies with temperature and chemical potential using the PNJL model, revealing features related to phase transitions and implications for hydrodynamics.

Contribution

It introduces new features of the speed of sound in QCD matter, including hierarchy and near-zero values near critical points, and relates different definitions of sound velocity.

Findings

Speed of sound depends on temperature and chemical potential.

Hierarchy of sound velocity for different quark flavors at low temperature.

Squared sound velocity approaches zero near the critical region.

Abstract

We systematically investigate the speed of sound in QCD matter under different conditions in the grand canonical ensemble within the Polyakov loop improved Nambu--Jona-Lasinio (PNJL) model. The numerical results indicate that the dependence of speed of sound on parameters like temperature and chemical potential can be indicative of QCD phase transition. Some new features of speed of sound are discovered, for instance, the hierarchy of sound velocity for $u(d)$ and $s$ quark at low temperature with the increasing chemical potential and the squared sound velocity approaching to almost zero in the critical region. We also formulate the relations between differently defined sound velocity using the fundamental thermodynamic relations. Some conclusions derived are useful for hydrodynamics simulation and calculation of transport coefficient of bulk viscosity.