Speed of sound in QCD matter at finite temperature and density]{Speed of sound in QCD matter at finite temperature and density

TL;DR

This paper investigates how the speed of sound varies in QCD matter at different temperatures and densities using the PNJL model, revealing its relation to phase transitions and stability regions.

Contribution

It introduces a systematic analysis of the speed of sound across different QCD phases and derives the boundary where sound velocity vanishes in the phase diagram.

Findings

Speed of sound varies across stable, metastable, and unstable phases.

Boundary of zero sound velocity is identified in the temperature-density phase diagram.

Regions where the sound wave equation breaks down are pointed out.

Abstract

The speed of sound in QCD matter at finite temperature and density is investigated within the Polyakov loop improved Nambu--Jona-Lasinio (PNJL) model. The spinodal structure associated with the chiral first-order chiral phase transition is considered to describe the continuous variation of the speed of sound. The behaviors of the squared sound speed in different phases, including the stable, metastable and unstable phases, are derived. The relation between speed of sound and QCD phase transitions is systematically explored. In particular, the boundary of vanishing sound velocity is derived in the temperature-density phase diagram, and the region where the sound wave equation being broken is pointed out. Some interesting features of speed of sound under different definitions are also discussed.