Speed of sound of QCD matter at chiral crossover

TL;DR

This paper develops a unified equation of state for hot QCD matter using a generalized Beth-Uhlenbeck approach, analyzing the speed of sound and chiral symmetry restoration effects across a wide temperature range.

Contribution

It introduces a novel unified thermodynamic model of QCD matter that incorporates both hadronic and partonic degrees of freedom with chiral crossover behavior.

Findings

Speed of sound becomes negative in hadron gas due to chiral symmetry restoration.

Partonic excitations resolve the negative speed of sound issue.

Entropy density remains smooth across the chiral crossover.

Abstract

Based on a generalized Beth-Uhlenbeck approach to thermodynamics of QCD motivated by cluster decomposition we present a unified equation of state of hot strongly interacting matter and analyze its properties in a wide range of temperatures. The hadrons are treated as color singlet multiquark clusters in medium with a background gluon field in the Polyakov gauge. The confining aspect of QCD is accounted for by the Polyakov loop mechanism and by a large vacuum quark mass motivated by a confining density functional approach. We demonstrate that an abrupt switching between hadronic and partonic degrees of freedom, which is one of striking manifestations of dynamical restoration of chiral symmetry, is accompanied by a smooth behavior of entropy density at chiral crossover. Individual contributions of different components of strongly interacting matter to its speed of sound are analyzed for the first time. It is shown that restoration of chiral symmetry drives speed of sound of hadron gas to negative values, manifesting its mechanical instability and being in a strike disagreements with the lattice QCD data. Accounting for the partonic excitations naturally resolves this contradiction.