Bulk viscosity of QCD matter near the critical temperature

TL;DR

This paper derives a relation between bulk viscosity and thermodynamic quantities in QCD matter near the critical temperature, using lattice data to show that bulk viscosity peaks near deconfinement.

Contribution

It introduces a new formula linking bulk viscosity to energy density and pressure in QCD, and uses lattice data to quantify its temperature dependence near the critical point.

Findings

Bulk viscosity increases sharply near the deconfinement temperature.

The viscosity-to-entropy ratio zeta/s approaches 1 near the critical point.

The derived relation provides a way to estimate bulk viscosity from thermodynamic data.

Abstract

Kubo's formula relates bulk viscosity to the retarded Green's function of the trace of the energy-momentum tensor. Using low energy theorems of QCD for the latter we derive the formula which relates the bulk viscosity to the energy density and pressure of hot matter. We then employ the available lattice QCD data to extract the bulk viscosity as a function of temperature. We find that close to the deconfinement temperature bulk viscosity becomes large, with viscosity-to-entropy ratio zeta/s about 1.