Viscosity of a net-baryon fluid near the QCD critical point

TL;DR

This paper analytically investigates the shear and bulk viscosities of the net-baryon fluid near the QCD critical point, revealing bounds on shear viscosity and a small but increasing bulk viscosity approaching the critical temperature.

Contribution

It provides a theoretical analysis of the viscosities of the net-baryon fluid near the QCD critical point, incorporating universality and strong coupling constraints.

Findings

Shear viscosity is bounded between 1.6 and 3.7 in 4πη/s for temperatures above T_c.

Bulk viscosity remains small but increases sharply as temperature approaches T_c.

Viscosity behavior is consistent with critical phenomena near the QCD critical point.

Abstract

In the dynamics of the QCD critical point, the net-baryon fluid, linked to the slow component of the order parameter, relaxes to a 3d Ising system in equilibrium. An analytical study of shear and bulk viscosity, with constraints imposed by the dynamics of the critical net-baryon fluid, the universality property and the requirements of a class of strong coupling theories, is performed in the neighbourhood of the critical point. It is found that the shear viscosity of the net-baryon fluid is restricted in the domain $1.6\leq 4\pi\frac{\eta}{s}\leq 3.7$ for $T_c < T \leq 2T_c$ whereas the bulk viscosity is small, $4\pi\frac{\zeta}{s} < 0.05$ (for $T>1.27 T_c$) but rising towards the singularity at $T=T_c$.