Chiral restoration effects on the shear viscosity of a pion gas

TL;DR

This paper studies how chiral symmetry restoration at finite temperature affects the shear viscosity of a pion gas, revealing significant medium modifications and a notable reduction in the viscosity-to-entropy ratio near the crossover region.

Contribution

It introduces a relativistic kinetic theory approach using the Nambu-Jona-Lasinio model to incorporate medium effects on pion interactions and mass at finite temperature.

Findings

Shear viscosity decreases significantly near the crossover region.

The eta/s ratio is strongly reduced due to medium modifications.

Results are sensitive to the approximations used.

Abstract

We investigate the shear viscosity of a pion gas in relativistic kinetic theory, using the Nambu-Jona-Lasinio model to construct the pion mass and the pi-pi interaction at finite temperature. Whereas at low temperatures the scattering properties and, hence, the viscosity are in agreement with lowest-order chiral perturbation theory, we find strong medium modifications in the crossover region. Here the system is strongly coupled and the scattering lengths diverge, similarly as for ultra-cold Fermi gases at a Feshbach resonance. As a consequence, the ratio eta/s is found to be strongly reduced as compared to calculations without medium-modified masses and scattering amplitudes. However, the quantitative results are very sensitive to the details of the applied approximations.