The Bulk Viscosity of a Pion Gas

TL;DR

This paper calculates the bulk viscosity of a pion gas at low temperatures using chiral perturbation theory, revealing an exponential increase in viscosity as temperature decreases due to slow number-changing processes.

Contribution

It provides the first calculation of pion gas bulk viscosity at low temperatures within chiral perturbation theory, highlighting the exponential growth near the dilute limit.

Findings

Bulk viscosity increases exponentially as temperature decreases.

Number-changing processes slow down exponentially at low temperatures.

Viscosity formula involves exponential dependence on inverse temperature.

Abstract

We compute the bulk viscosity of a gas of pions at temperatures below the QCD crossover temperature, for the physical value of pion mass, to lowest order in chiral perturbation theory. Bulk viscosity is controlled by number-changing processes which become exponentially slow at low temperatures when the pions become exponentially dilute, leading to an exponentially large bulk viscosity zeta ~ (F_0^8/m_\pi^5) exp(2m_\pi/T), where F_0 = 93 MeV is the pion decay constant.