Bulk Viscosity of a Gas of Massless Pions

Understand

In the hadronic phase, the dominant configuration of QCD with two flavors of massless quarks is a gas of massless pions.

• We calculate the bulk viscosity (zeta) using the Boltzmann equation with the kinetic theory generalized to incorporate the trace anomaly.
• We find that the dimensionless ratio zeta/s, s being the entropy density, is monotonic increasing below T=120 MeV, where chiral perturbation theory is applicable.
• This, combined with previous results, shows that zeta/s reaches its maximum near the phase transition temperature Tc, while eta/s, eta being the shear viscosity, reaches its minimum near Tc in QCD with massless quarks.