Bulk viscosity of a gas of massless pions

In the hadronic phase, the dominant configuration of quantum chromodynamics (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 T-c while eta/s, eta being the shear viscosity, reaches its minimum near T-c in QCD with massless quarks.