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 (ζ) using the Boltzmann equation with the kinetic theory generalized to incorporate the trace anomaly. We find that the dimensionless ratio ζ/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 ζ/s reaches its maximum near the phase-transition temperature Tc, while η/s, η being the shear viscosity, reaches its minimum near Tc in QCD with massless quarks.