Topology in the SU(N_f) chiral symmetry restored phase of unquenched QCD and axion cosmology II

Abstract

We investigate the physical consequences of the survival of the effects of the U(1)_A anomaly in the chiral symmetric phase of QCD, and show that the free energy density is a singular function of the quark mass m, in the chiral limit, and that the $\sigma$ and $\bar\pi$ susceptibilities diverge in this limit at any $T\ge T_c$. We also show that the difference between the $\bar\pi$ and $\bar\delta$ susceptibilities diverges in the chiral limit at any $T\ge T_c$, a result which seems to be excluded by recent results of Tomiya et al. from numerical simulations of two-flavor QCD. We also discuss on the generalization of these results to the $N_f\ge 3$ model.