Parity Odd Bubbles in Hot Qcd

We consider the topological susceptibility for an SU(N) gauge theory in the limit of a large number of colors, N → ∞. At nonzero temperature, the behavior of the topological susceptibility depends upon the order of the deconfining phase transition. The most interesting possibility is if the deconfining transition, at T = Td, is of second order. Then we argue that Witten’s relation implies that the topological susceptibility vanishes in a calculable fashion at Td. As noted by Witten, this implies that for sufficiently light quark masses, metastable states which act like regions of nonzero θ — parity odd bubbles — can arise at temperatures just below Td. Experimentally, parity odd bubbles have dramatic signatures: the η meson, and especially the η meson, become light, and are copiously produced. Further, in parity odd bubbles, processes which are normally forbidden, such as η → ππ, are allowed. The most direct way to detect parity violation is by measuring a parity odd global asymmetry for charged pions, which we define.