QCD at Large $\theta$ Angle and Axion Cosmology
  We use the chiral Lagrangian to investigate the global properties of the $N$-flavor QCD vacuum as a function of the $\theta$ parameter. In the case of exact quark degeneracy we find evidence for first order phase transitions at $\theta = \pi \cdot ({\rm odd~ integer})$. The first order transitions are smoothed by quark mass splittings, although interesting effects remain at realistic quark masses. We emphasize the role of the $\eta'$ condensate in our analysis. Finally, we discuss the implications of our results on the internal hadronic structure of axion domain walls and axion cosmology. 
N. Evans, S.D.H. Hsu, A. Nyffeler and M. Schwetz (Yale University)