QCT AT LARGE THETA-ANGLE AND AXION COSMOLOGY

We use the chiral Lagrangian to investigate the global properties of t he N-flavor QCD vacuum as a function of the theta parameter, in the ca se of exact quark degeneracy we find evidence for first-order phase tr ansitions at tau = pi . (odd integer). The first-order transitions are smoothed by quark mass splittings, although interesting effects remai n at realistic quark masses. We emphasize the role of the eta' condens ate in our analysis. Finally, we discuss the implications of our resul ts on the internal hadronic structure of axion domain walls and axion cosmology. (C) 1997 Elsevier Science B.V.