Casas-Ibarra Parametrization and Unflavored Leptogenesis

The Casas-Ibarra parametrization is a description of the Dirac neutrino mass matrix M_D in terms of the neutrino mixing matrix V, an orthogonal matrix O and the diagonal mass matrices of light and heavy Majorana neutrinos in the type-I seesaw mechanism. Because M^\dagger_D M_D is apparently independent of V but dependent on O in this parametrization, a number of authors have taken it for grant that unflavored leptogenesis has nothing to do with CP violation at low energies. Here we question this observation by clarifying the physical meaning of O. We establish a clear relationship between O and the observable quantities, and find that O does depend on V. Our general conclusion is that both unflavored leptogenesis and flavored leptogenesis have no direct connection with low-energy CP violation.