Constraints on leptogenesis from a symmetry viewpoint

It is shown that type-I seesaw models based on the Standard Model Lagrangian extended with three heavy Majorana right-handed fields do not have leptogenesis in leading order, if the symmetries of mass matrices are also the residual symmetry of the Lagrangian. In particular, flavor models that lead to a mass-independent leptonic mixing have a vanishing leptogenesis CP-asymmetry. Based on symmetry arguments, we prove that in these models the Dirac-neutrino Yukawa coupling combinations relevant for leptogenesis are always real in the physical basis where the charged leptons and heavy Majorana neutrinos are diagonal.