Vanishing of the $CP$ asymmetry in leptogenesis due to form dominance

We emphasize that the vanishing of the $CP$ asymmetry in leptogenesis, previously observed for models with tribimaximal mixing and family symmetry, may be traced to a property of the type I seesaw mechanism satisfied by such models known as form dominance, corresponding to the case of a diagonal Casas-Ibarra $R$-matrix. Form dominance leads to vanishing flavor-dependent $CP$ asymmetries irrespective of whether one has tribimaximal mixing or a family symmetry. Successful leptogenesis requires violation of form dominance, but not necessarily violation of tribimaximal mixing. This may be achieved in models where the family symmetry responsible for tribimaximal mixing is implemented indirectly and a strong neutrino mass hierarchy is present with the form dominance broken only softly by the right-handed neutrino responsible for the lightest neutrino mass, as in constrained sequential dominance.