We emphasize that the vanishing of the CP asymmetry in leptogenesis,previously observed for models with tri-bimaximal mixing and family symmetry,may be traced to a property of the type I see-saw mechanism satisfied by suchmodels known as Form Dominance, corresponding to the case of a diagonalCasas-Ibarra R-matrix. Form Dominance leads to vanishing flavour-dependent CPasymmetries irrespective of whether one has tri-bimaximal mixing or a familysymmetry. Successful leptogenesis requires violation of Form Dominance, but notnecessarily violation of tri bimaximal mixing. This may be achieved in modelswhere the family symmetry responsible for tri-bimaximal mixing is implementedindirectly and a strong neutrino mass hierarchy is present with the FormDominance broken only softly by the right-handed neutrino responsible for thelightest neutrino mass, as in constrained sequential dominance.
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