On the Vanishing of the CP Asymmetry in Leptogenesis due to Form Dominance

TL;DR

This paper explains why CP asymmetry vanishes in certain leptogenesis models due to Form Dominance, and discusses how breaking this property can enable successful leptogenesis.

Contribution

It identifies Form Dominance as the key reason for vanishing CP asymmetry and explores how its violation can restore leptogenesis in models with tri-bimaximal mixing.

Findings

Form Dominance causes zero flavor-dependent CP asymmetries.

Breaking Form Dominance allows successful leptogenesis.

Models with indirect family symmetry and neutrino hierarchy can break Form Dominance.

Abstract

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 such models known as Form Dominance, corresponding to the case of a diagonal Casas-Ibarra R-matrix. Form Dominance leads to vanishing flavour-dependent CP asymmetries irrespective of whether one has tri-bimaximal mixing or a family symmetry. Successful leptogenesis requires violation of Form Dominance, but not necessarily violation of tri bimaximal mixing. This may be achieved in models where the family symmetry responsible for tri-bimaximal 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.