Reconciling leptogenesis with observable mu --> e gamma rates

TL;DR

This paper demonstrates that in seesaw models with approximate lepton number conservation, large lepton flavor violation rates can coexist with successful thermal leptogenesis, especially when right-handed neutrino mass splittings are small and stable.

Contribution

It shows that large Yukawa couplings and low seesaw scales can produce observable lepton flavor violation without hindering leptogenesis, due to small, stable neutrino mass splittings.

Findings

Large Yukawa couplings do not prevent leptogenesis due to small washout effects.

Small, radiatively stable neutrino mass splittings enable observable lepton flavor violation.

Leptogenesis can be compatible with observable mu --> e gamma rates in these models.

Abstract

We perform a detailed analysis of thermal leptogenesis in the framework of seesaw models which approximately conserve lepton number. These models are known to allow for large Yukawa couplings and a low seesaw scale in agreement with neutrino mass constraints, and hence to lead to large lepton flavour violating rates that can be probed experimentally. Although large Yukawa couplings lead to (inverse) decay rates much larger than the Hubble expansion rate, we show that the leptogenesis washout induced is generically small if the mass splitting between the right-handed neutrinos is small enough. As a result, large lepton flavour violating rates are compatible with successful leptogenesis. We emphasize that this scenario does not require any particular flavour structure. A small splitting is natural and radiatively stable in this context because it is protected by the lepton number symmetry.