Minimal flavour violation extensions of the seesaw

TL;DR

This paper explores minimal lepton flavour violation models compatible with type-I seesaw, introducing a new flavor symmetry that enhances certain muon-to-electron transitions, and discusses implications for neutrino mass scales.

Contribution

It proposes a novel flavor symmetry realization within minimal lepton flavour violation compatible with type-I seesaw, enabling enhanced LFV effects and insights into neutrino mass scale suppression.

Findings

New flavor symmetry $SU(3)_e\times SU(3)_{\ell+N}$ identified

Enhanced $\mu \to e$ transitions in normal hierarchy scenario

Seesaw symmetries allow lower N mass scale without suppressing lepton number breaking

Abstract

We analyze the most natural formulations of the minimal lepton flavour violation hypothesis compatible with a type-I seesaw structure with three heavy singlet neutrinos N, and satisfying the requirement of being predictive, in the sense that all LFV effects can be expressed in terms of low energy observables. We find a new interesting realization based on the flavour group $SU(3)_e\times SU(3)_{\ell+N}$ (being $e$ and $\ell$ respectively the SU(2) singlet and doublet leptons). An intriguing feature of this realization is that, in the normal hierarchy scenario for neutrino masses, it allows for sizeable enhancements of $\mu \to e$ transitions with respect to LFV processes involving the $\tau$ lepton. We also discuss how the symmetries of the type-I seesaw allow for a strong suppression of the N mass scale with respect to the scale of lepton number breaking, without implying a similar suppression for possible mechanisms of N production