The Seesaw with Many Right-Handed Neutrinos

TL;DR

This paper explores the theoretical and phenomenological implications of models with many right-handed neutrinos in the seesaw mechanism, highlighting potential enhancements in lepton-flavour violation and relaxed leptogenesis bounds.

Contribution

It investigates the effects of having up to O(100) right-handed neutrinos on neutrino mass scales and related phenomenology, extending traditional seesaw models.

Findings

Lepton-flavour violation may be significantly enhanced with many right-handed neutrinos.

The seesaw mass scale can be much higher than in traditional models.

Bounds on reheating temperature and neutrino mass are relaxed.

Abstract

There are no upper limits on the possible number of massive, singlet (right--handed) neutrinos that may participate in the seesaw mechanism, and some string constructions motivate seesaw models with up to O(100) right--handed neutrinos. In this case, the seesaw mass scale can be significantly higher than that in the traditional scheme with just 3 right--handed neutrinos. We consider the possible phenomenological implications of such models, in particular, for lepton-flavour violation and electric dipole moments. Since the neutrino masses depend on the Majorana mass scale linearly, while supersymmetric loop corrections depend on it logarithmically, the magnitude of lepton-flavour- and CP-violating transitions may increase with the multiplicity of the right--handed neutrinos and may be enhanced by orders of magnitude. We also point out that, in the context of leptogensis, the bounds on the reheating temperature and the lightest neutrino mass get relaxed compared to those in the case of 3 right--handed neutrinos.