Perturbativity in the seesaw mechanism

TL;DR

This paper analyzes how the perturbativity constraints in the seesaw mechanism limit the mixing of the lightest right-handed neutrino, impacting models like leptogenesis and linking high-energy theories to low-energy experiments.

Contribution

It provides a detailed analysis of perturbativity bounds on right-handed neutrino mixing, highlighting their implications for neutrino mass models and high-energy phenomena.

Findings

Perturbativity imposes strict upper bounds on lightest neutrino mixing.

Heavier neutrinos require larger Yukawa couplings, constrained by perturbativity.

Low-energy experiments can test high-energy leptogenesis models.

Abstract

We consider the Standard Model extended by right-handed neutrinos to explain massive neutrinos through the seesaw mechanism. The new fermion can be observed when it has a sufficiently small mass and large mixings to left-handed neutrinos. If such a particle is the lightest right-handed neutrino, its contribution to the mass matrix of active neutrinos needs to be canceled by that of a heavier one. Yukawa couplings of the heavier one are then larger than those of the lightest one. We show that the perturbativity condition gives a severe upper bound on the mixing of the lightest right-handed neutrino, depending on the masses of heavier ones. Models of high energy phenomena, such as leptogenesis, can be constrained by low energy experiments.