Type I seesaw mechanism for quasi degenerate neutrinos

TL;DR

This paper explores how symmetries and the minimal flavour violation principle can lead to quasi-degenerate neutrino masses in type-I seesaw models, successfully explaining neutrino properties and baryon asymmetry.

Contribution

It introduces a general framework for quasi-degenerate neutrinos in type-I seesaw models based on symmetry principles and minimal flavour violation, applicable beyond specific Yukawa structures.

Findings

Neutrino mass matrix approximates to a simple form compatible with observed mixing.

The scenario can explain the baryon asymmetry via thermal leptogenesis with flavor effects.

Lepton flavor violating processes may be observable in supersymmetric versions.

Abstract

We discuss symmetries and scenarios leading to quasi-degenerate neutrinos in type-I seesaw models. The existence of degeneracy in the present approach is not linked to any specific structure for the Dirac neutrino Yukawa coupling matrix $y_D$ and holds in general. Basic input is the application of the minimal flavour violation principle to the leptonic sector. Generalizing this principle, we assume that the structure of the right handed neutrino mass matrix is determined by $y_D$ and the charged lepton Yukawa coupling matrix $y_l$ in an effective theory invariant under specific groups ${\cal G}_F$ contained in the full symmetry group of the kinetic energy terms. ${\cal G}_F$ invariance also leads to specific structure for the departure from degeneracy. The neutrino mass matrix (with degenerate mass $m_0$) resulting after seesaw mechanism has a simple form ${\cal M}_\nu\approx m_0(I-p y_ly_l^T)$ in one particular scenario based on supersymmetry. This form is shown to lead to correct description of neutrino masses and mixing angles. The thermal leptogenesis after inclusion of flavour effects can account for the observed baryon asymmetry of the universe within the present scenario. Rates for lepton flavour violating processes can occur at observable levels in the supersymmetric version of the scenario.