Non-unitary Leptonic Mixing and Leptogenesis

TL;DR

This paper explores how non-unitarity in leptonic mixing matrices influences leptogenesis, revealing new mechanisms for generating baryon asymmetry with lower right-handed neutrino mass scales than traditional models.

Contribution

It demonstrates the connection between non-unitarity, dimension six operators, and leptogenesis, providing a novel framework for understanding baryon asymmetry at lower neutrino mass scales.

Findings

Leptogenesis can be driven by non-unitarity effects without resonance.

The lower bound on right-handed neutrino mass is reduced to about 10^8 GeV.

Flavor-dependent decay asymmetries are enhanced in low-energy seesaw scenarios.

Abstract

We investigate the relation between non-unitarity of the leptonic mixing matrix and leptogenesis. We discuss how all parameters of the canonical type-I seesaw mechanism can, in principle, be reconstructed from the neutrino mass matrix and the deviation of the effective low-energy leptonic mixing matrix from unitary. When the mass M' of the lightest right-handed neutrino is much lighter than the masses of the others, we show that its decay asymmetries within flavour-dependent leptogenesis can be expressed in terms of two contributions, one depending on the unique dimension five (d=5) operator generating neutrino masses and one depending on the dimension six (d=6) operator associated with non-unitarity. In low-energy seesaw scenarios where small lepton number violation explains the smallness of neutrino masses, the lepton number conserving d=6 operator contribution generically dominates over the d=5 operator contribution which results in a strong enhancement of the flavour-dependent decay asymmetries without any resonance effects. To calculate the produced final baryon asymmetry, the flavour equilibration effects directly related to non-unitarity have to be taken into account. In a simple realization of this non-unitarity driven leptogenesis, the lower bound on M' is found to be about 10^8 GeV at the onset of the strong washout regime, more than one order of magnitude below the bound in "standard" thermal leptogenesis.