Resonant enhancement in leptogenesis

TL;DR

This paper reviews the mechanisms of resonant enhancement in leptogenesis, focusing on the conditions under which CP asymmetry is maximized when sterile neutrino states are nearly degenerate, and compares different theoretical formalisms for this regime.

Contribution

It systematically compares various methods to describe resonant leptogenesis and discusses when simplified formulas for CP asymmetry are valid.

Findings

Resonant enhancement allows lower right-handed neutrino mass scales in leptogenesis.

Coherent flavor transitions are crucial near the resonance condition.

Different formalisms can be used to accurately describe the resonant regime.

Abstract

Vanilla leptogenesis within the type I seesaw framework requires the mass scale of the right-handed neutrinos to be above 10^9 GeV. This lower bound can be avoided if at least two of the sterile states are almost mass degenerate, which leads to an enhancement of the decay asymmetry. Leptogenesis models that can be tested in current and upcoming experiments often rely on this resonant enhancement, and a systematic and consistent description is therefore necessary for phenomenological applications. In this review article, we give an overview of different methods that have been used to study the saturation of the resonant enhancement when the mass difference becomes comparable to the characteristic width of the Majorana neutrinos. In this limit, coherent flavor transitions start to play a decisive role, and off-diagonal correlations in flavor space have to be taken into account. We compare various formalisms that have been used to describe the resonant regime and discuss under which circumstances the resonant enhancement can be captured by simplified expressions for the CP asymmetry. Finally, we briefly review some of the phenomenological aspects of resonant leptogenesis.