Neutrino Masses and Leptogenesis from Extra Fermions

TL;DR

This paper explores how extending the Standard Model with heavy Majorana fermions can generate neutrino masses and enable leptogenesis, introducing new scenarios and revising key bounds to better understand early universe conditions.

Contribution

It classifies leptogenesis scenarios based on new fermion spectra, introduces singlet-triplet leptogenesis, and revises the Davidson-Ibarra bound, including models that violate it.

Findings

Classification of leptogenesis scenarios by fermion spectra

Introduction of singlet-triplet leptogenesis for the first time

Revised upper bound on CP asymmetry, dependent on neutrino hierarchy

Abstract

Generation of the neutrino masses and leptogenesis (LG) in the Standard Model extended by the heavy Majorana fermions is considered. Classification of LG scenarios according to the new fermion mass spectra is given, where singlet-triplet LG is considered for the first time. The upper bound on the CP asymmetry relevant for LG with hierarchical heavy neutrinos (Davidson-Ibarra bound) is revised, and shown that in the case of one massless neutrino it essentially depends on the type of the light neutrino mass hierarchy. The resonant scenarios, which help to avoid the problem of extremely high reheating temperature in the early universe, are discussed. In particular, we present new simplified, generalized and detailed formulation of freed LG, which violates Davidson-Ibarra bound in a special class of models.