MeV-scale Seesaw and Leptogenesis

TL;DR

This paper investigates the parameter space of the MeV-scale type-I seesaw model with three right-handed neutrinos, showing that constraints from cosmology and experiments significantly limit viable scenarios and that low-scale leptogenesis can explain the baryon asymmetry.

Contribution

It provides a comprehensive analysis of the MeV-scale seesaw model, incorporating new cosmological constraints and demonstrating the viability of low-scale leptogenesis without additional tuning.

Findings

Photodisintegration constraints rule out certain decay scenarios.

Heavy neutrinos decaying before BBN can explain baryon asymmetry.

Motivates experimental searches in pion decay experiments.

Abstract

We study the type-I seesaw model with three right-handed neutrinos and Majorana masses below the pion mass. In this mass range, the model parameter space is not only strongly constrained by the requirement to explain the light neutrino masses, but also by experimental searches and cosmological considerations. In the existing literature, three disjoint regions of potentially viable parameter space have been identified. In one of them, all heavy neutrinos decay shortly before big bang nucleosynthesis. In the other two regions, one of the heavy neutrinos either decays between BBN and the CMB decoupling or is quasi-stable. We show that previously unaccounted constraints from photodisintegration of nuclei practically rule out all relevant decays that happen between BBN and the CMB decoupling. Quite remarkably, if all heavy neutrinos decay before BBN, the baryon asymmetry of the universe can be quite generically explained by low-scale leptogenesis, i.e. without further tuning in addition to what is needed to avoid experimental and cosmological constraints. This motivates searches for heavy neutrinos in pion decay experiments.