MeV-scale seesaw and leptogenesis

Valerie Domcke,Marco Drewes,Marco Hufnagel,Michele Lucente

doi:10.1007/jhep01(2021)200

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.

Highlights

JHEP01(2021)200 experimentally viable parameter space [7], cosmological constraints [8], and the perspectives for leptogenesis [9]
If all heavy neutrinos decay before big bang nucleosynthesis (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
Sterile neutrinos in the O(10 − 100) MeV mass range can alter our cosmological history and are strongly constrained by observations related to BBN and the cosmic microwave background (CMB)

Summary

Introduction

JHEP01(2021)200 experimentally viable parameter space [7], cosmological constraints [8], and the perspectives for leptogenesis [9]. A intriguing feature of this model is the fact that the same νR that give masses to the light neutrinos can explain the observed matter-antimatter asymmetry in the early universe, which is believed to be the origin of all baryonic matter that is present today.1 This is realized via the process of leptogenesis [12], which is feasible for a very wide range of possible Mi For Mi above the electroweak scale, the asymmetry is typically generated during the freeze-out and decay of the heavy neutrinos [12] (“freeze-out scenario”), while for Mi below the electroweak scale, it is instead generated during their production [14,15,16] (“freeze-in scenario”).2 It is well-known that leptogenesis is in principle feasible with Mi in the range of a few MeV [18]. In this mass range, the model parameter space is strongly constrained by laboratory experiments, cosmology, and astrophysics

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Discussion

Conclusion