MeV-scale Seesaw and Leptogenesis
  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.
