Leptogenesis from Lighter-than-Pion Neutrinos

We address the Type-I Seesaw mechanism with three right-handed neutrinos and Majorana masses below the pion mass: we identify three distinct potentially viable regions in the parameter space, characterised by the value of the sterile neutrino lifetime, and show that the impact of the sterile neutrino decay products on the abundance of light elements via photodisintegration (together with complementary existing bounds) effectively rules out solutions where the decays take place between BBN and the CMB decoupling. Remarkably, solutions in which all the three heavy neutrinos decay before BBN can generally account for the observed baryon asymmetry of the Universe via freeze-in leptogenesis, without further requirements other than complying with laboratory and cosmological constraints. We conclude that leptogenesis can be tested in pion decay experiments.