Leptogenesis without on-shell right-handed neutrinos

Abstract

We propose a novel mechanism for generating the baryon asymmetry of the Universe through leptogenesis in a scenario where the right-handed neutrinos are heavier than the maximal temperature of the Universe, and are never produced on-shell neither by thermal nor by non-thermal mechanisms. We introduce a new scalar field, $\phi$, lighter than the right-handed neutrinos, that couples to the latter via a Yukawa coupling, so that it decays into two lepton doublets and two higgs doublets via off-shell right-handed neutrinos. Then, we derive the CP asymmetry arising from the interference between tree-level and loop diagrams in the four-body decay, and we show that the generated baryon asymmetry can reproduce the observed value both in a scenario where $\phi$ is responsible for the reheating of the Universe, and in a scenario where $\phi$ is a generic scalar that remains in thermal equilibrium with the plasma.