Non-relativistic Majorana neutrinos in a thermal bath and leptogenesis

Neutrino oscillations, the evidence of dark matter and the baryon asymmetry in the universe can not be explained by the standard model of particle physics.Majorana fermions enter in many scenarios of physics beyond the standard model. For example,in the simplest leptogenesis framework, heavy Majorana neutrinos are at the origin of the baryon asymmetry. In the strong wash-out regime non-relativistic Majorana neutrinos produce the lepton asymmetry that is partially reprocessed into a baryon asymmetry. Moreover, all the interactions occur in a thermal medium, namely the universe in its early stage. We discuss an effective field theory approach to study the dynamics of non-relativistic Majorana particles in a thermal bath made of standard model particles. In particular, the decay width of Majorana neutrinos and the CP asymmetries are key ingredients for most leptogenesis models. We address the derivation of such quantities at finite temperature. We provide a formalism to calculate the thermal corrections to the CP asymmetry in the case of a hierarchical mass spectrum for heavy Majorana neutrinos