Further Considerations on the CP Asymmetry in Heavy Majorana Neutrino   Decays
  We work out the thermodynamic equations for the decays and scatterings of heavy Majorana neutrinos including the constraints from unitarity. The Boltzmann equations depend on the CP asymmetry parameter which contains both, a self-energy and a vertex correction. At thermal equilibrium there is no net lepton asymmetry due to the CPT theorem and the unitarity constraint. We show explicitly that deviations from thermal equilibrium create the lepton asymmetry. 
Marion Flanz, Emmanuel A. Paschos (Dortmund U.)