  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.
