  We study the CP asymmetries in lepton number violating two body scattering
processes and show explicitly how they vanish, in agreement with unitarity
constraints. We relate these cross section asymmetries to the CP decay rate
asymmetries of the intermediate massive neutrinos and show how the inclusion of
the Universe expansion via Boltzmann equations is the key ingredient to allow
the production of a non-vanishing asymmetry in spite of the unitarity
constraint on the cross sections. We then show that the absorptive parts of
both the one loop vertex and self energy corrections do contribute to the CP
decay asymmetries.
