On the CP asymmetries in Majorana neutrino decays

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.