CP violation from scatterings with gauge bosons in leptogenesis

We present an explicit computation of the CP asymmetry in scattering processes involving the heavy right-handed neutrinos of the type I seesaw framework and the Standard Model gauge bosons. Compared to CP violation in two-body decays and in scatterings with top quarks there are new contributions at one loop in the form of new type of vertex corrections as well as of box diagrams. We show that their presence implies that, unlike the CP asymmetry in scatterings with top quarks, the CP asymmetry in scatterings with gauge bosons is different from the two-body decay asymmetry even for hierarchical right-handed neutrinos. This also holds for the L-conserving CP asymmetry in scatterings with U(1)Y gauge bosons. Quantitatively however, the effects are not very different in size from those of scatterings with top quarks: the CP asymmetry per scattering is O(0.5-2)× the CP asymmetry per decay in all the relevant temperature range for leptogenesis and the generated baryon asymmetry can only be sizeably affected in leptogenesis models that are sensible to the T≳M epoch.