  We obtain the Boltzmann equations for leptogenesis including decay and
scattering processes with two and three body initial or final states. We
present an explicit computation of the CP violating scattering asymmetries. We
analyze their possible impact in leptogenesis, and we discuss the validity of
their approximate expressions in terms of the decay asymmetry. In scenarios in
which the initial heavy neutrino density vanishes, the inclusion of CP
asymmetries in scatterings can enforce a cancellation between the lepton
asymmetry generated at early times and the asymmetry produced at later times.
We argue that a sizeable amount of washout is crucial for spoiling this
cancellation, and we show that in the regimes in which the washouts are
particularly weak, the inclusion of CP violation in scatterings yields a
reduction in the final value of the lepton asymmetry. In the strong washout
regimes the inclusion of CP violation in scatterings still leads to a
significant enhancement of the lepton asymmetry at high temperatures; however,
due to the independence from the early conditions that is characteristic of
these regimes, the final value of the lepton asymmetry remains approximately
unchanged.
