  We consider the simplest realization of leptogenesis with one heavy Majorana
neutrino species much lighter than the other ones. In this scenario, when the
temperature of the early universe is smaller than the lightest Majorana
neutrino mass, we compute at first order in the Standard Model couplings and,
for each coupling, at leading order in the termperature the CP asymmetry in the
decays of the lightest neutrino into leptons and anti-leptons. We perform the
calculation using a hierarchy of two effective field theories organized as
expansions in the inverse of the heavy-neutrino masses. In the ultimate
effective field theory, leading thermal corrections proportional to the Higgs
self coupling and the gauge couplings are encoded in one single operator of
dimension five, whereas corrections proportional to the top Yukawa coupling are
encoded in four operators of dimension seven, which we compute.
