  We calculate the decay rate of the lightest heavy Majorana neutrino in a
thermal bath using finite temperature cutting rules and effective Green's
functions according to the hard thermal loop resummation technique. Compared to
the usual approach where thermal masses are inserted into the kinematics of
final states, we find that deviations arise through two different leptonic
dispersion relations. The decay rate differs from the usual approach by more
than one order of magnitude in the temperature range which is interesting for
the weak washout regime. This work summarizes the results of Ref. [1], to which
we refer the interested reader.
