The measure of CP violation in the early Universe is a crucial function needed for a reasonable description of low-scale leptogenesis.
As it is proportional to the imaginary part of decay amplitudes of e.g. heavy neutral leptons, it can be calculated for example by using finite
temperature cutting rules. In this talk we present results where all cuts of the vertex function and of the self-energy are included in case of
massive propagators. We also show how the result obtained from cutting rules of Kobes and Semenoff is connected with the result obtainable from the imaginary time formalism.