This thesis discusses the construction of an effective field theory (EFT) for non-relativistic Majorana fermions, shows how to use it to calculate observables in a thermal medium, and derives the effects of these thermal particles on the CP asymmetry.

The methods described in this thesis are the only ones to date that allow a systematic and effective description of the non-relativistic dynamics of a heavy Majorana fermion at finite temperature. The CP asymmetry is studied for hierarchical and nearly degenerate heavy-neutrino masses and the analysis includes the treatment of...