  In a model where Majorana neutrinos heavier than the electroweak scale couple
to Standard Model Higgs bosons and leptons, we compute systematically thermal
corrections to the direct and indirect CP asymmetries in the Majorana neutrino
decays. These are key ingredients entering the equations that describe the
thermodynamic evolution of the induced lepton-number asymmetry eventually
leading to the baryon asymmetry in the universe. We compute the thermal
corrections in an effective field theory framework that assumes the temperature
smaller than the masses of the Majorana neutrinos and larger than the
electroweak scale, and we provide the leading corrections in an expansion of
the temperature over the mass. In this work, we consider the case of two
Majorana neutrinos with nearly degenerate masses.
