Thesis defense of Sinan Zeißner

In this thesis, different ways to use right-handed Majorana neutrinos to explain the active neutrino masses as well as the baryon number asymmetry observed in our universe are investigated. It is shown that the induced lepton number violation can assist baryon number asymmetry generating models, whose asymmetry without additional lepton number violation is completely washed out by sphaleron transitions. Extending this investigation to the scotogenic model, also the relic dark matter abundance observed in our universe can be explained. Furthermore, different mechanisms for generating a lepton number asymmetry directly via interactions involving right-handed Majorana neutrinos are investigated. It is discussed that for models in which the Yukawa couplings of the neutrino sector can be larger than in the type-I seesaw mechanism, the parameter space for thermal leptogenesis is enlarged. Moreover, it is pointed out that considering thermal mass corrections, only an induced mass degeneracy of right-chiral neutrinos enables new prospects of leptogenesis. Additionally, loop corrections for massive vector bosons interacting with each other are investigated in time-dependent perturbation theory. Studying the case of two interacting massive vector bosons in a simple example in detail, it is shown that the degree of divergence in the S-matrix is the same as expected from Rξ gauge.

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