Some theory and phenomenology of neutrino masses and the baryon asymmetry

Abstract

This thesis provides some theoretical and phenomenological work on two of the outstanding problems in modern physics, namely, the extreme smallness of neutrino masses and the relative abundance of matter over antimatter. A theorem is developed relating the smallness of the light neutrino masses to the degree of lepton number violation in some seesaw extensions of the Standard Model of particle physics. It is shown that for exactly massless light neutrinos there must be an exact lepton number symmetry. Then the viability of thermal leptogenesis as a resolution to the baryon asymmetry problem at different scales is assessed using more sophisticated numerical tools than have previously been applied. It is shown that, if fine-tuned solutions are allowed, the scale may be lowered to GeV. Using these results, it is shown that, if CP violation comes purely from the phases of the PMNS matrix, thermal leptogenesis may still be viable over a range of scales covering --- GeV. It is also shown that thermal leptogenesis is viable in the Neutrino Option, in which the Higgs potential has its dimensionful parameter provided by loop corrections from the heavy Majorana neutrinos in the type I seesaw at a mass scale GeV.