In this thesis, we explore the seesaw mechanism for neutrino masses in the context of supersymmetry and in the early Universe. In the case of the former, we extend the Minimal Supersymmetric Standard Model with a singlet superfield that contains right-handed neutrinos and sneutrinos. We derive analytical expressions for one-loop corrections to neutrino masses in the flavour and weak bases, and we find that the smallness of the observed light neutrino masses may be explained by a soft SUSY-screening effect from a nearly supersymmetric singlet neutrino sector. This effect implies that the neutrino mass constraints on lepton number and lepton flavour violation can be drastically relaxed in the screening region. Lastly, we discuss the phenomenological, and cosmological implications of this model. Regarding the early Universe implications of the seesaw mechanism, we study a class of leptogenesis models that may naturally include three nearly degenerate heavy Majorana neutrinos, which can strongly mix with one another, and have mass differences comparable to their decay widths. Within this quasi-degenerate scenario, we point out effects of tri-resonance, i.e., constructive contributions to the CP asymmetry coming from the three heavy neutrinos. Solving both the heavy neutrino and lepton asymmetry Boltzmann equations, we find that the dynamical evolution of the baryon asymmetry is heavily impacted by corrections on the temperature dependence of the relativistic degrees of freedom of the plasma when the mass of the lightest Majorana neutrino is lower than 100 GeV. We explore the parameter space for this leptogenesis model, and identify regions that could be probed in future experimental facilities searching for signals of lepton number and flavour violation.
- seesaw mechanism
- radiative neutrino masses