Radiative neutrino masses: A window to new physics

Abstract: Neutrino oscillations constitute the first experimental confirmation of new physics, as they require a new piece that is absent in the Standard Model, non-zero neutrino masses. Questions such as how these masses are generated or why they are so small compared to other masses, are still under investigation. In this thesis, we study radiative neutrino mass models, a class of models where small neutrino masses are generated naturally, while the energy scale at which new physics appears is still low enough to be tested by current and future experiments, offering a falsifiable window to test beyond the Standard Model physics. In these models, neutrino masses are forbidden at the tree-level, but allowed at a certain order in loops. Consequently, we get masses suppressed by loop factors without requiring a large new physics scale and/or small couplings. In particular, we study the systematic classifications of the $d=7$ Majorana neutrino mass operator at one-loop order, the Weinberg operator at three-loop order and the $d=4$ Dirac neutrino mass models at the two-loop level. We then move to more specific models and mechanisms, analysing the connection between the generation of radiative neutrino masses and the stability of dark matter from a model-independent point of view, and examine a particular realisation of the type-I seesaw which relies on the radiative generation of Dirac Yukawa coupling. Finally, we analyse the phenomenology of some of the models discussed in the classifications, as well as a very particular case of neutrinoless double-$\beta$ decay where electrons are emitted with opposite chiralities along with a light scalar (Majoron).