Decadimento radiativo di leptoni carichi e neutrini sterili pesanti

The Standard Model is nowadays the best theory available in elementary particle physics, able to describe extremely accurately a wide range of phenomena. However, over the years, some discrepancies have emerged between its predictions and experimental verifications, so that today we talk about Minimal Standard Model (MSM) to differentiate it from those models that try to extend it. Among these there are also models with sterile neutrinos, whose presence could solve, at least in part, some problems of MSM including dark matter and neutrino mass evidence. There isn't one single model that incorporates sterile neutrinos; for example we find both models with light sterile neutrinos (also called nuMSM) and with heavy sterile neutrinos, as in our case. In this thesis we focus on a possible process, studied today in the MEG experiment, that could confirm the existence of heavy sterile neutrinos and give us information about their mixing: mu to e + gamma (forbidden in the MSM). Specifically we will calculate the decay amplitude of the process, its decay rate and the branching ratio (BR), comparing the latter with the experimental BR of MEG. From this comparison we will first find the limits on mixing known in the literature for n=1 sterile neutrinos and then find new ones by extending the model to n=2.