Neutrinos and Lepton - Flavour – Violation ∗

The standard model (SM) of particle physics successfully describes a variety of phenomena in terms of a few fundamental quantities. Besides the gauge coupling constants associated to the fundamental interactions, and the scalar sector responsible for spontaneous electroweak symmetry breaking, the major part of adjustable parameters are related to the fermion sector describing the masses and mixings of quark and lepton flavours. The experimental observation of neutrino oscillations implies that neutrinos have (tiny) masses, and the SM – which in its original version is restricted to massless neutrinos – has to be extended. The most popular approaches are based on “see-saw” scenarios, where neutrino masses are suppressed by a large scale which originates from integrating out heavy particles. In particular, heavy Majorana neutrinos appear naturally when embedding the SM in grand unified theories (GUTs). The violation of lepton number L at the Majorana mass scale allows to explain the baryon asymmetry in the universe by leptogenesis, where an L-asymmetry is built by out-of-equilibrium decays of Majorana neutrinos, which is subsequently transformed into a baryon asymmetry by electroweak sphaleron processes. The required CP-violation enters through intereference effects of tree-level and loop diagrams and depends on the phases in the Majorana sector. While the above phenomena relate the lepton flavour sector to highenergy scales and physics near the GUT scale, low-energy observables from