Looking Beyond the Standard Model in Flavour Violating Neutrino Physics and in Ordinary-Exotic Fermion Mixing

Almost all the present accelerator data are consistent with the Glashow-Weinberg-Salam 'Standard Model' (SM) of electroweak interactions. One exception is given by tau lepton decay measurements, but even in this case the disagreement is not dramatic. The minimal SM is now tested at the quantum (1-loop) level in several precise experiments. There are some strong, but still controversial, evidences of a mixing of the electron neutrino with a 17Ke V neutrino. Important missing points are the top-quark and the Higgs-boson, which have not yet been found and are predicted to exist by the model. There is the hope that at least the top will be produced in the next few years. This would allow to reduce the uncertanties, that arise from the lack of knowledge of the top mass, that affect the theoretical predictions of the SM, so that the large amount of precise experiments will be more effective to test the SM itself, and eventually to indicate signals of new physics. Then, why look for new physics while waiting for the discovery of top-quark and Higgs boson? There are two main reasons for such an effort. First, the SM is not completely satisfactory for several theoretical reasons. It has a lot of free parameters, the hierarchy of fermion masses is introduced by hand, and the mechanism of spontaneous breaking of the gauge symmetry suffers a serious naturalness 'hierarchy' problem. All the above difficulties are related to the Higgs sector of the model, introduced to ensure the renormalizability of the theory, but for which there is no experimental evidence yet. Furthermore, the SM does not achieve a complete unification of the known interactions, nor describe gravity. So, it is hard to believe that it is the final theory of everything. Second, solar neutrino observations and possibly some astrophysical evidences (Dark Matter, ionization of interstellar matter) require non standard physics, as neutrino masses and oscillations and/or neutrino magnetic moments, and/or some non standard leptonic flavour changing mechanism. For these reasons, it is of interest to study non-standard physics, with two different aims. First, to constrain the possible extensions of the SM, which solve some of its theoretical drawbacks, by analyzing the effects of the new physics on the several precise experiments available. Second, to study the possibilities to explain solar neutrino and astrophysical observations in the currently viable extensions of the SM. In this thesis, we have examined some aspects of both these approaches. In the first chapter, we study the neutrino properties in the presence of nonstandard interactions violating the leptonic numbers. We will consider in particular some Flavour Changing Neutral Current (FCNC) effects occurring in supersymmetric models, which predict new scalars interacting with the known matter. In the remaining three chapters, we will present the available precise experimental data, including the recent LEP results, and use all of them to constrain general extensions of the SM where new (heavy) fermions, that could mix the known leptons and quarks, are introduced.