Models of fermion masses

Since the time the Standard Model (SM) has been proposed by Weinberg, Glashow and Salam, it has been tested to high precision showing an unexpected solidity. However the SM contains many free parameters which have to be introduced as input from the experimental data. It is a common opinion that the SM is only an effective theory which might break down at an energy scale somewhere above the electroweak scale. How much bigger will the energy scale be before effects of new physics appear is one of the fundamental questions in high energy physics.; The usual approach to predict the energy scale of new physics is to look at processes which in the extensions of the SM are not forbidden but from experimental evidences are very suppressed. Also very small quantities like the neutrino masses are believed to be the effect of high energy scales.; In the work of this thesis we will deal with two completely different approaches to extend the SM, one of which has been introduced by P.Q. Hung to explain small neutrino masses and the other by Arkani-Hamed, Dimopoulos and Dvali to predict a small Newton constant.; In the model of Hung the SM is extended by adding new gauge symmetries and the neutrinos acquire a small mass when these symmetries are broken at energies much higher than the electroweak scale. The work presented in the first part of this thesis consists in estimating a common parameter in this model, shared by the lepton sector and the quark sector, by looking at rare K decays.; The approach of Arkani-Hamed, Dimopoulos and Dvali postulates the existance of new extra compactified dimensions where only gravity can propagate loosing its strength, with the result that four dimensional gravitational interaction is very weak. In the second part of this thesis we present the phenomenology of a model proposed by P. Q. Hung and M. Seco in the framework of extra dimensions to derive a quark mass matrix. We will show that with the form of the mass matrix predicted by Hung and Seco one is able to fit the experimental data. In the last part of the thesis a possible idea to solve the Strong CP problem without axion, which has been suggested by Hung and Seco, has been also investigated in light of the results obtained in this thesis.