Broken Flavor Symmetries in High-Energy Particle Phenomenology

Over the past couple of decades, the Standard Model of high energy particle physics has clearly established itself as an invaluable tool in the analysis of high energy particle phenomenon. However, from a field theorist's point of view, there are many dissatisfying aspects to the model. One of these, is the large number of free parameters in the theory arising from the Yukawa couplings of the Higgs doublet. In this thesis, we examine various issues relating to the Yukawa coupling structure of high energy particle field theories. We begin by examining extensions to the Standard Model of particle physics which contain additional scalar fields. By appealing to the flavor structure observed in the fermion mass and Kobayashi-Maskawa matrices, we propose a reasonable phenomenological parameterization of the new Yukawa couplings based on the concept of approximate flavor symmetries. It is shown that such a parameterization eliminates the need for discrete symmetries which limit the allowed couplings of the new scalars. New scalar particles which can mediate exotic flavor changing reactions can have masses as low as the weak scale. Next, we turn to the issue of neutrino mass matrices, where we examine a particular texture which leads to matter independent neutrino oscillation results for solar neutrinos. Using a nonstandard basis for our parameterization, we argue that such a mass matrix has a far larger allowed parameter space than the standard see-saw mass matrices. We propose a model which gives rise to such a matrix, finding that approximate flavor symmetries are an important tool in its construction. The experimental consequences of this model are discussed in detail. We, then, examine the basis for extremely strict limits placed on flavor changing interactions which also break lepton- and/or baryon-number. These limits are derived from cosmological considerations. Such interactions, when in equilibrium simultaneously with electroweak instantons, can destroy an existing asymmetry in baryon number. We find that it is a simple matter to avoid these limits entirely, and that one need not impose a symmetry which has a baryon number component in order to do so. Finally, we embark on an extended analysis of proton decay in super-symmetric SO(10) grand unified theories. In such theories, the dominant decay diagrams involve the Yukawa couplings of a heavy triplet superfield. We argue that past calculations of proton decay which were based on the minimal supersymmetric SU(5) model require reexamination because the Yukawa couplings of that theory are known to be wrong. By analyzing the flavor structure of a class of SO(10) theories which do not suffer from this problem, we determine that proton decay branching ratios and dominant diagrams can differ substantially from previous expectations. We discuss, in some generality, the circumstances in which charged lepton decay modes have large branching ratios and in which gluino diagrams become dominant. We find that both possibilities are more likely for the models we analyze. In addition, we examine some commonly made assumptions concerning squark mass matrices and discuss how the possible changes arising from very high energy radiative effects and nonminimal boundary conditions can effect proton decay.