B-L in Grand Unified Theories.

In this thesis, we study (B-L)-violation in grand unified theories. The thesis is divided into two sections. In the first section, we study six-quark operators of a single generation, that contribute to n-n transitions, and we calculate their matrix elements in the MIT bag model. This calculation is needed to relate experimental bounds on the phenomenological parameter (tau)(,n-n) to parameters that appear in the effective Lagrangian. The second section deals with the construction and study of (B-L)-violating operators of arbitrary generational structure. We construct the general set of (B-L)-violating six-fermion operators that are invariant under SU(3)(,c) x U(1)(,em), SU(3)(,c) x SU(2)(,L) x U(1)(,y) and SU(5). We also prove several theorems about (B-L)-violation and the number of fermion fields in the operator. The study of higher generation operators is important even for first generation processes, because they can contribute through mixing. We find that some of the higher generation operators dominate over purely first generation operators, despite mixing angle suppression, because they can be mediated by lighter Higgs. Finally, we comment on the absolute n-n transition rate and derive a new lower bound on the masses of colour-sextet (B-L) -violating Higgs bosons.