Proton decay in supersymmetric grand unified theories

The instability of the proton is a crucial prediction of supersymmetric GUTs, and the long-lasting search for proton decay makes it possible to constrain such models. We consider the decay in minimal supersymmetric SU(5), which is dominated by dimension-five operators, and analyze the implications of the failure of Yukawa unification for the decay rate. In a consistent SU(5) model with higher dimensional operators, where SU(5) relations among Yukawa couplings hold, the proton decay rate can be several orders of magnitude smaller than the present experimental bound. We extend the operator analysis to SO(10) as well. Finally, we discuss a 6D SO(10) orbifold GUT model, where proton decay is mediated by dimension-six operators. The branching ratios differ significantly from those in four dimensions. (orig.)