Testing horizontal gauge symmetries from nucleon-decay experiments

We discuss the impact of horizontal gauge symmetries on nucleon decay and demonstrate that the ongoing proton-decay experiments may be used to distinguish between different horizontal gauge theories. Under very general assumptions, it is shown that although a minimal $\mathrm{U}{\mathrm{}\left(1\right)\mathrm{}}_H$ horizontal symmetry allows dimension-six operators and leads to the decay of the nucleon into leptons of the first generation only, an $\mathrm{SU}{\mathrm{}\left(3\right)\mathrm{}}_H$ horizontal symmetry forbids any baryon-nonconserving operator of dimension less than nine.