Proton decay and related processes models with gauged baryon number

In unification models based on SU(15) or SU(16), baryon number is part of the gauge symmetry, broken spontaneously. In such models, we discuss various scenarios of important baryon-number-violating processes such as proton decay and neutron-antineutron oscillation. Our analysis depends on the effective operator method, and covers many variations of symmetry breaking, including different intermediate groups and different Higgs-boson content. We discuss processes mediated by gauge bosons and Higgs bosons in parallel. We show how accidental global or discrete symmetries present in the full gauge invariant Lagrangian restrict baryon-number-violating processes in these models. In all cases we find that baryon-number-violating interactions are sufficiently suppressed to allow grand unification at energies much lower than the usual ${10}^{16}$ GeV.