B-physics constraints on baryon number violating couplings: grand unification or R-parity violation

We investigate the role that baryon number violating interactions may play in $B$ phenomenology. Present in various grand unified theories, supersymmetric theories with R-parity violation and composite models, a diquark state could be quite light. Using the data on B decays as well as $B - {\bar B}$ mixing, we find strong constraints on the couplings that such a light diquark state may have with the Standard Model quarks.