A General Lower Bound for Radiative B Decay

In a wide class of models – including the standard model – flavour-changing neutral currents are suppressed by an approximate chiral symmetry of the low-energy lagrangian. This symmetry allows the derivation of general relations among low-energy flavour-changing processes. We derive one such: the relative sizes of the decay rates b → sγ, b → dγ and s → dγ. Together with a unitarity-related lower bound on the rate for the strangenesschanging hyperon decay Ω → Ξγ we obtain a reasonably model-independent lower bound for the inclusive b → sγ rate.