A measurement of the inclusive b→sγ branching ratio

Abstract The flavour changing neutral current decay b → sγ has been detected in hadronic Z decays collected by ALEPH at LEP. The signal is isolated in lifetime-tagged b b events by the presence of a hard photon associated with a system of high momentum and high rapidity hadrons. The background processes are normalised from the data themselves. The inclusive branching ratio is measured to be (3.11±0.80 stat ±0.72 syst )×10 −4 , consistent with the Standard Model expectation via penguin processes.