A MEASUREMENT OF THE INCLUSIVE B ---> S GAMMA BRANCHING RATIO.

The flavour changing neutral current decay b --> s gamma has been detected in hadronic Z decays collected by ALEPH at LEP. The signal is isolated in lifetime-tagged b (b) over bar 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)) x 10(-4),consistent with the Standard Model expectation via penguin processes. (C) 1998 Published by Elsevier Science B.V. All rights reserved.