FCNC transitions c -> u gamma and s -> d gamma in B-c -> B-u*gamma and B-s-> B-d*gamma decays - art. no. 114003

We propose the B-c-->B-u*gamma decay as the most suitable probe for the fla vor changing neutral transition c-->u gamma. We estimate the short and long distance contributions to this decay within the standard model and we find them to be comparable; this is in contrast with radiative decays of D meso ns that are completely dominated by the long distance contributions. Since the c-->u gamma transition Is very sensitive to the physics beyond the stan dard model, the standard model prediction Br(B-c-->B-u*gamma)similar to 10( -8) obtained here opens a new window for future experiments. The detection of B-c-->B-u*gamma decay at a branching ratio well above 10(-8) would signa l new physics. In addition we study the s-->d gamma transition in B-s-->B-d *gamma decay and we find it to be dominated by the long distance contributi on. We use the Isgur-Scora-Grinstein-Wise constituent quark model for the c alculation of these decays. [S0556-2821(99)08409-X].