FCNC transitions c→uγ and s→dγ in Bc→Bu*γ and Bs→Bd*γ decays -: art. no. 114003

We propose the B-c-->B-u*gamma decay as the most suitable probe for the flavor 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 mesons that are completely dominated by the long distance contributions. Since the c-->u gamma transition Is very sensitive to the physics beyond the standard 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 signal 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 contribution. We use the Isgur-Scora-Grinstein-Wise constituent quark model for the calculation of these decays. [S0556-2821(99)08409-X].