RARE DECAYS B-]XD+GAMMA IN THE STANDARD MODEL

We present an estimate of the inclusive decay rate and photon energy- and hadron mass-spectrum for the CKM-suppressed radiative rare decays B --> X(d) + gamma, based on perturbative QCD and a phenomenological model for the B-meson wave function (here X(d) denotes nonstrange hadrons). Present constraints on \V(td)\ are used to predict BR(B --> X(d) + gamma) = (0.8-4) X 10(-5) for the top quark mass in the range 100 less-than-or-equal-to m(t) less-than-or-equal-to 200 GeV. From the inclusive invariant hadron-mass spectrum and using vector meson dominance in the region M (X(d)) less-than-or-equal-to 1 GeV, we estimate the branching ratio for the exclusive rare decay mode B --> rho + gamma and the ratio GAMMA(B --> rho + gamma)/GAMMA(B --> K* + gamma). The importance of measuring these decays in determining the CKM matrix element \V(td)\ is emphasized.