Isospin breaking in B → K*γ decays

A calculation of the leading isospin-breaking contributions to the B --> K*gamma decay amplitudes based on the QCD factorization approach is presented. They arise at order Lambda/m(b) in the heavy-quark expansion and are due to annihilation contributions from 4-quark operators, the chromo-magnetic dipole operator, and charm penguins. In the Standard Model the decay rate for (B) over bar (0) --> (K) over bar (*0)gamma is predicted to be about 10-20% larger than that for B- --> K(*-)gamma. Isospin-breaking effects are a sensitive probe of the penguin sector of the effective weak Hamiltonian. New Physics models in which the hierarchy of B --> K*gamma decay rates is either flipped or greatly enhanced could be ruled out with more precise data. (C) 2002 Elsevier Science B.V. All rights reserved.