Long-distance effects in B→Vγ radiative weak decays

A systematic approach to long-distance effects in exclusive radiative weak B decays is presented, based on a combination of the heavy quark limit with perturbative QCD. The dominant long-distance effects, connected with weak annihilation and W-exchange topologies, can be computed in a model-independent way using experimental data on B radiative leptonic decays. Nonfactorizable corrections vanish in the chiral limit and to leading twist. The left-handed photon amplitudes are shown to be enhanced relative to the right-handed ones, both in the long- and short-distance parts of the B̄ decay amplitudes. Recent CLEO data on B→K*γ decays are consistent with standard model estimates of the long-distance contributions, and disfavor an enhanced gluonic penguin contribution. We discuss the implications of our results for the extraction of |V_td|.