Long-distance effects in $B\to Vγ$ 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. 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 amplitude. Recent CLEO data on $B\to K^{*}\gamma$ 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 $|V_{td}|$. A new method for determining the weak phase $\alpha$ using radiative weak decays is presented.