Constraining the unitarity triangle with B → Vγ -: art. no. 035

We discuss the exclusive radiative decays B -> K*gamma, B -> rho gamma, and B ->omega gamma in QCD factorization within the Standard Model. The analysis is based on the heavy-quark limkit of QCD. Our results for these decays are complete to next-to-leading order in QCD and to leading order in the heavy-quark limit. Special emphasis is placed on constraining the CKM-unitarity triangle from these observables. We propose a theoretically clean method to determine CKM parameters from the ratio of the B -> rho iota nu decay spectrum to the branching fraction of B -> rho gamma. The method is based on the cancellation of soft hadronic form factors in the large energy limit, which occurs in a suitable region of phase space. The ratio of the B -> rho gamma and B -> K*gamma branching fractions determines the side R-t of the standard unitarity triangle with reduced hadronic uncertainties. The recent Babar bound on B(B-0 -> rho(0)gamma) implies R-t < 0.82 (xi/1.3), with the limiting uncertainty coming only from the SU(3) breaking form factor ratio xi. This constraint is already getting competitive with the constraint from B-s-(B) over bar (s) mixing. Phenomenological implications from isospin-breaking effects are briefly discussed.