We discuss the exclusive radiative decays B→K∗γ, B→ργ, and B→ωγ in QCD factorization within the Standard
Model. The analysis is based on the heavy-quark limit 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→ρlν decay
spectrum to the branching fraction of B→ργ. 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→ργ
and B→K∗γ branching fractions determines the side Rt of the
standard unitarity triangle with reduced hadronic uncertainties. The recent
Babar bound on B(B0→ρ0γ) implies Rt<0.81(ξ/1.3), with the
limiting uncertainty coming only from the SU(3) breaking form factor ratio
ξ. This constraint is already getting competitive with the constraint from
Bs-Bˉs mixing. Phenomenological implications from
isospin-breaking effects are briefly discussed.Comment: 23 pages, 8 figure