Threshold resummation for exclusiveBmeson decays

We argue that double logarithmic corrections αₛln²x need to be resummed in the perturbative QCD factorization theorem for exclusive B meson decays, when the end-point region with a momentum fraction →x0 is important. These double logarithms, being of collinear origin, are absorbed into a quark jet function, which is defined by a matrix element of a quark field attached by a Wilson line. The factorization of the jet function from the decay →Bγlν is proved to all orders. Threshold resummation for the jet function leads to a universal, i.e., process-independent, Sudakov factor, whose qualitative behavior is analyzed and found to smear the end-point singularities in heavy-to-light transition form factors.