Radiative leptonic decays of B mesons in QCD - art. no. 114510

We compute the form factors parametrizing radiative leptonic decays of heav y mesons B+ --> gamma e(+)nu for photon energies much larger than Lambda(QC D), where perturbative QCD methods for exclusive processes can be combined with the heavy quark effective theory. The form factors can be reliably obt ained in this region in an expansion in powers of Lambda/E gamma. The leadi ng term in this expansion displays an additional spin symmetry manifested i n the equality of form factors of vector and axial vector currents. The len ding twist form factors can be written as the convolution of the B meson li ght-cone wave function with a hard scattering amplitude, which is explicitl y calculated to one-loop order. The Sudakov double logarithms of the form [ ( alpha(s) / pi)log(2)(2E gamma/Lambda)](n) are resummed to all orders. As an application we present a method for determining the CKM matrix element \ V-ub\ from a comparison of photon spectra in B and D radiative leptonic dec ays.