Spectator interactions in B→Vγ decays and QCD factorisation

We study the radiative decays B --> Vgamma (where V = rho, K*) in the framework of QCD factorisation, and in particular the hard-spectator contributions to the decay amplitudes. For the phenomenologically significant chromomagnetic operator, we show by an explicit next-to-leading-order computation that the spectator interactions factorise in the heavy-quark limit, i.e., that they can be written as the convolution of a hard-scattering kernel, computable in perturbation theory, and of the light-cone distribution amplitudes for the B and V mesons which contain the soft physics. The presence of an intermediate scale of O(M(B)Lambda(QCD)) leads to the presence of Sudakov logarithms. We indicate how the demonstration of factorisation can be extended to other (four-quark) operators. (C) 2004 Elsevier B.V. All rights reserved.