Perturbative QCD factorization of πγ*→γ(π) and B→γ(π)l(ν)over-bar -: art. no. 014019

We prove the factorization theorem for the processes pi gamma* --> gamma and pi gamma* --> pi to leading twist in the covariant gauge by means of the Ward identity. Soft divergences cancel and collinear divergences are grouped into a pion wave function defined by a nonlocal matrix element. The gauge invariance and universality of the pion wave function are confirmed. The proof is then extended to the exclusive B meson decays B --> gamma/<(<nu>)over bar> and B --> pi/<(<nu>)over bar> in the heavy quark limit. It is shown that a light-cone B meson wave function, though absorbing soft dynamics, can be defined in an appropriate frame. Factorization of the B --> pi/<(<nu>)over bar> decay in k(T) space, k(T) being parton transverse momenta, is briefly discussed. We comment on the extraction of the leading-twist pion wave function from experimental data.