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We prove factorization theorem for the processes $\pi\gamma^*\to\gamma$ and$\pi\gamma^*\to\pi$ to leading twist in the covariant gauge by means of theWard identity. Soft divergences cancel and collinear divergences are groupedinto a pion wave function defined by a nonlocal matrix element. The gaugeinvariance and universality of the pion wave function are confirmed. The proofis then extended to the exclusive $B$ meson decays $B\to\gamma l\bar\nu$ and$B\to\pi l\bar\nu$ in the heavy quark limit. It is shown that a light-cone $B$meson wave function, though absorbing soft dynamics, can be defined in anappropriate frame. Factorization of the $B\to\pi l\bar\nu$ decay in $k_T$space, $k_T$ being parton transverse momenta, is briefly discussed. We commenton the extraction of the leading-twist pion wave function from experimentaldata.Comment: 21 pages in Latex file, version to appear in Phys. Rev.

physical review. d. particles, fields, gravitation, and cosmology/physical review. d. particles and fields

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Perturbative QCD factorization ofπγ*→γ(π)andB