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We formulate and prove a QCD factorization theorem for hard exclusive electroproduction of mesons in QCD. The proof is valid for the leading power in Q and all logarithms. This generalizes previous work on vector meson production in the diffractive region of small x. The amplitude is expressed in terms of off-diagonal generalizations of the usual parton densities. The full theorem applies to all kinds of mesons and not just to vector mesons. The parton densities used include not only the ordinary parton density, but also the helicity density (g₁ or) and the transversity density (h₁ or), and these can be probed by measuring the polarization of the produced mesons with unpolarized protons.
Collins et al. (Mon,) studied this question.