FACTORIZATION FOR HARD EXCLUSIVE ELECTROPRODUCTION OF MESONS IN QCD

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 vect or meson production in the diffractive region of small x. The amplitud e is expressed in terms of off-diagonal generalizations of the usual p arton densities. The full theorem applies to all kinds of mesons and n ot just to vector mesons. The parton densities used include not only t he ordinary parton density, but also the helicity density (g(1) or Lam bda q) and the transversity density (h(1) or delta q), and these can b e probed by measuring the polarization of the produced mesons with unp olarized protons.