Proof of factorization for deeply virtual Compton scattering in QCD

We show that factorization holds for the deeply virtual Compton scattering amplitude in QCD, up to power suppressed terms, to all orders in perturbation theory. The theorem applies to the production of off-shell photons as well as real photons. We give a detailed treatment of the situation where one of the two partons joining the parton density to the hard scattering has zero longitudinal momentum. {copyright} {ital 1999} {ital The American Physical Society}