We calculate large mass diphoton exclusive photoproduction in the framework of collinear QCD factorization at next to leading order in {alpha}s and at leading twist. Collinear divergences of the coefficient function are absorbed by the evolution of the generalized parton distributions (GPDs). This result enlarges the existing factorization proofs to 2 -> 3 processes, opening new reactions to a trustable extraction of GPDs.
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