Exclusive production of a pair of high transverse momentum photons in pion-nucleon collisions for extracting generalized parton distributions*

We show that exclusive massive photon-pair production in pion-nucleon collisions can be systematically studied in terms of QCD factorization approach if the photon's transverse momentum with respect to the colliding pion ??_?? ≫ Λ_{QCD}. We demonstrate that leading power non-perturbative contributions to the scattering amplitudes of this exclusive process are process-independent and can be systematically factorized into universal pion's distribution amplitudes (DAs) and nucleon's generalized parton distributions (GPDs), which are convoluted with corresponding infrared safe and perturbatively calculable short-distance hard parts. The correction to this factorized expression is suppressed by powers of 1/??_??. We also demonstrate quantitatively that this new type of exclusive processes is not only complementary to existing processes for extracting GPDs, but also capable of providing an enhanced sensitivity to the momentum fraction ??-dependence of both DAs and GPDs.