Proton-antiproton annihilation into two photons at large s

Exclusive proton-antiproton annihilation into two photons can be viewed as the Compton process in the crossed channel. At large s (~10 GeV^2) and |t|,|u| s this process can be described by a generalized partonic picture, analogous to the 'soft mechanism' in wide-angle real Compton scattering. The two photons are emitted in the annihilation of a single fast quark and antiquark ('handbag graph'). The transition of the p-pbar system to a q-qbar pair through soft interactions is described by double distributions, which can be related to the timelike proton elastic form factors as well as, by crossing symmetry, to the usual quark--antiquark distributions in the nucleon. We estimate that this reaction should be observable with reasonable statistics at the proposed 1.5..15 GeV high-luminosity antiproton storage ring (HESR) at GSI.