  Exclusive proton-antiproton annihilation into two photons at large s (~10
GeV^2) and |t|,|u| ~ s can be described by a generalized parton 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. The matrix element describing the transition of the p-pbar system to
a q-qbar pair can be related to the timelike proton elastic form factors as
well as to the quark/antiquark distributions measured in inclusive
deep-inelastic scattering. The reaction could be studied with the proposed
1.5-15 GeV high-luminosity antiproton storage ring (HESR) at GSI.
