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¯ p system to a ¯ qq 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. Compton scattering, i.e., the scattering of real or virtual photons off hadrons, has proven to be a useful tool for investigating the structure of the nucleon. Of particular interest are certain extreme kinematical situations where the reaction mechanism simplifies. In the limit of large virtuality of the incoming photon and fixed t (" deeply virtual Compton scattering ") QCD implies that the amplitude factorizes into a hard photon–quark amplitude and a generalized parton distribution, representing the information about the structure of the nucleon. Similarly, it has been argued that real Compton scattering at large s (≈ 10 GeV 2) and |t|, |u| ∼ s (wide–angle scattering) is dominated by contributions in which the photon scatters off a single quark/antiquark in the nucleon (" handbag graph "). 1,2,3 The soft interactions responsible for the emission and absorption of the active quark/antiquark by the nucleon are parametrized by double distributions, which can be related to both the usual quark/antiquark distributions measured in inclusive deep–inelastic scattering and the elastic form factors of the proton. This so-called " soft mechanism " describes well the existing