Tübingen 2003 – scientific programme
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HK: Physik der Hadronen und Kerne
HK 32: Theorie V
HK 32.2: Group Report
Wednesday, March 19, 2003, 14:30–15:00, D
Proton–antiproton annihilation into two photons in a generalized parton picture — •C. Weiss1, A. Freund1, A.V. Radyushkin2, and A. Schäfer1 — 1Institut für Theoretische Physik, Universität Regensburg — 2Theory Group, Jefferson Lab, Newport News, VA, USA, and Old Dominion University, Norfolk, VA, USA
Exclusive proton–antiproton annihilation into two photons p p → γγ is the cross–channel process to real Compton scattering off the proton. This process could be studied at the planned 1.5−15 GeV high–luminosity antiproton storage ring (HESR) at GSI [1]. We argue that at moderately large s (∼ 10 GeV2 ) and |t|, |u| ∼ s (wide–angle scattering) the process can be described by a generalized parton picture analogous to that for wide–angle real Compton scattering [2]. The two photons are emitted in the annihilation of a single quark and antiquark (“handbag graph”). The matrix element describing the conversion of the p p system to a q q pair can be related to the timelike proton form factors, as well as to the quark/antiquark distributions measured in inclusive deep–inelastic scattering off the proton. We discuss the qualitative predictions of this picture and estimate the cross section. We also compare with data on the time–reversed reaction γγ → pp obtained in e+ e− annihilation experiments.
[1] “An International Accelerator Facility for Beams of Ions and Antiprotons”, GSI Conceptual Design Report, November 2001
[2] A. Freund, A. V. Radyushkin, A. Schäfer and C. Weiss, hep-ph/0208061