Münster 2002 – scientific programme
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HK: Physik der Hadronen und Kerne
HK 38: Theory VI
HK 38.6: Talk
Thursday, March 14, 2002, 15:15–15:30, C
Proton–antiproton annihilation and the partonic structure of the nucleon — •A. Freund1, A.V. Radyushkin2, A. Schäfer1, O.V. Teryaev3,1, and C. Weiss1 — 1Institut für Theoretische Physik, Universität Regensburg, D–93053 Regensburg, Germany — 2Theory Group, Jefferson Lab, Newport News, VA 23606, USA, and Old Dominion University, Norfolk, VA 23529, USA — 3Laboratory of Theoretical Physics, JINR Dubna, Russia
We consider exclusive proton–antiproton annihilation into two photons, p p → γγ, at s ≫ mN2, which could be studied with the proposed 1–15 GeV antiproton storage ring (HESR) at GSI [1]. We argue that in QCD this process is dominated by contributions in which the two photons are emitted in a single quark–antiquark annihilation process, accompanied by soft annihilation of the spectators (“handbag graph”). This description is analogous to the “soft mechanism” dominating wide–angle real Compton scattering off the proton [2]. The amplitude for p p annihilation can thus be expressed in terms of a function describing the decay of the pp system into a q q pair. This function is the deeply timelike variant of the double distribution of quarks measured in wide–angle real and also deeply virtual Compton scattering (generalized parton distributions). We present an estimate of the expected annihilation cross section based on a simple model for the decay function, and discuss experimental signatures for the dominance of the “handbag” mechanism.
[1] “An International Accelerator Facility for Beams of Ions and Antiprotons”, GSI Conceptual Design Report, November 2001
[2] A. V. Radyushkin, Phys. Rev. D58 (1998) 114008