The process pp̄ → γ π 0 within the handbag approch

We analyse the exclusive channel p¯ p → γ π 0 , assuming handbag dominance. The soft parts are parametrized in terms of CGLN amplitudes for the q ¯ q → γπ 0 transition and form factors for the p¯ p → q ¯ q ones, the latter represent moments of Generalized Distribution Amplitudes. We present a combined fit to Fermilab data from E760 taking simultaneously into account information from other exclusive reactions, especially from p¯ p → γγ data. Overall a nicely consistent picture emerges, such that one can hope, that our theoretical analysis will be reliable also for the kinematics of GSI/FAIR, which hopefully will provide much more precise and complete data. Consequently, data from this facility should improve our knowledge both on the proton-antiproton distribution amplitudes and the pion production mechanism. 1 Introduction The reliable theoretical treatment of hard exclusive processes has been a challenge for QCD for many years. With the advent of the generalized parton distribution formalism [1] a large class of such processes, all involving some hard scale Q 2 , can now be treated on a firm, perturbative QCD basis, absorbing all non-perturbative soft physics in suitable generalized parton distributions. This success is made possible by the dominance of the handbag diagrams in leading twist, as demonstrated in the factorization proofs. There are other hard exclusive processes for which these rigorous proofs do not apply. Still, however, there are good arguments for the dominance of the handbag contribution in certain regions of phase space also for many of these reactions. Examples of such processes are two-photon annihilations into pair of hadrons for large but not asymptotically large Mandelstam variables, s, −t, −u, for applications see [2, 3, 4]. The FAIR project at GSI with the HESR anti-proton program [5] will offer ideal possibilities to study exclusive channels in p¯ p annihilation, e.g. p¯ p → γγ, the time reversed of