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The PAX Collaboration has successfully performed a spin-filtering experiment with protons at the COSY-ring. The measurement allowed the determination of the spin-dependent polarizing cross section, that compares well with the theoretical prediction from the nucleon-nucleon potential. The test confirms that spin-filtering can be adopted as a method to polarize a stored beam and that the present interpretation of the mechanism in terms of the proton-proton interaction is correct. The outcome of the experiment is of utmost importance in view of the possible application of the method to polarize a beam of stored antiprotons.

QC 20121212

Abstract. Baryon-to-meson Transition Distribution Amplitudes (TDAs) encoding valuable new information on hadron structure appear as building blocks in the collinear factorized description for several types of hard exclusive reactions. In this paper, we address the possibility of accessing nucleon-to-pion (πN ) TDAs frompp → e + e − π 0 reaction with the futurePANDA detector at the FAIR facility. At high centerof-mass energy and high invariant mass squared of the lepton pair q 2 , the amplitude of the signal channel pp → e + e − π 0 admits a QCD factorized description in terms of πN TDAs and nucleon Distribution Amplitudes (DAs) in the forward and backward kinematic regimes. Assuming the validity of this factorized description, we perform feasibility studies for measuringpp → e + e − π 0 with thePANDA detector. Detailed simulations on signal reconstruction efficiency as well as on rejection of the most severe background channel, i.e.pp → π + π − π 0 were performed for the center-of-mass energy squared s = 5 GeV 2 and s = 10 GeV 2 , in the kinematic regions 3.0 < q 2 < 4.3 GeV 2 and 5 < q 2 < 9 GeV 2 , respectively, with a neutral pion scattered in the forward or backward cone | cos θ π 0 | > 0.5 in the proton-antiproton center-of-mass frame. Results of the simulation show that the particle identification capabilities of thePANDA detector will allow to achieve a background rejection factor of 5 · 10 7 (1 · 10 7 ) at low (high) q 2 for s = 5 GeV 2 , and of 1 · 10 8 (6 · 10 6 ) at low (high) q 2 for s = 10 GeV 2 , while keeping the signal reconstruction efficiency at around 40%. At both energies, a clean lepton signal can be reconstructed with the expected statistics corresponding to 2 fb −1 of integrated luminosity. The cross sections obtained from the simulations are used to show that a test of QCD collinear factorization can be done at the lowest order by measuring scaling laws and angular distributions. The future measurement of the signal channel cross section withPANDA will provide a new test of the perturbative QCD description of a novel class of hard exclusive reactions and will open the possibility of experimentally accessing πN TDAs.

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