We report on a new experimental method based on initial-state radiation (ISR) in e-p scattering, in which the radiative tail of the elastic e-p peak contains information on the proton charge form factor (G p E ) at extremely small Q 2 . The ISR technique was validated in a dedicated experiment using the spectrometers of the A1-Collaboration at the Mainz Microtron (MAMI). This provided first measurements of G p E for 0.001 ≤ Q 2 ≤ 0.004 (GeV/c) 2 .

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The (anti-Proton ANnihiliation at DArmstadt) experiment will be one of the four flagship experiments at the new international accelerator complex FAIR (Facility for Antiproton and Ion Research) in Darmstadt, Germany. will address fundamental questions of hadron physics and quantum chromodynamics using high-intensity cooled antiproton beams with momenta between 1.5 and 15 GeV/c and a design luminosity of up to 2 × 1032 cm−2 s−1. Excellent particle identification (PID) is crucial to the success of the physics program. Hadronic PID in the barrel region of the target spectrometer will be performed by a fast and compact Cherenkov counter using the detection of internally reflected Cherenkov light (DIRC) technology. It is designed to cover the polar angle range from 22° to 140° and will provide at least 3 standard deviations (s.d.) π/K separation up to 3.5 GeV/c, matching the expected upper limit of the final state kaon momentum distribution from simulation. This documents describes the technical design and the expected performance of the Barrel DIRC detector. The design is based on the successful BaBar DIRC with several key improvements. The performance and system cost were optimized in detailed detector simulations and validated with full system prototypes using particle beams at GSI and CERN. The final design meets or exceeds the PID goal of clean π/K separation with at least 3 s.d. over the entire phase space of charged kaons in the Barrel DIRC.

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.

The exclusive charmonium production process inpp annihilation with an associated π 0 mesonpp → J=ψπ 0 is studied in the framework of QCD collinear factorization. The feasibility of measuring this reaction through the J=ψ → e þ e − decay channel with the AntiProton ANnihilation at DArmstadt (PANDA) experiment is investigated. Simulations on signal reconstruction efficiency as well as the background rejection from various sources including thepp → π þ π − π 0 andpp → J=ψπ 0 π 0 reactions are performed with PANDAROOT, the simulation and analysis software framework of thePANDA experiment. It is shown that the measurement can be done atPANDA with significant constraining power under the assumption of an integrated luminosity attainable in four to five months of data taking at the maximum design luminosity.