We studied simultaneously the 4 He(e, e p), 4 He(e, e pp), and 4 He(e, e pn) reactions at Q 2 = 2 (GeV/c) 2 and xB > 1, for an (e, e p) missing-momentum range of 400 to 830 MeV/c. The knocked-out proton was detected in coincidence with a proton or neutron recoiling almost back to back to the missing momentum, leaving the residual A = 2 system at low excitation energy. These data were used to identify two-nucleon short-range correlated pairs and to deduce their isospin structure as a function of missing momentum, in a region where the nucleon-nucleon (N N ) force is expected to change from predominantly tensor to repulsive. The abundance of neutron-proton pairs is reduced as the nucleon momentum increases beyond ∼500 MeV/c. The extracted fraction of proton-proton pairs is small and almost independent of the missing momentum. Our data are compared with calculations of two-nucleon momentum distributions in 4 He and discussed in the context of probing the elusive repulsive N N force.

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We have measured the beam-normal single-spin asymmetry An in the elastic scattering of 1-3 GeV transversely polarized electrons from 1 H and for the first time from 4 He, 12 C, and 208 Pb. For 1 H, 4 He and 12 C, the measurements are in agreement with calculations that relate An to the imaginary part of the two-photon exchange amplitude including inelastic intermediate states. Surprisingly, the 208 Pb result is significantly smaller than the corresponding prediction using the same formalism. These results suggest that a systematic set of new An measurements might emerge as a new and sensitive probe of the structure of heavy nuclei.

Beam-Line FIG. 3. Implementation of detectors in the πM1 area in a Geant4 [10] simulation. The beam strikes the thin scintillator beam hodoscope and three GEM chambers, passes through a hole in the annular veto scintillator, enters the cryotarget vacuum chamber and strikes one of the targets, then exits the vacuum chamber and goes through the beam monitor. Scattered particles are detected by two symmetric spectrometers, each with two straw chambers wrapped in RF shielding and two planes of scintillator paddles.

Measurement of the ω → π 0 e + e − and η → e + e − γ Dalitz decays with the A2 setup at MAMI The Dalitz decays η → e + e − γ and ω → π 0 e + e − have been measured in the γp → ηp and γp → ωp reactions, respectively, with the A2 tagged-photon facility at the Mainz Microtron, MAMI. The value obtained for the slope parameter of the electromagnetic transition form factor of η, Λ −2 η = (1.97 ± 0.11tot ) GeV −2 , is in good agreement with previous measurements of the η → e + e − γ and η → µ + µ − γ decays. The uncertainty obtained in the value of Λ −2 η is lower than in previous results based on the η → e + e − γ decay. The value obtained for the ω slope parameter, Λ −2 ωπ 0 = (1.99 ± 0.21tot) GeV −2 , is somewhat lower than previous measurements based on ω → π 0 µ + µ − , but the results for the ω transition form factor are in better agreement with theoretical calculations, compared to earlier experiments.