Differential and total cross sections for the quasifree reactions γp → ηp and γn → ηn have been determined at the MAMI-C electron accelerator using a liquid deuterium target. Photons were produced via bremsstrahlung from the 1.5 GeV incident electron beam and energy-tagged with the Glasgow photon tagger. Decay photons of the neutral decay modes η → 2γ and η → 3π 0 → 6γ and coincident recoil nucleons were detected in a combined setup of the Crystal Ball and the TAPS calorimeters. The η-production cross sections were measured in coincidence with recoil protons, recoil neutrons, and in an inclusive mode without a condition on recoil nucleons, which allowed a check of the internal consistency of the data. The effects from nuclear Fermi motion were removed by a kinematic reconstruction of the final-state invariant mass and possible nuclear effects on the quasifree cross section were investigated by a comparison of free and quasifree proton data. The results, which represent a significant improvement in statistical quality compared to previous measurements, agree with the known neutron-to-proton cross-section ratio in the peak of the S11(1535) resonance and confirm a peak in the neutron cross section, which is absent for the proton, at a center-of-mass energy W = (1670 ± 5) MeV with an intrinsic width of Γ ≈ 30 MeV.

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Beam asymmetry and differential cross-section for the reaction γp → ηp were measured from production threshold to 1500 MeV photon laboratory energy. The two dominant neutral decay modes of the η-meson, η → 2γ and η → 3π 0 , were analyzed. The full set of measurements is in good agreement with previously published results. Our data were compared with three models. They all fit satisfactorily the results but their respective resonance contributions are quite different. The possible photoexcitation of a narrow state N (1670) was investigated and no evidence was found. PACS. 13.60.Le Meson production-13.88.+e Polarization in interactions and scattering-25.20.Lj Pho-toproduction reactions

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Coherent photoproduction of η-mesons off 3 He, i.e. the reaction γ 3 He → η 3 He, has been investigated in the near-threshold region. The experiment was performed at the Glasgow tagged photon facility of the Mainz MAMI accelerator with the combined Crystal Ball -TAPS detector. Angular distributions and the total cross section were measured using the η → γγ and η → 3π 0 → 6γ decay channels. The observed extremely sharp rise of the cross section at threshold and the behavior of the angular distributions are evidence for a strong η 3 He final state interaction, pointing to the existence of a resonant state. The search for further evidence of this state in the excitation function of π 0 -proton back-to-back emission in the γ 3 He → π 0 pX reaction revealed a very complicated structure of the background and could not support previous conclusions.