Abstract. Quasi-free photoproduction of η -mesons off nucleons bound in the deuteron has been measured with the combined Crystal Barrel -TAPS detector. The experiment was done at a tagged photon beam of the ELSA electron accelerator in Bonn for incident photon energies from the production threshold up to 2.5 GeV. The η -mesons have been detected in coincidence with recoil protons and recoil neutrons. The quasi-free proton data are in good agreement with the results for free protons, indicating that nuclear effects have no significant impact. The coincidence with recoil neutrons provides the first data for the γn → nη reaction. In addition, also first estimates for coherent η -production off the deuteron have been obtained. In agreement with model predictions, the total cross-section for this channel is found to be very small, at most at the level of a few nb. The data are compared to model calculations taking into account contributions from nucleon resonances and t-channel exchanges.

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Quasifree photoproduction of eta mesons off nucleons bound in the deuteron has been measured with the CBELSA/TAPS detector for incident photon energies up to 2.5 GeV at the Bonn ELSA accelerator. The eta mesons have been detected in coincidence with recoil protons and recoil neutrons, which allows a detailed comparison of the quasifree n(gamma,eta)n and p(gamma,eta)p reactions. The excitation function for eta production off the neutron shows a pronounced bumplike structure at W=1.68 GeV (E{gamma} approximately 1 GeV), which is absent for the proton.

Abstract. Photoproduction off protons of the pπ 0 η three-body final state was studied with the Crystal Barrel/TAPS detector at the electron stretcher accelerator ELSA at Bonn for incident energies from the π 0 η production threshold up to 2.5 GeV. Differential cross sections and the total cross section are presented. The use of linearly polarized photons gives access to the polarization observables Σ, Is and I c , the latter two characterize beam asymmetries in case of three-body final states. ∆(1232)η, N (1535)1/2 − π, and pa0(980) are the dominant isobars contributing to the reaction. The partial wave analysis confirms the existence of some nucleon and ∆ resonances for which so far only fair evidence was reported. A large number of decay modes of known nucleon and ∆ resonances is presented. It is shown that detailed investigations of decay branching ratios may provide a key to unravelling the structure of nucleon and ∆ resonances.

The beam asymmetry, Σ, was measured at ELSA in the reaction γ p → η p using linearly polarised tagged photon beams, produced by coherent bremsstrahlung off a diamond. The crystal was oriented to provide polarised photons in the energy range Eγ = 800 to 1400 MeV with the maximum polarisation of Pγ = 49 % obtained at 1305 MeV. Both dominant decay modes of the η into two photons and 3π 0 were used to extract the beam asymmetry from the azimuthal modulation of the cross section. The measurements cover the angular range Θcm ≃ 50 -150 degrees. Large asymmetries up to 80 % are observed, in agreement with a previous measurement. The eta-MAID model and the Bonn-Gatchina partial wave analysis describe the measurements, but the required partial waves differ significantly.PACS. 13.60.-r Photon and charged-lepton interactions with hadrons -13.60.Le Meson production -13.88.+e Polarization in interactions and scattering -14.20.Gk Baryon resonances with S=0