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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The photoproduction of ω mesons on nuclei has been investigated using the Crystal Barrel/TAPS experiment at the ELSA tagged photon facility in Bonn. The aim is to study possible in-medium modifications of the ω meson via the reaction γ + A → ω + X → π 0 γ + X ′ . Results obtained for Nb are compared to a reference measurement on a LH2 target. While for recoiling, long-lived mesons (π 0 , η and η ′ ), which decay outside of the nucleus, a difference in the lineshape for the two data samples is not observed, we find a significant enhancement towards lower masses for ω mesons produced on the Nb target. For momenta less than 500 MeV/c an in-medium ω meson mass of M medium = [722 +2 −2 (stat) +35 −5 (syst)] MeV/c 2 has been deduced at an estimated average nuclear density of 0.6 ρ0.

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

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At the electron accelerator ELSA a linearly polarised tagged photon beam is produced by coherent bremsstrahlung off a diamond crystal. Orientation and energy range of the linear polarisation can be deliberately chosen by accurate positioning of the crystal with a goniometer. The degree of polarisation is determined by the form of the scattered electron spectrum. Good agreement between experiment and expectations on basis of the experimental conditions is obtained. Polarisation degrees of Pγ = 40% are typically achieved at half of the primary electron energy. The determination of Pγ is confirmed by measuring the beam asymmetry, Σ, in π 0 photoproduction and a comparison of the results to independent measurements using laser backscattering.PACS. 13.60.-r Photon and charged-lepton interactions with hadrons -13.60.Le Meson production -13.88.+e Polarization in interactions and scattering a present address: FZ