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.

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Total and differential cross sections for the reaction p(gamma,eta)p have been measured for photon energies in the range from 750 MeV to 3 GeV. The low-energy data are dominated by the S11 wave which has two poles in the energy region below 2 GeV. Eleven nucleon resonances are observed in their decay into peta. At medium energies we find evidence for a new resonance N(2070)D15 with (M,Gamma)=(2068+/-22, 295+/-40) MeV. At gamma energies above 1.5 GeV, a strong peak in the forward direction develops, signaling the exchange of vector mesons in the t channel.

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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