Linearly polarised photons at ELSA and measurement of the beam asymmetry in η-photoproduction off the proton

Abstract

The beam asymmetry, Σ, was measured at ELSA with the Crystal-Barrel- and TAPS detector setup 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 E _γ = 800 to 1400 MeV with the maximum polarisation of P _γ = 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 Θ _cm ≃ 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.

For the CBELSA-TAPS Collaboration