Measurement of the beam asymmetry in η photoproduction off the proton

The beam asymmetry, Sigma, was measured at ELSA in the reaction gamma p -> eta p using linearly polarised tagged photon beams, produced by coherent bremsstrahlung o ff a diamond. The crystal was oriented to provide polarised photons in the energy range E-gamma = 800 to 1400MeV with the maximum polarisation of P-gamma = 49% obtained at 1305MeV. Both dominant decay modes of the eta into two photons and 3 pi(0) were used to extract the beam asymmetry from the azimuthal modulation of the cross- section. The measurements cover the angular range circle minus cm similar or equal to 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 di ff er significantly.