Measurement of the beam asymmetry in eta-photoproduction off the proton

The beam asymmetry, $\Sigma$, was measured at ELSA in the reaction $\vec \gamma p \to \eta p$ 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 $E_\gamma = 800$ to 1400 MeV with the maximum polarisation of $P_\gamma = 49$ % obtained at 1305 MeV. 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 $\Theta_\text{cm}\simeq 50$ -- 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.