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

Abstract

The beam asymmetry, $Σ$, was measured at ELSA in the reaction $\vec γp \to ηp$ 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 $Θ_\text{cm}\simeq 50$ -- 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.