Recoil Proton Polarization and Differential Cross Section in Eta-Meson Photoproduction at 890 MeV

The differential cross section and recoil proton polarization for the process γ+ p →η°+ p were measured by detecting photons from η-decay and recoil protons. The differential cross sections are 0.75±0.03 and 1.04±0.07 µb/sr at the CM angles θ η * =82.3° and 108.3°, respectively, at the incident photon energy k =890 MeV, † and 0.20±0.05 µb/sr at θ η * =83.1° and k =1050 MeV. Thus the differential cross sections at k =890 MeV seem to show a backward peaking. The value of recoil proton polarization is -0.15±0.30 at θ η * =82.3°, whereas it is 0.27±0.25 at θ η * =108.3°, both at k =890 MeV, thus suggesting a sign change around θ η * =90°. In addition to the S 11 (1535), a comparably large contribution of the D 13 (1520) is needed to explain the experimental results.