Exclusive measurement of quasi-free η-photoproduction from deuterium

Quasi-free photoproduction of η-mesons from the deuteron has been measured at the tagged photon facility of the Mainz microtron MAMI with the photon spectrometer TAPS for incident photon energies from the production threshold at 630 MeV up to 820 MeV. In a fully exclusive measurement η-mesons and recoil nucleons were detected in coincidence. At incident photon energies above the production threshold on the free nucleon, where final-state interaction effects are negligible, an almost constant ratio of σn/σp = 0.66±0.10 was found. At lower incident photon energies the ratio rises due to re-scattering effects. The average ratio agrees with the value extracted from a comparison of the inclusive d (γ,η)X cross section to the free-proton cross-section via a participant-spectator model for the deuteron cross-section ( σn/σp = 0.67±0.07). The angular dependence of the ratio agrees with the expected small deviation of neutron and proton cross-section from isotropy due to the influence of the D13(1520)-resonance. The energy dependence of the ratio in the excitation energy range of the S11(1535)-resonance disfavors model predictions which try to explain this resonance as KΣ bound state.