Quasi-free photoproduction of eta-mesons off the the deuteron

Precise data for quasi-free photoproduction of eta-mesons off the deuteron have been measured at the Bonn ELSA accelerator with the combined Crystal Barrel/TAPS detector for incident photon energies up to 2.5 GeV. The eta-mesons have been detected in coincidence with recoil protons and neutrons. Possible nuclear effects like Fermi motion and re-scattering can be studied via a comparison of the quasi-free reaction off the bound proton to eta-production off the free proton. No significant effects beyond the folding of the free cross-section with the momentum distribution of the bound protons have been found. These Fermi motion effects can be removed by an analysis using the invariant mass of the eta-nucleon pairs reconstructed from the final-state four-momenta of the particles. The total cross-section for quasi-free eta-photoproduction off the neutron reveals even without correction for Fermi motion a pronounced bump-like structure around 1 GeV of incident photon energy, which is not observed for the proton. This structure is even narrower in the invariant-mass spectrum of the.-neutron pairs. Position and width of the peak in the invariant-mass spectrum are W approximate to 1665MeV and FWHM Gamma approximate to 25MeV. The data are compared to the results of different models.