Photoproduction of eta-mesons off light nuclei

The experimental study of photoproduction of mesons off the neutron is under way at the tagged photon facilities of the ELSA accelerator in Bonn and the MAMI accelerator in Mainz with the Crystal Barrel/TAPS and Crystal Ball/TAPS detector systems. It aims mainly at the iso-spin structure of the electromagnetic resonance excitations of the neutron. The experiments exploit quasi-free meson photoproduction off neutrons bound in light nuclei, in particular the deuteron, for many different final states. Data for some channels (eta, eta`) have already been published, results for single pi(0), pi(0)pi(0), pi(0)pi(perpendicular to), pi(0)eta, pi(+/-)eta are in preparation. The most surprising finding is a pronounced, very narrow (Gamma > 40 MeV), peak-like structure around W approximate to 1670 MeV in the excitation function of eta-photoproduction off the neutron.