Eta photoproduction on nucleons and nuclei

The photoproduction of η-mesons provides a very selective tool for the study of N* resonances. The growing interest into this field has prompted significant improvements of the theoretical interpretation of η-photoproduction. However, the progress was severely hampered by the lack of experimental data. We have therefore studied η-photoproduction on the proton, deuterium and some complex nuclei (C, Ca, Nb, Pb) in a series of experiments carried out at the Mainz MAMI accelerator with the TAPS detector system. Precise total and differential cross sections from the production thresholds (707 MeV on proton, 548 MeV on lead) up to 790 MeV can be derived from this set of data. They provide information about such crucial questions as the nature of the ηNN coupling and the respective coupling constant, the contribution of different N* resonances to the elementary cross section, the ratio of proton and neutron cross section and the absorption properties of nuclear matter for η-mesons. In the present paper we report on first preliminary results. (author)