Narrow Structure in the Excitation Function of η Photoproduction off the Neutron

The photoproduction of eta mesons off nucleons bound in H-2 and He-3 has been measured in coincidence with recoil protons and recoil neutrons for incident photon energies from threshold up to 1.4 GeV. The experiments were performed at the Mainz MAMI accelerator, using the Glasgow tagged photon facility. Decay photons from the eta -> 2 gamma and eta -> 3 pi(0) decays and the recoil nucleons were detected with an almost 4 pi electromagnetic calorimeter combining the Crystal Ball and TAPS detectors. The data from both targets are of excellent statistical quality and show a narrow structure in the excitation function of gamma n -> n eta. The results from the two measurements are consistent, taking into account the expected effects from nuclear Fermi motion. The best estimates for position and intrinsic width of the structure are W = (1670 +/- 5) MeV and Gamma = (30 +/- 15) MeV. For the first time precise results for the angular dependence of this structure have been extracted.