Evidence for narrorw N(1685) Nucleon Resonance

Description
Recently four groups, GRAAL, CB/TAPS@ELSA, LNS@Tokhoku, and A2@MAMIC, reported the evidence for a narrow structure at W~1.67−1.68 GeV in the eta photoproduction on the neutron. The structure was observed as a bump in the quasi-free gamma n -> eta n cross section and as a peak in the invariant mass spectrum of the final-state eta and the neuron. This observation was interpreted as a signal of a new narrow resonance. Alternatively, the bump in the quasi-free cross section was explained in terms of interference of known resonances. Review of available experimental data will be presented together with new results on Compton scattering on the neutron from GRAAL. The combination of experimental findings, if to consider them altogether, seem to support the existence of a new nucleon resonance with unusual properties: the mass near 1.685 GeV the possibly narrow <30 MeV width, and the much stronger photocoupling to the neutron than to the proton. The interpretation of the structure in gamma n -> eta n in terms of interference of known resonances will be discussed as well.