N*(1670) Observed At Lns, Sendai

A resonance-like structure has been observed in the total cross section for the gamma d --> eta np reaction at E-gamma similar to 1.02 GeV. The experiment was conducted at Laboratory of Nuclear Science, Tohoku University, by utilizing a tagged photon beam provided with an internal radiator in a 1.2 GeV electron synchrotron. A solid deuterium target was employed as well as a solid hydrogen target to extract information on the process gamma n --> eta n. A clear bump has been observed in the total cross section for the gamma n' --> eta n reaction, where n' means a neutron in Fermi motion. A simple analysis gives the position of about 1670 MeV and the width of less than 40 MeV for the bump, respectively, taking into account the Fermi motion. There is no bump structure observed, in contrast, in the gamma p --> eta p process at the same energy region. The U-spin conservation may naturally account for this phenomenon, if the bump is an indication of a hidden-strangeness pentaquark baryon N-5(1670) in the anti-decuplet state.