Search for Narrow Nucleon Resonance in $\gamma p\to \eta p$
  Results of a partial wave analysis of new high-statistics data on $\gamma p\to p\eta$ from MAMI are presented. A fit using known broad resonances and only standard background amplitudes can not describe the relatively narrow peaking structure in the cross section in the mass region of 1660-1750 MeV which follows a minimum. An improved description of the data can be reached by either assuming the existence of a narrow resonance at a mass of about 1700 MeV with small photo-coupling or by a threshold effect. In the latter case the observed structure is explained by a strong (resonant or non-resonant) $\gamma p\to\omega p$ coupling in the $S_{11}$ partial wave. When the beam asymmetry data, published by part of the GRAAL collaboration, are included in the fit, the solution with a narrow $P_{11}$ state is slightly preferred. In that fit, mass and width of the hypothetical resonance are determined to $M\sim$1694 MeV and $\Gamma\sim 40$ MeV, respectively, and the photo-coupling to $\sqrt{{\rm Br}_{\eta N}} A_{1/2}^p \sim 2.6\cdot 10^{-3}$ GeV$^{-1/2}$. High precision measurements of the target asymmetry and $F$-observable are mandatory to establish the possible existence of such a narrow state and to provide the necessary information to define which partial wave is responsible for the structure observed in the data. 
A. V. Anisovich, E. Klempt, V. Kuznetsov, V. A. Nikonov, M. V.   Polyakov, A. V. Sarantsev, U. Thoma
hep-ph hep-ex nucl-ex nucl-th