Study of the narrow structure at 1685 MeV in γp→ηp

Results of a coupled channel isobar analysis of new high-statistics data on γp→pη from MAMI are presented. A fit using known broad resonances and only standard background amplitudes cannot 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) γp→ωp coupling in the S11 partial wave. When the beam asymmetry data, published by part of the GRAAL Collaboration, are included in the fit, the solution with a narrow P11 state is slightly preferred. In that fit, mass and width of the hypothetical resonance are determined to M∼1694 MeV and Γ∼40 MeV, respectively, and the photo-coupling to BrηNA1/2p∼2.6⋅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.