Photoproduction off the nucleon revisited: Evidence for a narrow N(1688) resonance?

Abstract

Revised analysis of $\Sigma$ beam asymmetry for the $\eta$ photoproduction on the free proton reveals a structure at $W\sim 1.69$ GeV. Fit of the experimental data based on the E429 solution of the SAID partial wave analysis suggests a narrow ($\Gamma \leq 25$ MeV) resonance. Possible candidates are $P_{11}, P_{13}$, or $D_{13}$ resonances. The result is considered in conjunction with the recent evidence for a bump-like structure at $W\sim 1.67 - 1.68$ GeV in the quasi-free $\eta$ photoproduction on the neutron.