Insight into the narrow structure in {\boldmath{$\eta$}}-photoproduction on the neutron from helicity dependent cross sections

The double polarization observable E and the helicity dependent cross sections sigma(1/2) and sigma(3/2) were measured for eta photoproduction from quasifree protons and neutrons. The circularly polarized tagged photon beam of the A2 experiment at the Mainz MAMI accelerator was used in combination with a longitudinally polarized deuterated butanol target. The almost 4 pi detector setup of the Crystal Ball and TAPS is ideally suited to detect the recoil nucleons and the decay photons from eta -> 2 gamma and eta -> 3 pi(0). The results show that the narrow structure previously observed in eta photoproduction from the neutron is only apparent in sigma(1/2) and hence, most likely related to a spin-1/2 amplitude. Nucleon resonances that contribute to this partial wave in eta production are only N1/2(-) (S-11) and N1/2(+) (P-11). Furthermore, the extracted Legendre coefficients of the angular distributions for sigma(1/2) are in good agreement with recent reaction model predictions assuming a narrow resonance in the P-11 wave as the origin of this structure.