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 quasi-free 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 $4pi$ detector setup of the Crystal Ball and TAPS is ideally suited to detect the recoil nucleons and the decay photons from $etarightarrow 2gamma$ and $etarightarrow 3pi^0$. 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.