Insight into the Narrow Structure in η Photoproduction on the Neutron from Helicity-Dependent Cross Sections

The double polarization observable E and the helicity dependent cross sections σ1=2 and σ3=2 were measured for η 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 4π detector setup of the Crystal Ball and TAPS is ideally suited to detect the recoil nucleons and the decay photons from η → 2γ and η → 3π0. The results show that the narrow structure previously observed in η photoproduction from the neutron is only apparent in σ1=2 and hence, most likely related to a spin-1=2 amplitude. Nucleon resonances t…