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Insight into the Narrow Structure in η Photoproduction on the Neutron from Helicity-Dependent Cross Sections
Witthauer, L.; Dieterle, M.; Abt, S.; Achenbach, P.; Afzal, F.; Ahmed, Z.; Annand, J. R. M.; Arends, H. J.; Bashkanov, M.; Beck, R.; Biroth, M.; Borisov, N. S.; Braghieri, A.; Briscoe, W. J.; Cividini, F.; Costanza, S.; Collicott, C.; Denig, A.; Downie, E. J.; Drexler, P.; Ferretti-Bondy, M. I.; Gardner, S.; Garni, S.
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 that contribute to this partial wave in η production are only N1/2− (S11) and N 1/2+ (P11). Furthermore, the extracted Legendre coefficients of the angular distributions for σ1/2 are in good agreement with recent reaction model predictions assuming a narrow resonance in the P11 wave as the origin of this structure.