Photoproduction of $\eta$ η mesons from the neutron: Cross sections and double polarization observable E

Abstract

Results from measurements of the photoproduction of η mesons from quasifree protons andneutrons are summarized. The experiments were performed with the CBELSA/TAPS detector at theelectron accelerator ELSA in Bonn using the η → 3π0 → 6γ decay. A liquid deuterium target was used forthe measurement of total cross sections and angular distributions. The results confirm earlier measurementsfrom Bonn and the MAMI facility in Mainz about the existence of a narrow structure in the excitationfunction of γn → nη. The current angular distributions show a forward-backward asymmetry, which waspreviously not seen, but was predicted by model calculations including an additional narrow P11 state.Furthermore, data obtained with a longitudinally polarized, deuterated butanol target and a circularlypolarized photon beam were analyzed to determine the double polarization observable E. Both data setstogether were also used to extract the helicity-dependent cross sections σ1/2 and σ3/2. The narrow structurein the excitation function of γn → nη appears associated with the helicity-1/2 component of the reaction