The differential cross section for the photoproduction reaction
γ + p → η° + p
has been measured in the energy region about the N*** (1688, 5/2 +) and at an angle θ*η=45°. The eta was detected through its two photon decay mode
η° → γ + γ
Two thin foil spark chambers and an aluminum plate range chamber are used to measure the lab angle and energy of the outgoing proton and two shower counters measure the energies of the decay photons and the lab angle of the η°. Overdetermined kinematical parameters allow the mass of the two photon object to be measured. A striking peak centered at 549.9 ± 1.5 Mev on top of a small, broad background indicates a clean selection of eta mesons.Comparison of the measured ratio
N*** → η°p ⁄ N*** → π°p
provides an experimental measure for the ratio
GηNN*** ⁄ GπNN***
to be compared with predictions of unbroken unitary symmetry.This ratio confirms that the N*** (1688) belongs to a J = 5⁄2 + baryon octet, and gives experimental limits for the F⁄D coupling ratio for (PS B B*) couplings.
