η-Nucleon Interaction andηProduction inπ-Nucleon Scattering

A dynamical model of the $\eta$-nucleon interaction is constructed which satisfies the requirements of analyticity and unitarity as well as including the most important single-particle exchange terms as an interaction. The resulting model depends on two parameters: the $\eta$-nucleon coupling constant $g_{\eta }$ and a cutoff. It is found that a reasonably good fit to the $\eta$-production data and the $\pi$-nucleon phase-shift analysis can be obtained. Finally it is shown that the strong rise in $\eta$ production just above the threshold implies the existence of a $J=\frac{1}{2}$ odd-parity nonstrange baryon of mass 1420-1460 MeV.