The Reaction $pp\to ppη$ and the eta-Nucleon and Nucleon-Nucleon Interactions

The $\pi$, $\eta$ and $\rho$ exchange contributions to the cross-section for the reaction $pp\to pp\eta$ near threshold are calculated with a phenomenological description of the intermediate S11 (N(1535)) resonance for all exchange mechanisms. The final state interaction in the $pp$ system is described by realistic nucleon-nucleon interaction models. The sensitivity of the results to the phenomenological models for the $\eta$-nucleon transition amplitude and the nucleon-nucleon interaction is explored. The $\eta$ exchange mechanism is found to play a dominant role. The off-shell behavior of the $\eta N \to \eta N$ amplitude within the $\eta-$exchange amplitude leads to a result significantly different from that obtained with the constant scattering length approximation. The two-nucleon amplitudes that are associated with the short range components of the nucleon-nucleon interaction contribute significantly to the cross section.