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

The $π$, $η$ and $ρ$ exchange contributions to the cross-section for the reaction $pp\to ppη$ 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 $η$-nucleon transition amplitude and the nucleon-nucleon interaction is explored. The $η$ exchange mechanism is found to play a dominant role. The off-shell behavior of the $ηN \to ηN$ amplitude within the $η-$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.