Measurements of the analyzing power for the $\overrightarrow{p}p\to pp\eta$ reaction have been performed at excess energies of $Q=10$ and 36 MeV. The determined analyzing power is essentially consistent with zero, implying dominance of the $s$ wave at both excess energies. The angular dependence of the analyzing power, combined with the isospin dependence of the total cross section for the $\eta$ meson production in nucleon-nucleon collisions, reveal that the excitation of the nucleon to the $S_{11}(1535)$ resonance is predominantly due to the exchange of the $\pi$ meson between the colliding nucleons.

• Received 7 November 2006

DOI:https://doi.org/10.1103/PhysRevLett.98.122003