Differential cross sections for the exclusive reaction $\vec{p}p\to pp\eta$ observed via the $\eta \to {\pi }^{+}{\pi }^{-}{\pi }^0$ decay channel have been measured at $T_{\text{beam}}=2.15\text{GeV}$, $2.50\text{GeV}$, and $2.85\text{GeV}$ (excess energies $324\text{MeV}$, $412\text{MeV}$, and $554\text{MeV}$). The influence of the $N\left(1535\right)S_{11}$ resonance is clearly seen in the invariant mass and momentum dependent differential cross sections. The extracted resonance parameters are compatible with existing data. No significant evidence for further resonance contributions has been found. In addition, angular distributions of the $pp\eta$ final state have been measured. The polar angle distribution of the $\eta$ shows an anisotropy with respect to the beam axis for the lowest beam energy, which vanishes for the higher energies. The sign of this anisotropy is negative and expected to be sensitive to the dominant production mechanism. In contrast, the proton polar angle in the $pp$ rest frame tends to be more strongly aligned along the beam axis with increasing beam energy. The analyzing power $A_y$ is compatible with zero for all beam energies.

• Received 30 April 2003

DOI:https://doi.org/10.1103/PhysRevC.69.064003