Exclusive $\eta$ production in pp reactions

Angular distributions for the exclusive reaction \(pp\rightarrow pp\eta\) observed via the \(\pi^{ + }\pi^{-}\pi^{0}\) decay channel have been measured at \(T_{\rm beam} = 2.15\) GeV, 2.50 GeV and 2.85 GeV (excess energies Q = 324 MeV, 412 MeV and 554 MeV). The polar angle of the \(\eta\) shows an anisotropy with respect to the beam direction for the lowest energy, which vanishes for the higher energies. The anisotropy of the pp polar angle increases slightly with the beam energy.