Higher partial waves in pp→ppη near threshold

Abstract Exclusive measurements of the production of η -mesons in the pp → ppη reaction have been carried out at excess energies of 16 and 37 MeV above threshold. The deviations from phase space are dominated by the proton-proton final state interaction and this influences particularly the energy distribution of the η -meson. However, evidence is also presented at the higher energy for the existence of an anisotropy in the angular distributions of the η -meson and also of the final proton-proton pair, probably to be associated with D -waves in this system interfering with the dominant S -wave term. The sign of the η angular anisotropy suggests that ρ -exchange is important for this reaction.