Experimental data from the reaction close to threshold have been used to investigate time-dependent properties of the WASA pellet target and the stored beam of 892.8 MeV protons in the CELSIUS accelerator. The detection of ³He ions in solid-state detectors provides an excellent almost background free trigger for η production. Pellet interactions with the surrounding material and with the beam lead to evaporation of deuterium gas into the beam pipe. Approximately 30% of the registered η decay events are due to interactions in deuterium gas outside the pellets. Due to beam–target interaction the beam is transversely heated and appears to acquire a component of a slightly lower energy.

A zero-degree spectrometer for tagging η mesons on the CELSIUS/WASA facility is described, and its characteristics are presented. Tagging of η mesons is performed by detecting ³He ions produced by reaction pd → ³Heη close to the production threshold. The low background level (<2%) is in good agreement with the data obtained earlier on the SATURNE accelerator.

The reaction at threshold was used to provide a clean source of η mesons for decay studies with the WASA detector at CELSIUS. The branching ratio of the decay η→π⁺π⁻e⁺e⁻ is measured to be (4.3±1.3±0.4)×10⁻⁴.

A search for rare lepton decays of the η meson was performed using the WASA detector at CELSIUS. Two candidates for double Dalitz decay η→e⁺e^-e⁺e^- events are reported with a background of 1.3±0.2 events. This allows to set an upper limit to the branching ratio of 9.7×10^-5 (90% CL). The branching ratio for the decay η→e⁺e^-γ is determined to (7.8±0.5_stat±0.8_syst)×10^-3 in agreement with world average value. An upper limit (90% CL) for the branching ratio for the η→e⁺e^- decay is 2.7×10^-5 and a limit for the sum of the η→μ⁺μ^-μ⁺μ^- and η→π⁺π^-μ⁺μ^- decays is 3.6×10^-4.