The ABC effect-a puzzling low-mass enhancement in the pipi invariant mass spectrum, first observed by Abashian, Booth, and Crowe-is well known from inclusive measurements of two-pion production in nuclear fusion reactions. Here we report on the first exclusive and kinematically complete measurements of the most basic double-pionic fusion reaction pn-->dpi;{0}pi;{0} at beam energies of 1.03 and 1.35 GeV. The measurements, which have been carried out at CELSIUS-WASA, reveal the ABC effect to be a (pipi)_{I=L=0} channel phenomenon associated with both a resonancelike energy dependence in the integral cross section and the formation of a DeltaDelta system in the intermediate state. A corresponding simple s-channel resonance ansatz provides a surprisingly good description of the data.

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The WASA 4π multidetector system, aimed at investigating light meson production in light ion collisions and η meson rare decays at the CELSIUS storage ring in Uppsala is presented. A detailed description of the design, together with the anticipated and achieved performance parameters are given.

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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 ± 0.5stat ± 0.8syst) × 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 .

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