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.

The production of η mesons in proton-proton collisions has been studied with the WASA detector using internal pellet targets in the CELSIUS storage ring at the The Svedberg Laboratory in Uppsala. Data were taken at two beam energies, 1360 MeV and 1445 MeV, corresponding to CM excess energies of 40 and 72 MeV, respectively. The η was detected via its 2γ decay in a near-4π electromagnetic calorimeter, whereas the protons were measured by a combination of straw chambers and plastic scintillator planes in the forward direction. The measurements were kinematically complete. The analysis yielded 69•10 3 events at 1360 MeV and 93•10 3 events at 1445 MeV, with a background contribution of less than 5%. Data were acceptance-corrected using a parametrization of a matrix element which includes all states up to two units of total angular momentum. The final state interaction between protons in the 1 S 0 state was included by a momentum-dependent enhancement factor in the relevant amplitudes. Angular distributions of the final state, invariant mass spectra and Dalitz plots are presented. The cos θ * η-distribution is found to be anisotropic with its maximum at 90 • at both energies. From the parametrization it is inferred that this is due to interference between the Ss and Sd final states. A significant contribution from the Pp final state is also needed to describe data.

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 .