Abstract
A search for the decay μ + → e +e +e −, which violates the conservation of muon number, was done at the πE3 channel of the Swiss Institute for Nuclear Research (SIN). The experiment used a large-solid-angle magnetic spectrometer (SINDRUM), equipped with cylindrical multiwire proportional chambers and a scintillator hodoscope. A total of 7.3 × 10 12 μ + were stopped in an 11 mg/cm 2 foam target. Accidental background was reduced by requiring e +e +e − events to be prompt and to have a good vertex. The remaining 7443 events are compatible both in number and momentum distribution with the decay μ + → e + e + e −ν e ν μ , if one assumes the standard electroweak theory. No candidate for the decay μ + → e +e +e − was found. Taking into account the overall acceptance for this decay of (13.8 ± 0.5)% a new upper limit for the branching ratio B μ → 3 e = μ(μ + → e + e + e −)/μ(μ + → e +ν e ν μ) < 2.4 × 10 −12 (90% confidence) is derived.
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