A search for the reaction [mu]⁻ + Cu [arrow] e⁺ + Co

A search has been performed for the reaction µ⁻ + Cu ⟶ e⁺ + Co, which is allowed by the lepton conservation scheme that assigns the same lepton number to the µ⁻, e⁺, v_µ , and vₑ. Momentum analysis of particles emitted following muon capture was performed using a magnetic spectrometer with magnetostrictive readout wire spark chambers. The time of flight through the spectrometer and energy loss in a scintillation counter were measured for each event. One event was observed, at momentum 89.9 ± 3.5 MeV/c, which is consistent with the above process. Background from radiative muon capture was expected to produce ~ 0.01 events that could simulate the searched for reaction. However, there exists uncertainty in the relative yield and cutoff for high energy photons from radiative muon capture. Although we cannot be certain whether or not the reaction µ⁻ + Cu ⟶ e⁺ + Co has been detected, we can set an upper limit on the branching ratio relative to ordinary muon capture of R < 2.6 x 10⁻⁸ (90% confidence level). According to a calculation by Kisslinger, this can be related to an upper limit on the coupling constant of this type of interaction relative to the vector coupling constant of (G_Δ I Gᵥ) ≤ 0.10^+0.05_-0.03.