The SINDRUM-I Experiment

SINDRUM-I was the first nearly 4\pi 4 π spectrometer at SIN. It was initially designed to search for the forbidden decay \mu^+ \rightarrow e^+e^-e^+ μ + → e + e − e + , but also successfully studied various other processes with high precision. The upper limit obtained for the branching ratio of B_{\mu \rightarrow 3e} = \Gamma (\mu^+ \rightarrow e^+e^-e^+)/\Gamma(\mu^+ \rightarrow e^+ \nu_e \bar{\nu}_{\mu}) < 1.0 \times 10^{-12} B μ → 3 e = Γ ( μ + → e + e − e + ) / Γ ( μ + → e + ν e ν ‾ μ ) < 1.0 × 10 − 12 (90% CL) from 1988 is still the best. The first statistically significant observation of the rare decay \mu^+ \rightarrow e^+e^-e^+ \nu_e {\bar{\nu}_{\mu}} μ + → e + e − e + ν e ν ‾ μ , achieved in 1985, yielded a branching ratio of B_{\mu\to3e2\nu} = (3.4 \pm 0.2 \pm 0.2) \times 10^{-5} B μ → 3 e 2 ν = ( 3.4 ± 0.2 ± 0.2 ) × 10 − 5 . Several other measurements of rare processes were undertaken. The first observation of the \pi π -decay \pi^+ \rightarrow e^+ \nu_e e^-e^+ π + → e + ν e e − e + resulted in the value \Gamma (\pi^+ \rightarrow e^+ \nu_e e^-e^+)/ \Gamma (\pi^+ \rightarrow \mu^+ \nu_{\mu}) Γ ( π + → e + ν e e − e + ) / Γ ( π + → μ + ν μ ) = (3.2 \pm 0.5 \pm 0.2) \times 10^{-9} ( 3.2 ± 0.5 ± 0.2 ) × 10 − 9 , also still the best measurement. The determination of the ratio of the weak axial- to vector-form factor F_A/F_V = (0.7 \pm 0.5) F A / F V = ( 0.7 ± 0.5 ) resolved a long-standing ambiguity. In addition, upper limits for \mu^+ \rightarrow e^+ \phi μ + → e + ϕ and \pi^+ \rightarrow e^+ \nu_e \phi π + → e + ν e ϕ with subsequent decay \phi \rightarrow e^+ e^- ϕ → e + e − (search for “massless” Goldstone bosons \phi ϕ ) and \pi^0 \rightarrow e^+e^- \gamma < 1.3 \times 10^{-7} π 0 → e + e − γ < 1.3 × 10 − 7 were obtained.