The SINDRUM-I Experiment

Abstract

SINDRUM-I was the first nearly 4\pi4π 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}Bμ→3e=Γ(μ+→e+e−e+)/Γ(μ+→e+νeν‾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+νeν‾μ, achieved in 1985, yielded a branching ratio of B_{\mu\to3e2\nu} = (3.4 \pm 0.2 \pm 0.2) \times 10^{-5}Bμ→3e2ν=(3.4±0.2±0.2)×10−5. Several other measurements of rare processes were undertaken. The first observation of the \piπ-decay \pi^+ \rightarrow e^+ \nu_e e^-e^+π+→e+νee−e+ resulted in the value \Gamma (\pi^+ \rightarrow e^+ \nu_e e^-e^+)/ \Gamma (\pi^+ \rightarrow \mu^+ \nu_{\mu})Γ(π+→e+νee−e+)/Γ(π+→μ+νμ) = (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)FA/FV=(0.7±0.5) resolved a long-standing ambiguity. In addition, upper limits for \mu^+ \rightarrow e^+ \phiμ+→e+ϕ and \pi^+ \rightarrow e^+ \nu_e \phiπ+→e+νeϕ with subsequent decay \phi \rightarrow e^+ e^-ϕ→e+e− (search for “massless” Goldstone bosons \phiϕ) and \pi^0 \rightarrow e^+e^- \gamma < 1.3 \times 10^{-7}π0→e+e−γ<1.3×10−7 were obtained.