Measurement of the tau branching fractions into leptons

Using data collected with the L3 detector near the Z resonance, corresponding to an integrated luminosity of 150 pb−1, the branching fractions of the tau lepton into electron and muon are measured to be B(τ→eν̄eντ)=(17.806±0.104(stat.)±0.076(syst.))%, B(τ→μν̄μντ)=(17.342±0.110(stat.)±0.067(syst.))%. From these results the ratio of the charged current coupling constants of the muon and the electron is determined to be gμ/ge=1.0007±0.0051. Assuming electron-muon universality, the Fermi constant is measured in tau lepton decays as GF=(1.1616±0.0058)×10−5 GeV−2. Furthermore, the coupling constant of the strong interaction at the tau mass scale is obtained as αs(mτ2)=0.322±0.009(exp.)±0.015(theory).