Measurement of the tau branching fractions into leptons

Abstract

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→eν̅_eν_τ) = (17.806±0.104 (stat.) ± 0.076 (syst.) %, B(τ→μν̅ν_τ) = (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 g_μ/g_e=1.0007±0.0051. Assuming electron-muon universality, the Fermi constant is measured in tau lepton decays as G_F=(1.1616±0.0058)×10⁻⁵ GeV⁻². Furthermore, the coupling constant of the strong interaction at the tau mass scale is obtained as α_s(m²_τ) = 0.322 ± 0.009(exp.) ± 0.015 (theory).