A determination of electroweak parameters from Z⁰ decays into charged leptons

Abstract

We have measured the partial widths for the three reactions e⁺e⁻ → Z⁰ → e⁺e⁻, μ⁺μ^-, τ⁺τ⁻. The results are γ_ee = 84.3±1.3 MeV, √γ_eeγ_μμ=83.9±1.4 MeV, and √γ_eeγ_π=83.9±1.4 MeV, where the errors are statistical. The systematic errors are estimated to be 1.0 MeV, 0.9 MeV, and 1.4 MeV, respectively. We perform a simultaneous fit to the cross sections for the e⁺e⁻ → Z⁰ → e⁺e^-, μ⁺μ⁻, τ⁺τ⁻ data, the differential cross section as a function of polar angle for the electron data, and the forward- backward asymmetry for the muon data. We obtain the leptonic partial with γ_ℓℓ=84.0±0.9 (stat.) MeV. The systematic error is estimated to be 0.8 MeV. Also, we obtain the axial-vector and vector weak coupling constants of charged leptons, g_A=−0.500±0.003 and g_Ν=−0.064_−0.013^+0.017.