Measurement of electroweak parameters from hadronic and leptonic decays of the Z⁰

Abstract

We have studied the reactions e+e−→hadrons, e+e−, μ+ μ− and τ+τ−, in the energy range 88.2 ≦ √s ≦ 94.2 GeV. A total luminosity of 5.5 pb⁻¹, corresponding to approximately 115000 hadronic and 10000 leptonic Z⁰ decays, has been recorded with the L3 detector. From a simultaneous fit to all of our measured cross section data, we obtain assuming lepton universality: M_z = 91.181 ± 0.010 ± 0.02 (LEP) GeV, Γ_z = 2501 ± 17 MeV, Γ_had= 1742 ± 19 MeV, Γ_t = 83.6 ± 0.8 MeV. If we do not assume lepton universality, we obtain for the partial decay widths of the Z⁰ into e+e−, μ+μ− and τ+ τ−: Γ_e = 83.3 ± 1.1 MeV, Γ_μ = 84.5 ± 2.0 MeV, Γ_τ = 84.0 ± 2.7 MeV. From the measured ratio of the invisible and the leptonic decay widths of the Z⁰, we determine the number of light neutrino species to be Nv =3.05±0.10. We include our measurements of the forward-backward asymmetry for the leptonic channels in a fit to determine the vector and axial-vector neutral current coupling constants of charged leptons to the Z⁰. We obtain g¯_V = −0.046^+0.015_−0.012 and g¯_A = −0.500 ± 0.003. In the framework of the Standard Model, we estimate the top quark mass to be m_t =193⁺⁵²₋₆₉ ± 16 (Higgs) GeV, and we derive a value for the weak mixing angle of sin²θ_W ≡ 1 − (M_W/M_Z)² = 0.222 ± 0.008, corresponding to an effective weak mixing angle of sin²θ¯_W = 0.2315 ± 0.0025.