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 94.2GeV. A total luminosity of 5.5 pb^-1, corresponding to approximately 115000 hadronic and 10000 leptonic Z ⁰ 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±17MeV, Γ^had=1742±19MeV, Γ^t=83.6±0.8MeV. If we do not assume lepton universality, we obtain for the partial decay widths of the Z ⁰ into e ⁺ e ^- μ⁺ μ^- and τ ⁺ τ^-: Γ_e=83.3 ± 1.1MeV, Γ_μ=84.5 ± 2.0MeV, Γ _τ=84.0 ± 2.7MeV. From the measured ratio of the invisible and the leptonic decay widths of the Z ⁰, we determine the number of light neutrino species to be N_v =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 Z ⁰. 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 bem t =193-69 ⁺⁵² ±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.