Measurement of electroweak parameters from hadronic and leptonic decays of theZ ⁰

Abstract

We have studied the reactionse ⁺ e ⁻→hadrons,e ⁺ e ⁻, μ⁺ μ⁻ and τ⁺ τ⁻, in the energy range 88.2$\leqq \sqrt s \leqq 94.2$ GeV. A total luminosity of 5.5 pb⁻¹, corresponding to approximately 115000 hadronic and 10000 leptonicZ 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:

$$\begin{gathered} M_z = 91.181 \pm 0.010 \pm 0.02 (LEP) GeV, \hfill \\ \Gamma _z = 2501 \pm 17 MeV, \hfill \\ \Gamma _{had} = 1742 \pm 19 MeV, \Gamma _t = 83.6 \pm 0.8 MeV. \hfill \\ \end{gathered}$$

If we do not assume lepton universality, we obtain for the partial decay widths of theZ ⁰ intoe ⁺ e ⁻ μ⁺ μ⁻ and τ⁺ τ⁻:

$$\begin{gathered} \Gamma _e = 83.3 \pm 1.1 MeV, \Gamma _\mu = 84.5 \pm 2.0 MeV, \hfill \\ \Gamma _\tau = 84.0 \pm 2.7 MeV. \hfill \\ \end{gathered}$$

From the measured ratio of the invisible and the leptonic decay widths of theZ ⁰, we determine the number of light neutrino species to beN _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 theZ ⁰. We obtain$\bar g_V = - 0.046_{ - 0.012}^{ + 0.015}$ and$\bar g_A = - 0.500 \pm 0.003$. In the framework of the Standard Model, we estimate the top quark mass to bem _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$\sin ^2 \bar \theta _W = 0.2315 \pm 0.0025$.