Update of electroweak parameters from Z decays

Based on 520 000 fermion pairs accumulated during the first three years of data collection by the ALEPH detector at LEP, updated values of the resonance parameters of theZ are determined to beM Z =(91.187±0.009) GeV, Γ Z =(2.501±0.012) GeV, σ had 0 =(41.60±0.27) nb, andRl=20.78±0.13. The corresponding number of light neutrino species isNν=2.97±0.05. The forward-backward asymmetry in lepton-pair decays is used to determine the ratio of vector to axial-vector couplings of leptons:g V 2 (M Z 2 )/g A 2 (M Z 2 )=0.0052±0.0016. Combining this with ALEPH measurements of theb andc quark asymmetries and τ polarization gives sin2θ W eff =0.2326±0.0013. Assuming the minimal Standard Model, and including measurements ofM W /M Z fromp\(\bar p\) colliders and neutrino-nucleon scattering, the mass of the top quark is\(M_{top} = 156 \pm \begin{array}{*{20}c} {22} \\ {25} \\ \end{array} \pm \begin{array}{*{20}c} {17} \\ {22Higgs} \\ \end{array} \) GeV.