Measurement of the Z resonance parameters at LEP

Abstract. The properties of the Z resonance are measured from the analysis of 4.5 million Z decays into fermion pairs collected with the Aleph detector at LEP. The data are consistent with lepton universality. The resonance parameters are measured to be $M_{\rm Z}=(91.1885 \pm 0.0031){\rm GeV}/c^2$, $\Gamma_{\rm Z}= (2.4951 \pm 0.0043)$ GeV, $\sigma^0_{\rm had}=(41.559 \pm 0.058)$ nb and, combining the three lepton flavours, $R_{\ell}= 20.725\pm 0.039$. The corresponding number of light neutrino species is $N_{\nu}=2.983\pm0.013$ and the strong coupling constant is $\alpha_s(M_{\rm Z}) = 0.114\pm{0.004} \pm 0.002_{\rm{QCD}} +\,0.005\,\log_{10}\left[\frac{M_{\rm H}}{150\,{\rm GeV}/c^2}\right]\,$. The lepton pair forward-backward asymmetry is measured to be ${\rm A}_{\rm FB}^{0, \ell}=0.0173 \pm 0.0016$ from which the effective weak mixing angle is derived: $\sin^2\theta_{\rm eff}^{\rm lept} = 0.23089\pm 0.00089$. The measurement of the leptonic width $\Gamma_{\ell\ell}=84.02\pm0.15$ MeV leads to a determination of the effective $\rho$ parameter $\rho_{\rm eff}^{\rm lept} = 1.0064 \pm 0.0018$. The data support the Standard Model and favour a light Higgs.