Measurement of the Z resonance parameters at LEP

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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.

Received: 16 July 1999 / Published online: 17 March 2000