A Precise Measurement of the Z Resonance Parameters through its Hadronic Decays

A measurement of the cross section for e+e-→ hadrons using 11 000 hadronic decays of the Z boson at ten different center-of-mass energies is presented. A three-parameter fit gives the following values for the Z mass MZ, the total width ΓZ, the product of the electronic and hadronic partial widths ΓeΓh, and the unfolded pole cross section σ0: MZ = 91.171 ± 0.030 (stat.) ± 0.030 (beam) GeV, ΓZ = 2.511 ± 0.065 GeV, ΓeΓh = 0.148 ± 0.006 (stat.) ± 0.004 (syst.) GeV2, σ0 = 41.6 ± 0.7 (stat.) ± 1.1 (syst.) nb, Good agreement with the predictions of the standard model is observed. From a two-parameter fit the number of massless neutrino generations is found to be Nv = 2.91 ± 0.26. Thus the hypothesis of a fourth neutrino with mass less than 40 GeV is excluded with 95% confidence level. Combining the cross section measurements with the ratio Γl/Γh reported in another DELPHI paper [Phys. Lett. B 241 (1990) 425], the hadronic, leptonic and invisible widths are found to be Γh = 1741 ± 61 MeV, Γl = 85.1 ± 2.9 MeV, Γh/Γl = 20.45 ± 0.98, Γinv = 515 ± 54 MeV, in good agreement with the standard model.0SCOPUS: ar.j