EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH CERN{PPE/94{08 12 January 1994 Measurements of the Lineshape of the Z and Determination of Electroweak Parameters from its Hadronic and Leptonic Decays

During the LEP running periods in 1990 and 1991 DELPHI has accumulated approximately 450,000 Z decays into hadrons and charged leptons. The increased event statistics coupled with improved analysis techniques and improved knowledge of the LEP beam energies permit signi cantly better measurements of the mass and width of the Z resonance. Model independent ts to the cross sections and leptonic forward-backward asymmetries yield the following Z parameters: the mass and total width M Z = 91:187 0:009 GeV, Z = 2:486 0:012 GeV, the hadronic and leptonic partial widths had = 1:725 0:012 GeV, l = 83:01 0:52 MeV, the invisible width inv = 512 10 MeV, the ratio of hadronic to leptonic partial widths Rl = 20:78 0:15, and the Born level hadronic peak cross section 0 = 40:90 0:28 nb. Using these results and the value of s determined from DELPHI data, the number of light neutrino species is determined to be 3:08 0:05. The individual leptonic widths are found to be: e = 82:93 0:70 MeV, = 83:20 1:11 MeV and = 82:89 1:31 MeV. Using the measured leptonic forward-backward asymmetries and assuming lepton universality, the squared vector and axial-vector couplings of the Z to charged leptons are found to be g Vl = (1:47 0:51) 10 3 and g Al = 0:2483 0:0016. A full Standard Model t to the data yields a value of the top mass mt = 115 +52 82(expt:) +23 24(Higgs) GeV, corresponding to a value of the weak mixing angle sin lept eff = 0:2339 0:0015(expt:) 0:0004(Higgs). Values are obtained for the variables S and T, or 1 and 3 which parameterize electroweak loop e ects. (To be submitted to Nuclear Physics B)