Precise determination of the Z resonance parameters at LEP: ``Zedometry''

This final analysis of hadronic and leptonic cross-sections and of leptonic forward-backward asymmetries in ^+- collisions with the OPAL detector makes use of the full LEP 1 data sample comprising 161 {pb}^-1 of integrated luminosity and 4.5×10⁶ selected Z decays. An interpretation of the data in terms of contributions from pure Z exchange and from γ /Z interference allows the parameters of the Z resonance to be determined in a model-independent way. Our results are in good agreement with lepton universality and consistent with the vector and axial-vector couplings predicted in the Standard Model. A fit to the complete dataset yields the fundamental Z resonance parameters: m_Z = (91.1852 +/- 0.0030) GeV, Γ _Z = (2.4948 +/- 0.0041) GeV, σ⁰_h = (41.501 +/- 0.055) nb, R_l = 20.823 +/- 0.044, and A_FB^0,l = 0.0145 +/- 0.0017. Transforming these parameters gives a measurement of the ratio between the decay width into invisible particles and the width to a single species of charged lepton, Γ _inv/Γ _ll = 5.942 +/- 0.027. Attributing the entire invisible width to neutrino decays and assuming the Standard Model couplings for neutrinos, this translates into a measurement of the effective number of light neutrino species, N_ν = 2.984 +/- 0.013. Interpreting the data within the context of the Standard Model allows the mass of the top quark, m_t = (162 ⁺²⁹_-16) GeV, to be determined through its influence on radiative corrections. Alternatively, utilising the direct external measurement of m_t as an additional constraint leads to a measurement of the strong coupling constant and the mass of the Higgs boson: α _s(m_Z) = 0.127 +/- 0.005 and m_H = (390⁺⁷⁵⁰_-280) GeV.