Precise determination of the Z resonance parameters at LEP: "Zedometry"

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Colleges, School and Institutes


This final analysis of hadronic and leptonic cross-sections and of leptonic forward-backward asymmetries in e(+)e(-) collisions With the OPAL detector makes use of the full LEP 1 data sample comprising 161pb(-1) of integrated luminosity and 4.5 x 10(6) selected Z decays. An interpretation of the data in terms of contributions from pure Z exchange and from gamma /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 anti 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. Gamma (Z) = 12.4948 +/- 0.0041) GeV. sigma (0)(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. Gamma (inv)/Gamma (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-nu = 2.984 +/- 0.013. Interpreting the data within the context of the Standard Model allows the mass of the top quark, m(t) = (162(-16)(+29)) GeV, to be determined through its influence oil 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: alpha (s)(m(Z)) = 0.127 +/- 0.005 and m(H) = (390(-280)(+750)) GeV.


Original languageEnglish
Pages (from-to)587-651
Number of pages65
JournalEuropean Physical Journal C
Issue number4
Publication statusPublished - 1 Mar 2001