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

The OPAL Collaboration G. Abbiendi; et al.

doi:doi:10.1007/s100520100627

2021 Impact factor 4.991

Particles and Fields

Eur. Phys. J. C 19, 587-651
DOI: 10.1007/s100520100627

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

The OPAL Collaboration
G. Abbiendi and et al.

Dipartimento di Fisica dell' Università di Bologna and INFN, 40126 Bologna, Italy

(Received: 1 December 2000 / Published online: 6 April 2001 -© Springer-Verlag 2001)

Abstract
This final analysis of hadronic and leptonic cross-sec tions and of leptonic forward-backward asymmetries in e⁺e^- collisions with the OPAL detector makes use of the full LEP1 data sample comprising 161 pb^-1 of integrated luminosity and $4.5\times10^6$ selected Z decays. An interpretation of the data in terms of contributions from pure Z exchange and from $\gamma/\mathrm{Z}$ 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: $m_{\mathrm{Z}} = (91.1852 \pm 0.0030) {GeV}$ , $\Gamma_{\mathrm{Z}} = (2.4948 \pm 0.0041) {GeV}$ , $\sigma^0_{\mathrm{h}} = (41.501 \pm 0.055) {nb}$ , $R_{\ell} = 20.823 \pm 0.044$ , and $A_{\mathrm{FB}}^{0,\ell} = 0.0145 \pm 0.0017$ . 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, $\Gamma_{\mathrm{inv}}/\Gamma_{\ell\ell} = 5.942 \pm 0.027$ . 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, $N_{\nu} = 2.984 \pm 0.013$ . 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: $\alpha_{\mathrm{s}}(m_{\mathrm{Z}}) = 0.127 \pm 0.005$ and m_H = (390⁺⁷⁵⁰_-280) GeV.