Measurement of the W boson mass and \({\rm W^+W^-}\) production and decay properties in \({\rm e}^+{\rm e}^-\) collisions at \({\sqrt s = 172}\) GeV

  • The OPAL Collaboration
  • K. Ackerstaff et al.
Experimental physics


This paper describes the measurement of the W boson mass, \(M_W\), and decay width, \(\Gamma_{\rm W}\), from the direct reconstruction of the invariant mass of its decay products in W pair events collected at a mean centre-of-mass energy of \(\sqrt s=172.12\) GeV with the OPAL detector at LEP. Measurements of the W pair production cross-section, the W decay branching fractions and properties of the W decay final states are also described. A total of 120 candidate \({\rm W^+W^-}\) events has been selected for an integrated luminosity of 10.36 pb\(^{-1}\). The \({\rm W^+W^-}\) production cross-section is measured to be \(\sigma_{\rm WW}=12.3\pm1.3{\rm (stat.)}\pm0.3{\rm (syst.)}\) pb, consistent with the Standard Model expectation. The \({\rm W^+W^-\to q\bar q \ell \bar \nu_\ell}\) and \({\rm W^+W^-} \to{\rm q \overline q q \overline q}\) final states are used to obtain a direct measurement of \(\Gamma_{\rm W} =1.30^{+0.70}_{-0.55}{\rm (stat.)}\pm0.18{\rm (syst.)}\) GeV. Assuming the Standard Model relation between \(M_{\rm W}\) and \(\Gamma_{\rm W}\), the W boson mass is measured to be \(M_{\rm W}=80.32\pm0.30{\rm (stat.)}\pm0.09{\rm (syst.)}\) GeV. The event properties of the fully-hadronic decays of \({\rm W^+W^-}\) events are compared to those of the semi-leptonic decays. At the current level of precision there is no evidence for effects of colour reconnection in the observables studied. Combining data recorded by OPAL at \(\sqrt s\sim161\)–172 GeV, the W boson branching fraction to hadrons is determined to be \(69.8^{+3.0}_{-3.2}{\rm (stat.)}\pm0.7{\rm (syst.)}\)%, consistent with the prediction of the Standard Model. The combined mass measurement from direct reconstruction and from the \({\rm W^+W^-}\) production cross-sections measured at \(\sqrt s\sim161\) and \(\sqrt s\sim172\) GeV is \(M_{\rm W} = 80.35 \pm 0.24{\rm (stat.)}\pm0.07{\rm (syst.)}\) GeV.


Invariant Mass Pair Production Mass Measurement Opal Detector Decay Width 
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Copyright information

© Springer-Verlag Berlin Heidelberg 1998

Authors and Affiliations

  • The OPAL Collaboration
  • K. Ackerstaff et al.
    • 1
  1. 1. CERN, European Organisation for Particle Physics, CH-1211 Geneva 23, Switzerland CH

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