2018 Impact factor 4.843
Particles and Fields
Eur. Phys. J. C 1, 395-424

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

The OPAL Collaboration, K. Ackerstaff 8 et al.

8 CERN, European Organisation for Particle Physics, CH-1211 Geneva 23, Switzerland

Received: 8 September 1997

Abstract
This paper describes the measurement of the W boson mass, \ensuremath{M_{{\rm W}}}, and decay width, \ensuremath{\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 \ensuremath{\sqrt{s}}=172.12GeV 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 \ensuremath{{\rm W}^+{\rm W}^-} events has been selected for an integrated luminosity of 10.36pb-1. The \ensuremath{{\rm W}^+{\rm W}^-} production cross-section is measured to be $\ensuremath{\sigma_{{\rm WW}}}=12.3\pm1.3 {\rm (stat.)}\pm0.3{\rm (syst.)}$pb, consistent with the Standard Model expectation. The \ensuremath{\ensuremath{{\rm W}^+{\rm W}^-}\rightarrow\ensuremath{{\rm q\overline{q}}}\ensuremath{\ell\overline{\nu}_{\ell}}} and ${\rm W^+W^-}$ $\to$ ${\rm q \overline q q \overline q}$ final states are used to obtain a direct measurement of $\ensuremath{\Gamma_{{\rm W}}}=1.30^{+0.70}_{-0.55}{\rm (stat.)}\pm0.18{\rm (syst.)}$GeV. Assuming the Standard Model relation between \ensuremath{M_{{\rm W}}} and \ensuremath{\Gamma_{{\rm W}}}, the W boson mass is measured to be $\ensuremath{M_{{\rm W}}}=80.32\pm0.30{\rm (stat.)}\pm0.09{\rm (syst.)}$GeV. The event properties of the fully-hadronic decays of \ensuremath{{\rm W}^+{\rm 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 $\ensuremath{\sqrt{s}}\sim161$-172GeV, 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 \ensuremath{{\rm W}^+{\rm W}^-} production cross-sections measured at $\ensuremath{\sqrt{s}}\sim161$ and $\ensuremath{\sqrt{s}}\sim172$GeV is $\ensuremath{M_{{\rm W}}}= 80.35 \pm 0.24{\rm (stat.)}\pm0.07{\rm (syst.)}$ GeV.


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