Abstract
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.
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