Measurement of the W Boson Mass and W + W − Production and Decay Properties in e + e − Collisions at √ s = 172 GeV

This paper describes the measurement of the W boson mass, MW, and decay width, ΓW, 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 √ 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 WW events has been selected for an integrated luminosity of 10.36 pb. The WW production cross-section is measured to be σWW = 12.3±1.3(stat.)±0.3(syst.) pb, consistent with the Standard Model expectation. The WW → qqlνl and WW → qqqq final states are used to obtain a direct measurement of ΓW = 1.30 −0.55(stat.) ± 0.18(syst.) GeV. Assuming the Standard Model relation between MW and ΓW, the W boson mass is measured to be MW = 80.32 ± 0.30(stat.) ± 0.09(syst.) GeV. The event properties of the fullyhadronic decays of WW 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 √ s ∼ 161–172 GeV, the W boson branching fraction to hadrons is determined to be 69.8 −3.2(stat.)±0.7(syst.)%, consistent with the prediction of the Standard Model. The combined mass measurement from direct reconstruction and from the WW production crosssections measured at √ s ∼ 161 and √s ∼ 172 GeV is MW = 80.35 ± 0.24(stat.) ± 0.07(syst.) GeV. To be submitted to Zeit. Phys. C The OPAL Collaboration K.Ackerstaff, G.Alexander, J.Allison, N. Altekamp, K.J.Anderson, S.Anderson, S.Arcelli, S.Asai, D. Axen, G.Azuelos, A.H. Ball, E.Barberio, R.J. Barlow, R. Bartoldus, J.R.Batley, S. Baumann, J. Bechtluft, C.Beeston, T.Behnke, A.N. Bell, K.W.Bell, G. Bella, S. Bentvelsen, S. Bethke, O.Biebel, A. Biguzzi, S.D. 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