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The mass and width of the W boson are measured using e+e– → W+W– events from the data sample collected by the OPAL experiment at LEP at centre-of-mass energies between 170 GeV and 209 GeV. The mass (mW) and width (ΓW) are determined using direct reconstruction of the kinematics of W+W– → and W+W– → events. When combined with previous OPAL measurements using W+W– → events and the dependence on of the WW production cross-section at threshold, the results are determined to be where the first error is statistical, the second systematic and the third due to uncertainties in the value of the LEP beam energy. By measuring in the channel using several different determinations of the direction of jets with differing sensitivities to soft particles, a limit is also obtained on possible final-state interactions due to colour reconnection effects inW+W– → events. The consistency of the results for the W mass and width with those inferred from other electroweak parameters provides an important test of the Standard Model of electroweak interactions.

A search is made for charged Higgs bosons predicted by Two-Higgs-Doublet extensions of the Standard Model (2HDM) using electron-positron collision data collected by the OPAL experiment at$\\sqrt{s}=189\\mbox{--}209\\ \\mbox{GeV}$, corresponding to an integrated luminosity of approximately 600 pb −1 . Charged Higgs bosons are assumed to be pair-produced and to decay into$\\mathrm{q} \\bar{\\mathrm{q}}$, τν τ or AW ± . No signal is observed. Model-independent limits on the charged Higgs-boson production cross section are derived by combining these results with previous searches at lower energies. Under the assumption$\\mathrm{BR} (\\mathrm{H}^{\\pm} \\to \\tau\\nu_{\\tau}) + \\mathrm{BR} (\\mathrm{H}^{\\pm} \\to \\mathrm{q} \\bar{\\mathrm{q}}) = 1$, motivated by general 2HDM type II models, excluded areas on the$[m_{\\mathrm{H}^{\\pm}} , \\mathrm{BR} (\\mathrm {H}^{\\pm} \\to \\tau\\nu_{\\tau})]$plane are presented and charged Higgs bosons are excluded up to a mass of 76.3 GeV at 95 % confidence level, independent of the branching ratio BR(H ± → τν τ ). A scan of the 2HDM type I model parameter space is performed and limits on the Higgs-boson masses$m_{\\mathrm{H}^{\\pm}}$and m A are presented for different choices of tan β .

A search is made for charged Higgs bosons predicted by Two-Higgs-Doublet extensions of the Standard Model (2HDM) using electron-positron collision data collected by the OPAL experiment at , corresponding to an integrated luminosity of approximately 600 pb −1 . Charged Higgs bosons are assumed to be pair-produced and to decay into , τν τ or AW ± . No signal is observed. Model-independent limits on the charged Higgs-boson production cross section are derived by combining these results with previous searches at lower energies. Under the assumption , motivated by general 2HDM type II models, excluded areas on the plane are presented and charged Higgs bosons are excluded up to a mass of 76.3 GeV at 95 % confidence level, independent of the branching ratio BR(H ± → τν τ ). A scan of the 2HDM type I model parameter space is performed and limits on the Higgs-boson masses and m A are presented for different choices of tan β .

Hadronic event shape distributions from e + e − annihilation measured by the OPAL experiment at centre-of-mass energies between 91 GeV and 209 GeV are used to determine the strong coupling α S . The results are based on QCD predictions complete to the next-to-next-to-leading order (NNLO), and on NNLO calculations matched to the resummed next-to-leading-log-approximation terms (NNLO + NLLA). The combined NNLO result from all variables and centre-of-mass energies is while the combined NNLO + NLLA result is The completeness of the NNLO and NNLO + NLLA results with respect to missing higher order contributions, studied by varying the renormalization scale, is improved compared to previous results based on NLO or NLO + NLLA predictions only. The observed energy dependence of α S agrees with the QCD prediction of asymptotic freedom and excludes the absence of running.

From a total data sample of 701.1 pb-1 recorded with e+e- centre-of-mass energies of \\(\\sqrt{s} =\\)161–209 GeV with the OPAL detector at LEP, 11693 W-pair candidate events are selected. These data are used to obtain measurements of the W-pair production cross sections at 10 different centre-of-mass energies. The ratio of the measured cross sections to the standard model expectation is found to be: \\({\\text{data}}/{{\\text{SM}}} = 1.002\\pm0.011 ({\\text{stat.}}) \\pm0.007 ({\\text{syst.}}) \\pm0.005 ({\\text{theory}})\\), where the uncertainties are statistical, experimental systematics and theory systematics respectively. The data are used to determine the W boson branching fractions, which are found to be consistent with lepton universality of the charged current interaction. Assuming lepton universality, the branching ratio to hadrons is determined to be 67.41±0.37(stat.)±0.23(syst.)%, from which the CKM matrix element |Vcs| is determined to be 0.969±0.017(stat.)±0.012(syst.). The differential cross section as a function of the W- production angle is measured for the qqeν and qqμν final states. The results described in this paper are consistent with the expectations from the standard model.

Data collected around  GeV by the OPAL experiment at the LEP e + e − collider are used to study the mechanism of baryon formation. As the signature, the fraction of Σ − hyperons whose baryon number is compensated by the production of a or antihyperon is determined. The method relies entirely on quantum number correlations of the baryons, and not rapidity correlations, making it more model independent than previous studies. Within the context of the JETSET implementation of the string hadronization model, the diquark baryon production model without the popcorn mechanism is strongly disfavored with a significance of 3.8 standard deviations including systematic uncertainties. It is shown that previous studies of the popcorn mechanism with and correlations are not conclusive, if parameter uncertainties are considered.

A search is described for the neutral Higgs bosons h0 and A0 predicted by models with two scalar field doublets and, in particular, the Minimal Supersymmetric Standard Model (MSSM). The search in the Z0h0 and h0A0 production channels is based on data corresponding to an integrated luminosity of 25 pb−1 from e+e− collisions at centre-of-mass energies between 130 and 172 GeV collected with the OPAL detector at LEP. The observation of a number of candidates consistent with Standard Model background expectations is used in combination with earlier results from data collected at the Z0 resonance to set limits on mh and ma in general models with two scalar field doublets and in the MSSM. For example, in the MSSM, for tan β > 1, minimal and maximal scalar top quark mixing and soft SUSY-breaking masses of 1 TeV, the 95% confidence level limits mh > 59.0 GeV and ma > 59.5 GeV are obtained. For the first time, the MSSM parameter space is explored in a detailed scan.

A search for the resonant production of high mass photon pairs associated with a leptonic or hadronic system has been performed using a total data sample of 25.7 pb−1 taken at centre-of-mass energies between 130 GeV and 172 GeV with the OPAL detector at LEP. The observed number of events is consistent with the expected number from Standard Model processes. The observed candidates are combined with search results from ∝s ≈ Mz to place limits onB(H0 → γγ within the Standard Model for Higgs boson masses up to 77 GeV, and on the production cross section of any scalar resonance decaying into di-photons. Upper limits onB(H0 →4 γγ x σ(e+e− → H°Z°) of 290 – 830 fb are obtained over 40

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