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Mercury's transit on the solar disk offers ideal conditions to determine the stray light level of instruments. We present here the results on the stray light level deduced from the observation of the Mercury transit on 2003 May 7th at the secondary focus of the THEMIS telescope with the broad-band and spectral channels of the IPM instrument. The scattered light in the broad-band channel is about 17% and about 25% in the spectral channel. The spread function was deduced for the two channels taking into account the observations on the limb and on Mercury's disk.The goal of this paper is to underline the limits of determining the spread function from limb measurements to correct disk observations. Indeed, we show that if a diaphragm is located in the optical path of scattering surfaces, then the spread function deduced from limb measurements can be underestimated compared to the one required for disk observations. The case is illustrated with the results of the IPM-THEMIS instrument. The spread function deduced from limb measurements is able to correct disk observations in the broad-band channel but not in the spectral channel, even if the two channels are illuminated through the same telescope configuration.

This paper describes topological searches for neutral scalar bosons S0 produced in association with a Z0 boson via the Bjorken process \\(e^+e^- \\to{\\rm S}^0{}{\\rm Z}^0\\) at centre-of-mass energies of 91 GeV and 183-209 GeV. These searches are based on studies of the recoil mass spectrum of \\({\\rm Z}^0 \\to e^+e^-\\) and \\(\\mu^+ \\mu^-\\) events and on a search for \\({\\rm S}^0{\\rm Z}^0\\) with \\({\\rm Z}^0 \\to \\nu\\bar{\\nu}\\) and S\\(^0 \\to e^+ e^-\\) or photons. They cover the decays of the S0 into an arbitrary combination of hadrons, leptons, photons and invisible particles as well as the possibility that it might be stable. No indication for a signal is found in the data and upper limits on the cross section of the Bjorken process are calculated. Cross-section limits are given in terms of a scale factor k with respect to the Standart Model cross section for the Higgs-strahlung process \\(e^+ e^-\\to{\\rm H}_{\\rm SM}{\\rm Z}^0\\). These results can be interpreted in general scenarios independently of the decay modes of the S0. The examples considered here are the production of a single new scalar particle with a decay width smaller than the detector mass resolution, and for the first time, two scenarios with continuous mass distributions, due to a single very broad state or several states close in mass.

A study of \\(\\mathrm{b}\\) quark hadronisation is presented using inclusively reconstructed B hadrons in about four million hadronic Z decays recorded in 1992-2000 with the OPAL detector at LEP. The data are compared to different theoretical models, and fragmentation function parameters of these models are fitted. The average scaled energy of weakly decaying B hadrons is determined to be \\[\\langle{x}_{ \\mathrm{E}}\\rangle = 0.7193 \\pm 0.0016 (stat)^{+0.0038}_{-0.0033}(syst) \\ . \\]

A search was performed for charginos with masses close to the mass of the lightest neutralino in \\(\\mathrm{e}^{+}\\mathrm{e}^{-}\\) collisions at centre-of-mass energies of 189-209 GeV recorded by the OPAL detector at LEP. Events were selected if they had an observed high-energy photon from initial state radiation, reducing the dominant background from two-photon scattering to a negligible level. No significant excess over Standard Model expectations has been observed in the analysed data set corresponding to an integrated luminosity of 570 pb-1. Upper limits were derived on the chargino pair-production cross-section, and lower limits on the chargino mass were derived in the context of the Minimal Supersymmetric Extension of the Standard Model for the gravity- and anomaly-mediated Supersymmetry breaking scenarios.

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 .

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 .

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.