A study of QCD coherence is presented based on a sample of about 397,000 \(e^{+}e^{-}\) hadronic annihilation events collected at \(\sqrt{s}=91\) GeV with the OPAL detector at LEP. The study is based on four recently proposed observables that are sensitive to coherence effects in the perturbative regime. The measurement of these observables is presented, along with a comparison with the predictions of different parton shower models. The models include both conventional parton shower models and dipole antenna models. Different ordering variables are used to investigate their influence on the predictions.

The four LEP collaborations, ALEPH, DELPHI, L3 and OPAL, have searched for pair-produced charged Higgs bosons in the framework of Two Higgs Doublet Models (2HDMs). The data of the four experiments have been statistically combined. The results are interpreted within the 2HDM for Type I and Type II benchmark scenarios. No statistically significant excess has been observed when compared to the Standard Model background prediction, and the combined LEP data exclude large regions of the model parameter space. Charged Higgs bosons with mass below 80 \({\rm GeV}/c^{2}\) (Type II scenario) or 72.5 \({\rm GeV}/c^{2}\) (Type I scenario, for pseudo-scalar masses above 12 \({\rm GeV}/c^{2}\)) are excluded at the 95 % confidence level.

AbstractA 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 $\mathrm{q} \bar{\mathrm{q}}$, τντ 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±→τντ). A scan of the 2HDM type I model parameter space is performed and limits on the Higgs-boson masses $m_{\mathrm{H}^{\pm}}$ and mA are presented for different choices of tanβ.

Data collected around sqrt{s}=91 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 Sigma-hyperons whose baryon number is compensated by the production of a Sigma-, Lambda or Xi- 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. The diquark fragmentation 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 are not conclusive if parameter uncertainties are considered.

This letter describes a direct search for pair produced magnetic monopoles in e+e collisions. The analysis is based on 62.7 pb 1 of data collected with the OPAL detector at an average centre- of-mass energy of √ s= 206.3 GeV. The monopole signal was assumed to be characterized by two back-to-back particles with an anomalously high ionization energy loss dE/dx in the tracking chambers. No evidence for production of monopoles was observed. Upper limits were obtained on the magnetic monopole pair-production cross-section (σ) in the mass range 45 GeV/c2 < mM < 102 GeV/c2. The average limit is σ < 0.05 pb and is essentially independent of the magnetic monopole mass. The cross-section limit is derived at the 95% confidence level and is valid for spin-1/2 magnetic monopoles.

Hadronic final states with a hard isolated photon are studied using data taken at centre-of-mass energies around the mass of the Z boson with the OPAL detector at LEP. The strong coupling α s is extracted by comparing data and QCD predictions for event shape observables at average reduced centre-of-mass energies ranging from 24 GeV to 78 GeV, and the energy dependence of α s is studied. Our results are consistent with the running of α s as predicted by QCD and show that within the uncertainties of our analysis event shapes in hadronic Z decays with hard and isolated photon radiation can be described by QCD at reduced centre-of-mass energies. Combining all values from different event shape observables and energies gives α s (M Z )=0.1182±0.0015(stat.)±0.0101(syst.).

This paper describes a topological search for an invisibly decaying Higgs boson, H, produced via the Bjorken process (e + e - →HZ). The analysis is based on data recorded using the OPAL detector at LEP at centre-of-mass energies from 183 to 209 GeV corresponding to a total integrated luminosity of 629 pb -1 . In the analysis only hadronic decays of the Z boson are considered. A scan over Higgs boson masses from 1 to 120 GeV and decay widths from 1 to 3000 GeV revealed no indication for a signal in the data. From a likelihood ratio of expected signal and standard model background we determine upper limits on cross-section times branching ratio to an invisible final state. For moderate Higgs boson decay widths, these range from about 0.07 pb (M H =60 GeV) to 0.57 pb (M H =114 GeV). For decay widths above 200 GeV the upper limits are of the order of 0.15 pb. The results can be interpreted in general scenarios predicting a large invisible decay width of the Higgs boson. As an example we interpret the results in the so-called stealthy Higgs scenario. The limits from this analysis exclude a large part of the parameter range of this scenario experimentally accessible at LEP 2.

A study of Bose-Einstein correlations in pairs of identically charged pions produced in e+e- annihilations at the Z0 peak has been performed for the first time assuming a non-static emitting source. The results are based on the high statistics data obtained with the OPAL detector at LEP. The correlation functions have been analyzed in intervals of the average pair transverse momentum and of the pair rapidity, in order to study possible correlations between the pion production points and their momenta (position-momentum correlations). The Yano-Koonin and the Bertsch-Pratt parameterizations have been fitted to the measured correlation functions to estimate the geometrical parameters of the source as well as the velocity of the source elements with respect to the overall centre-of-mass frame. The source rapidity is found to scale approximately with the pair rapidity, and both the longitudinal and transverse source dimensions are found to decrease for increasing average pair transverse momenta.

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.

The talk covers three contributions on (i) the final measurement of branching ratios and spectral functions of t decays using the full LEP-I data from ALEPH, (ii) a preliminary measurement of the t hadronic branching ratios from DELPHI and (iii) a measurement of the strange spectral function in hadronic t decays from OPAL. These measurements are discussed and the relevant physics topics are briefly reviewed.

Hadronic events produced in e+e- collisions by the LEP collider and recorded by the OPAL detector were used to form distributions based on the number of reconstructed jets. The data were collected between 1995 and 2000 and correspond to energies of 91 GeV, 130-136 GeV and 161-209 GeV. The jet rates were determined using four different jet-finding algorithms (Cone, JADE, Durham and Cambridge). The differential two-jet rate and the average jet rate with the Durham and Cambridge algorithms were used to measure in the LEP energy range by fitting an expression in which \({\user1{\mathcal{O}}}{\left( {\alpha ^{2}_{{\text{s}}} } \right)}\)calculations were matched to a NLLA prediction and fitted to the data. Combining the measurements at different centre-of-mass energies, the value of () was determined to be

. 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 ( m W) and width ( Γ W ) are determined using direct reconstruction of the kinematics of W + W – → qqℓν and W + W – → qqqq events. When combined with previous OPAL measurements using W + W – → ℓνℓν events and the dependence on of the WW production cross-section at threshold, the results are determined to be [ m_W = 80.415 ± 0.042 ± 0.030 ± 0.009 GeV; Γ _W = 1.996 ± 0.096 ± 0.102 ± 0.003 GeV; ] 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 – → qqqq 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.

Using the OPAL detector at LEP, the running of the effective QED coupling alpha(t) is measured for space-like momentum transfer from the angular distribution of small-angle Bhabha scattering. In an almost ideal QED framework, with very favourable experimental conditions, we obtain: Delta alpha(-6.07GeV^2) - Delta alpha(-1.81GeV^2) = (440 pm 58 pm 43 pm 30) X 10^-5, where the first error is statistical, the second is the experimental systematic and the third is the theoretical uncertainty. This agrees with current evaluations of alpha(t).The null hypothesis that alpha remains constant within the above interval of -t is excluded with a significance above 5sigma. Similarly, our results are inconsistent at the level of 3sigma with the hypothesis that only leptonic loops contribute to the running. This is currently the most significant direct measurment where the running alpha(t) is probed differentially within the measured t range.

The effects of the final state interaction phenomenon known as colour reconnection are investigated at centre-of-mass energies in the range \(\sqrt s\) ≃ 189–209 using the OPAL detector at LEP. Colour reconnection is expected to affect observables based on charged particles in hadronic decays of . Measurements of inclusive charged particle multiplicities, and of their angular distribution with respect to the four jet axes of the events, are used to test models of colour reconnection. The data are found to exclude extreme scenarios of the Sjöstrand-Khoze Type I () model and are compatible with other models, both with and without colour reconnection effects. In the context of the model, the best agreement with data is obtained for a reconnection probability of 37%. Assuming no colour reconnection, the charged particle multiplicity in hadronically decaying W bosons is measured to be \({\left\langle {n^{{{\text{qq}}}}_{{{\text{ch}}}} } \right\rangle }\)=19.38±0.05(stat.)±0.08(syst.).

.Upper limits on the cross-section of the pair-production process \({\ensuremath{{\mathrm{e}}^ + {\mathrm{e}}^-}} \to \mathrm{h}^0 \mathrm{A}^0\), assuming 100 \(\%\) branching fraction to hadrons, are derived from a new search for the \(\mathrm{h}^0\mathrm{A}^0 \to \) hadrons final state, independently of the hadronic flavour of the decay products. This study, combined with previously published searches for the neutral Higgs bosons h0 and A0, is used to constrain the Type II Two Higgs Doublet Model (2HDM(II)) with no CP violation in the Higgs sector and no additional non-Standard Model particles besides the five Higgs bosons. The analysis combines LEP1 and LEP2 data collected with the OPAL detector up to the highest available centre-of-mass energies. The searches are sensitive to the \(\mathrm{h}^0, \mathrm{A}^0 \to \mathrm{q\bar{q}}, {\ensuremath{\mathrm{g}\mathrm{g}}}, {\ensuremath{\tau^ + \tau^-}}\) and \(\mathrm{h}^0\to\mathrm{A}^0\mathrm{A}^0\) decay modes of the Higgs bosons. A benchmark scan of the 2HDM(II) parameter space is performed. Large regions of the 2HDM(II) parameter space explored are excluded at the 95% CL in the (\(m_{\mathrm{h}}, m_{\mathrm{A}}), (m_{\mathrm{h}}, \tan{\beta})\) and \((m_{\mathrm{A}}, \tan{\beta})\) planes, using both direct neutral Higgs boson searches and indirect limits derived from Standard Model high precision measurements. The region \(1 \lesssim m_{\mathrm{h}} \lesssim 55\) GeV and \(3 \lesssim m_{\mathrm{A}} \lesssim 63\) GeV is excluded at 95 % CL, independently of \(m_{\mathrm{h}}, m_{\mathrm{A}}, \tan{\beta}\) and for selected values of \(\alpha\) which are representative of a complete \(\alpha\)-scan.

We have studied hadronic events from e + e- annihilation data at centre-of-mass energies from 91 to 209 GeV. We present distributions of event shape observables and their moments at each energy and compare with QCD Monte Carlo models. From the event shape distributions we extract the strong coupling \(\ensuremath{\alpha_{\mathrm{s}}}\) and test its evolution with energy scale. The results are consistent with the running of \(\ensuremath{\alpha_{\mathrm{s}}}\) expected from QCD. Combining all data, the value of \(\ensuremath{\alpha_{\mathrm{s}}} \ensuremath{(M_{\mathrm{Z}})}\) is determined to be \( \ensuremath{\alpha_{\mathrm{s}}}\ensuremath{M_{\mathrm{Z}}} = 0.1191 \pm0.0005 \mathrm{(stat.)} \pm0.0010\mathrm{(expt.)} \pm0.0011 \mathrm{(hadr.)} \pm0.0044 \mathrm{(theo.)}\,. \) The energy evolution of the moments is also used to determine a value of \(\ensuremath{\alpha_{\mathrm{s}}}\) with slightly larger errors: \(\ensuremath{\alpha_{\mathrm{s}}} \ensuremath{M_{\mathrm{Z}}}=0.1223\pm 0.0005(\mathrm{stat.})\pm 0.0014(\mathrm{expt.})\pm 0.0016(\mathrm{hadr.})^{+0.0054}_{-0.0036}(\mathrm{theo.})\).

Gluon jets with a mean energy of 22 GeV and purity of 95% are selected from hadronic Z0 decay events produced in e + e- annihilations. A subsample of these jets is identified which exhibits a large gap in the rapidity distribution of particles within the jet. After imposing the requirement of a rapidity gap, the gluon jet purity is 86%. These jets are observed to demonstrate a high degree of sensitivity to the presence of color reconnection, i.e. higher order QCD processes affecting the underlying color structure. We use our data to test three QCD models which include a simulation of color reconnection: one in the Ariadne Monte Carlo, one in the Herwig Monte Carlo, and the other by Rathsman in the Pythia Monte Carlo. We find the Rathsman and Ariadne color reconnection models can describe our gluon jet measurements only if very large values are used for the cutoff parameters which serve to terminate the parton showers, and that the description of inclusive Z0 data is significantly degraded in this case. We conclude that color reconnection as implemented by these two models is disfavored. The signal from the Herwig color reconnection model is less clear and we do not obtain a definite conclusion concerning this model. In a separate study, we follow recent theoretical suggestions and search for glueball-like objects in the leading part of the gluon jets. No clear evidence is observed for these objects.

.The branching ratio of beauty hadrons to final states containing two charm hadrons, Br(b \(\to \mathrm{D\bar{D}X}\), has been measured using an inclusive method in hadronic Z0 decays with the OPAL detector at LEP. The impact parameter significance of tracks opposite tagged b-jets is used to differentiate b \(\to \mathrm{D\bar{D}X}\) decays from other decays. The result is \( \mathrm{Br(b} \to \mathrm{D\bar{D}X}) = (10.0 \pm 3.2 (\mathrm{stat.})^{+2.4}_{-2.9}(\mathrm{det.})^{+10.4}_{-9.0} (\mathrm{phys.}))\%,\) where “det.” is the systematic uncertainty due to the modelling of the detector, and “\({\rm phys.}\)” is the systematic uncertainty due to the modelling of the underlying physics. Using this result, the average number of charm plus anti-charm quarks produced in a beauty quark decay, n c , is found to be 1.12 + 0.11 -0.10.

.Approximately 438 pb-1 of e + e- data from the OPAL detector, taken with the LEP collider running at centre-of-mass energies of 192-209 GeV, are analyzed to search for evidence of chargino pair production, \({\mathrm{e}}^ + {\mathrm{e}}^-\to\tilde\chi^ + _1\tilde\chi^-_1\), or neutralino associated production, \({\mathrm{e}}^ + {\mathrm{e}}^-\to\tilde\chi^0_2\tilde\chi^0_1\). Limits are set at the 95% confidence level on the product of the cross-section for the process \({\mathrm{e}}^ + {\mathrm{e}}^-\to\tilde\chi^ + _1\tilde\chi^-_1\) and its branching ratios to topologies containing jets and missing energy, or jets with a lepton and missing energy, and on the product of the cross-section for \({\mathrm{e}}^ + {\mathrm{e}}^-\to\tilde\chi^0_2\tilde\chi^0_1\) and its branching ratio to jets. R-parity conservation is assumed throughout this paper. When these results are interpreted in the context of the Constrained Minimal Supersymmetric Standard Model, limits are also set on the masses of the \(\tilde\chi^{\pm}_1, \tilde\chi^0_1\) and \(\tilde\chi^0_2\), and regions of the parameter space of the model are ruled out. Nearly model-independent limits are also set at the 95% confidence level on \(\sigma({\mathrm{e}}^ + {\mathrm{e}}^-\to\tilde\chi^ + _1\tilde\chi^-_1)\) with the assumption that each chargino decays via a W boson, and on \(\sigma({\mathrm{e}}^ + {\mathrm{e}}^-\to\tilde\chi^0_2\tilde\chi^0_1)\) with the \(\tilde\chi^0_2\) assumed to decay via a Z0.

Triple gauge boson couplings are measured from W-pair events recorded by the OPAL detector at LEP at centre-of-mass energies of 183 - 209 GeV with a total integrated luminosity of 680 pb(-1). Only CP-conserving couplings are considered and SU(2) x U(1) relations are used, resulting in four independent couplings, k(gamma), g(1)(z), lambdagamma and g(5)(z). Determining each coupling in a separate fit, assuming the other couplings to take their Standard Model values, we obtain k(gamma) = 0.88(-0.08)(+0.09), g(1)(z) = 0.987 (-0.033),(+0.034) lambda(gamma) = -0.060(-0.033)(+0.034) and g(5)(z) = -0.04(-0.12)(+0.13), where the errors include both statistical and systematic uncertainties. Fits are also performed allowing some of the couplings to vary simultaneously. All results are consistent with the Standard Model predictions.