C. Hajdú

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.

.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.

A study of 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 <xe>=0.7193+-0.0016(stat)+0.0036-0.0031(syst)

The scientific study of the harmful effects of indoor fires on building structures and on the environment is a top issue today. Indoor fires frequently occur all over the world. The goal of our research is to examine the effects of an average room fire on the survival possibility of a trapped person and on the building structure, taking into account features of the Eastern European architecture. First, a computational fluid dynamics (CFD) simulation was performed to examine the change of temperature, oxygen, and carbon monoxide concentration in a selected room in a vacant building used for military training. Based on the results, a 1:1 scale fire experiment was carried out with the parameters used in the simulation. The experiment was repeated once with the same settings. It was observed that without the intervention of firefighters, the temperature in the experiment could have rapidly reached 400 °C, as suggested by the simulation, which could have caused structural damage to the building. Furthermore, after 3 min the carbon monoxide concentration reached 400 ppm in both experiments and the simulation, which is a harmful level to people trapped inside the room. Also, in the experiment there was sufficient oxygen at the ground level with what people can survive 3 min.

We have studied hadronic events produced at LEP at centre-of-mass energies of 130 and 136 GeV. Distributions of event shape observables, jet rates, momentum spectra and multiplicities are presented and compared to the predictions of several Monte Carlo models and analytic QCD calculations. From fits of event shape and jet rate distributions to $${\mathcal{O}}(\alpha _s^2 ) + NLLA$$ QCD calculations, we determineα s (133 GeV)=0.110±0.005(stat.)±0.009(syst.). We measure the mean charged particle multiplicity 〈n ch〉=23.40±0.45(stat.) ±0.47(syst.) and the position ζ0 of the peak in the ζ p = ln(1/x p ) distribution ζ0=3.94±0.05(stat.)±0.11(syst.). These results are compared to lower energy data and to analytic QCD or Monte Carlo predictions for their energy evolution.

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– → ${\text{q\ifmmode\expandafter\bar\else\expandafter\=\fi{q}}}{\ell }\nu$ and W+W– → ${\text{q\ifmmode\expandafter\bar\else\expandafter\=\fi{q}q\ifmmode\expandafter\bar\else\expandafter\=\fi{q}}}$ events. When combined with previous OPAL measurements using W+W– → $ {\ell }\nu {\ell }\nu $ events and the dependence on of the WW production cross-section at threshold, the results are determined to be $ \begin{array}{*{20}c} {{m_{{\text{W}}} = 80.415 \pm 0.042 \pm 0.030 \pm 0.009\;{\text{GeV}}}} \\ {{\Gamma _{{\text{W}}} = 1.996 \pm 0.096 \pm 0.102 \pm 0.003\;{\text{GeV}}}} \\ \end{array} $ 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– → ${\text{q\ifmmode\expandafter\bar\else\expandafter\=\fi{q}q\ifmmode\expandafter\bar\else\expandafter\=\fi{q}}}$ 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.

The process $\mathrm{e}^+\mathrm{e}^- \to\gamma\gamma(\gamma)$ is studied using data collected by the OPAL detector at LEP between the years 1997 and 2000. The data set corresponds to an integrated luminosity of 672.3 pb-1 at centre-of-mass energies lying between 181 GeV and 209 GeV. Total and differential cross-sections are determined and found to be in good agreement with the predictions of QED. Fits to the observed angular distributions are used to set limits on parameters from several models of physics beyond the Standard Model such as cut-off parameters, contact interactions of the type $\mathrm{e}^+\mathrm{e}^- \gamma\gamma$ , gravity in extra spatial dimensions and excited electrons. In events with three photons in the final state the mass spectrum of photon pairs is investigated. No narrow resonance $X\to\gamma\gamma$ is found and limits are placed on the product of the $\rm X \gamma$ production cross-section and branching ratio.

A search for the Standard Model Higgs boson has been performed with the OPAL detector at LEP based on the full data sample collected at s≈192–209 GeV in 1999 and 2000, corresponding to an integrated luminosity of approximately 426 pb−1. The data are examined for their consistency with the background-only hypothesis and various Higgs boson mass hypotheses. A lower bound of 109.7 GeV is obtained on the Higgs boson mass at the 95% confidence level. At higher masses, the data are consistent with both the background and the signal-plus-background hypotheses.

A search for single top quark production via flavour changing neutral currents (FCNC) was performed with data collected by the OPAL detector at the e+e− collider LEP. Approximately 600 pb−1 of data collected at s=189–209 GeV were used to search for the FCNC process e+e−→tc(u)→bWc(u). This analysis is sensitive to the leptonic and the hadronic decay modes of the W boson. No evidence for a FCNC process is observed. Upper limits at the 95% confidence level on the single top production cross-section as a function of the centre-of-mass energy are derived. Limits on the anomalous coupling parameters κγ and κZ are determined from these results.

A search for stable and long-lived massive particles of electric charge |Q/e|=1 or fractional charges of 2/3, 4/3, and 5/3 is reported using data collected by the OPAL detector at LEP, at centre-of-mass energies from 130 to 209 GeV. These particles are assumed to be pair-produced in e+e− collisions and not to interact strongly. No evidence for the production of these particles was observed. Model-independent upper limits on the production cross-section between 0.005 and 0.028 pb have been derived for scalar and spin-1/2 particles with charge ±1. Within the framework of the Constrained Minimal Supersymmetric Standard Model (CMSSM), this implies a lower limit of 98.0 (98.5) GeV on the mass of long-lived right- (left-)handed scalar muons and scalar taus. Long-lived charged heavy leptons and charginos are excluded for masses below 102.0 GeV. For particles with fractional charge ±2/3, ±4/3 and ±5/3, the upper limit on the production cross-section varies between 0.005 and 0.020 pb. All mass and cross-section limits are derived at the 95% confidence level and are valid for particles with lifetimes longer than 10−6 s.

A search for pair-produced doubly charged Higgs bosons has been performed using data samples corresponding to an integrated luminosity of about 614 pb−1 collected with the OPAL detector at LEP at centre-of-mass energies between 189 GeV and 209 GeV. No evidence for a signal has been observed. A mass limit of 98.5 GeV/c2 at the 95% confidence level has been set for the doubly charged Higgs particle in left–right symmetric models. This is the first search for doubly charged Higgs bosons at centre-of-mass energies larger than 91 GeV.

This paper summarises the search for the Standard Model Higgs boson in e + e - collisions at centre-of-mass energies up to 209 GeV performed by the OPAL Collaboration at LEP. The consistency of the data with the background hypothesis and various Higgs boson mass hypotheses is examined. No indication of a signal is found in the data and a lower bound of 112.7 Gev/c2 is obtained on the mass of the Standard Model Higgs boson at the 95% CL.

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 {\alpha}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 {\alpha}S(mZ0) = 0.1201 {\pm} 0.0008(stat.) {\pm} 0.0013(exp.) {\pm} 0.0010(had.) {\pm} 0.0024(theo.). while the combined NNLO+NLLA result is {\alpha}S(mZ0) = 0.1189 {\pm} 0.0008(stat.) {\pm} 0.0016(exp.) {\pm} 0.0010(had.) {\pm} 0.0036(theo.). 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 {\alpha}S agrees with the QCD prediction of asymptotic freedom and excludes the absence of running.

This final analysis of hadronic and leptonic cross-sections and of leptonic forward-backward asymmetries in ${\mathrme}^+{\mathrme}^-$ collisions with the OPAL detector makes use of the full LEP 1 data sample comprising $161 \mathrm{pb}^{-1}$ of integrated luminosity and $4.5\times10^6$ selected Z decays. An interpretation of the data in terms of contributions from pure Z exchange and from $\gamma/$ Z interference allows the parameters of the Z resonance to be determined in a model-independent way. Our results are in good agreement with lepton universality and consistent with the vector and axial-vector couplings predicted in the Standard Model. A fit to the complete dataset yields the fundamental Z resonance parameters: $m_{\mathrm{Z}} = (91.1852 \pm 0.0030)$ GeV, $\Gamma_{\mathrm{Z}} = (2.4948 \pm 0.0041)$ GeV, $\sigma^0_{\mathrm{h}} = (41.501 \pm 0.055)$ nb, $R_{\ell} = 20.823 \pm 0.044$ , and $A_{\mathrm{FB}}^{0,\ell} = 0.0145 \pm 0.0017$ . Transforming these parameters gives a measurement of the ratio between the decay width into invisible particles and the width to a single species of charged lepton, $\Gamma_{\mathrm{inv}}/\Gamma_{\ell\ell} = 5.942 \pm 0.027$ . Attributing the entire invisible width to neutrino decays and assuming the Standard Model couplings for neutrinos, this translates into a measurement of the effective number of light neutrino species, $N_{\nu} = 2.984 \pm 0.013$ . Interpreting the data within the context of the Standard Model allows the mass of the top quark, $m_{\mathrm{t}} = (162 ^{+29}_{-16})$ GeV, to be determined through its influence on radiative corrections. Alternatively, utilising the direct external measurement of $m_{\mathrm{t}}$ as an additional constraint leads to a measurement of the strong coupling constant and the mass of the Higgs boson: $\alpha_{\mathrm{s}}(m_{\mathrm{Z}}) = 0.127 \pm 0.005$ and $m_{\mathrm{H}} = (390^{+750}_{-280})$ GeV.

We present the results of beam tests with high-energy (8–375 GeV) electrons, pions, protons and muons of a sampling calorimeter based on the detection of Cherenkov light produced by shower particles. The detector, a prototype for the very forward calorimeters in the CMS experiment, consists of thin quartz fibers embedded in a copper matrix. Results are given on the light yield of this device, on its energy resolution for electron and hadron detection, and on the signal uniformity and linearity. The signal generation mechanism gives this type of detector unique properties, especially for the detection of hadron showers: narrow, shallow shower profiles and extremely fast signals. These specific properties were measured in detail. The implications for measurements in the high-rate, high-radiation Large Hadron Collider (LHC) environment are discussed.

Using data from $e^+e^-$ annihilation into hadrons, taken with the OPAL detector at LEP at the Z pole between 1991 and 1995, we performed a simultaneous measurement of the colour factors of the underlying gauge group of the strong interaction, $C_F$ and $C_A$ , and the strong coupling, $\alpha_s$ . The measurement was carried out by fitting next-to-leading order perturbative predictions to measured angular correlations of 4-jet events together with multi-jet related variables. Our results, \[ C_A=3.02\pm 0.25\mathrm{(stat.)} \pm 0.49\mathrm{(syst.)}\:,\quad C_F=1.34\pm 0.13\mathrm{(stat.)} \pm 0.22\mathrm{(syst.)}\:, \] \[ \alpha_s (M_Z)=0.120\pm 0.011\mathrm{(stat.)} \pm 0.020\mathrm{(syst.)}\:, \] provide a test of perturbative QCD in which the only assumptions are non-abelian gauge symmetry and standard hadronization models. The measurements are in agreement with SU(3) expectations for $C_F$ and $C_A$ and the world average of $\alpha_s(M_{Z})$ .

.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, \(\kappa_{\gamma}, g^z_1, \lambda_{\gamma}\) and g z 5. Determining each coupling in a separate fit, assuming the other couplings to take their Standard Model values, we obtain \(\kappa_{\gamma} = 0.88^{ + 0.09}_{-0.08}\), g z 1 = 0.987 + 0.034 -0.033, \(\lambda_{\gamma} = -0.060^{ + 0.034}_{-0.033}\) and g z 5 = -0.04 + 0.13 -0.12, 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.

A search for the single production of doubly-charged Higgs bosons is performed using e+e− collision data collected by the OPAL experiment at centre-of-mass energies between 189 GeV and 209 GeV. No evidence for the existence of H±± is observed. Upper limits are derived on hee, the Yukawa coupling of the H±± to like-signed electron pairs. A 95% confidence level upper limit of hee< 0.071 is inferred for M(H±±)<160 GeV assuming that the sum of the branching fractions of the H±± to all lepton flavour combinations is 100%. Additionally, indirect constraints on hee from Bhabha scattering at centre-of-mass energies between 183 GeV and 209 GeV, where the H±± would contribute via t-channel exchange, are derived for M(H±±)<2 TeV. These are the first results both from a single production search and on constraints from Bhabha scattering reported from LEP.

Searches for a scalar top quark and a scalar bottom quark have been performed using a data sample of 438 pb−1 at centre-of-mass energies of s=192–209 GeV collected with the OPAL detector at LEP. No evidence for a signal was found. The 95% confidence level lower limit on the scalar top quark mass is 97.6 GeV if the mixing angle between the supersymmetric partners of the left- and right-handed states of the top quark is zero. When the scalar top quark decouples from the Z0 boson, the lower limit is 95.7 GeV. These limits were obtained assuming that the scalar top quark decays into a charm quark and the lightest neutralino, and that the mass difference between the scalar top quark and the lightest neutralino is larger than 10 GeV. The complementary decay mode of the scalar top quark decaying into a bottom quark, a charged lepton and a scalar neutrino has also been studied. The lower limit on the scalar top quark mass is 96.0 GeV for this decay mode, if the mass difference between the scalar top quark and the scalar neutrino is greater than 10 GeV and if the mixing angle of the scalar top quark is zero. From a search for the scalar bottom quark, a mass limit of 96.9 GeV was obtained if the mass difference between the scalar bottom quark and the lightest neutralino is larger than 10 GeV.

Anomalous quartic couplings between the electroweak gauge bosons may contribute to the $\ensuremath{\nu}\overline{\ensuremath{\nu}}\ensuremath{\gamma}\ensuremath{\gamma}$ and $q\overline{q}\ensuremath{\gamma}\ensuremath{\gamma}$ final states produced in ${e}^{+}{e}^{\ensuremath{-}}$ collisions. This analysis uses the LEP2 OPAL data sample at center-of-mass energies up to 209 GeV. Event selections identify $\ensuremath{\nu}\overline{\ensuremath{\nu}}\ensuremath{\gamma}\ensuremath{\gamma}$ and $q\overline{q}\ensuremath{\gamma}\ensuremath{\gamma}$ events in which the two photons are reconstructed within the detector acceptance. The cross section for the process ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}q\overline{q}\ensuremath{\gamma}\ensuremath{\gamma}$ is measured. Averaging over all energies, the ratio of the observed ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}q\overline{q}\ensuremath{\gamma}\ensuremath{\gamma}$ cross section to the standard model expectation is $\mathrm{R}(\mathrm{data}/\mathrm{SM})=0.92\ifmmode\pm\else\textpm\fi{}0.07\ifmmode\pm\else\textpm\fi{}0.04,$ where the errors represent the statistical and systematic uncertainties respectively. The $\ensuremath{\nu}\overline{\ensuremath{\nu}}\ensuremath{\gamma}\ensuremath{\gamma}$ and $q\overline{q}\ensuremath{\gamma}\ensuremath{\gamma}$ data are used to constrain possible anomalous ${W}^{+}{W}^{\ensuremath{-}}\ensuremath{\gamma}\ensuremath{\gamma}$ and $\mathrm{ZZ}\ensuremath{\gamma}\ensuremath{\gamma}$ couplings. Combining with previous OPAL results from the ${W}^{+}{W}^{\ensuremath{-}}\ensuremath{\gamma}$ final state, the 95% confidence level limits on the anomalous coupling parameters ${a}_{0}^{\mathrm{Z}},$ ${a}_{\mathrm{c}}^{\mathrm{Z}},$ ${a}_{0}^{\mathrm{W}}$ and ${a}_{\mathrm{c}}^{\mathrm{W}}$ are found to be $\ensuremath{-}0.007 {\mathrm{GeV}}^{\ensuremath{-}2}&lt;{a}_{0}^{\mathrm{Z}}/{\ensuremath{\Lambda}}^{2}&lt;0.023 {\mathrm{GeV}}^{\ensuremath{-}2},$ $\ensuremath{-}0.029 {\mathrm{GeV}}^{\ensuremath{-}2}&lt;{a}_{\mathrm{c}}^{\mathrm{Z}}/{\ensuremath{\Lambda}}^{2}&lt;0.029 {\mathrm{GeV}}^{\ensuremath{-}2},$ $\ensuremath{-}0.020 {\mathrm{GeV}}^{\ensuremath{-}2}&lt;{a}_{0}^{\mathrm{W}}/{\ensuremath{\Lambda}}^{2}&lt;0.020 {\mathrm{GeV}}^{\ensuremath{-}2},$ $\ensuremath{-}0.052 {\mathrm{GeV}}^{\ensuremath{-}2}&lt;{a}_{\mathrm{c}}^{\mathrm{W}}/{\ensuremath{\Lambda}}^{2}&lt;0.037 {\mathrm{GeV}}^{\ensuremath{-}2},$ where $\ensuremath{\Lambda}$ is the energy scale of the new physics. Limits found when allowing two or more parameters to vary are also presented.

.Using the OPAL detector at LEP, the running of the effective QED coupling α(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: \( \Delta \alpha {\left( { - 6.07\;{\text{GeV}}^{2} } \right)} - \Delta \alpha {\left( { - 1.81\;{\text{GeV}}^{2} } \right)} = {\left( {440 \pm 58 \pm 43 \pm 30} \right)} \times 10^{{ - 5}} ,\) 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 α(t). The null hypothesis that α remains constant within the above interval of -t is excluded with a significance above 5σ. Similarly, our results are inconsistent at the level of 3σ with the hypothesis that only leptonic loops contribute to the running. This is currently the most significant direct measurement where the running α(t) is probed differentially within the measured t range.

We measure the mass of the τ to be 1775.1±1.6(mcnstat.)±1.0(mcnsys.) MeV using τ from Z0 decays. To test CPT invariance we compare the masses of the positively and negatively charged τ. The relative mass difference is found to be smaller than 3.0×10−3 at the 90% confidence level.

The magnitude of the Cabibbo-Kobayashi-Maskawa matrix element Vcb has been measured using B̄0→D∗+ℓ−ν̄ decays recorded on the Z0 peak using the OPAL detector at LEP. The D∗+→D0π+ decays were reconstructed both in the particular decay modes D0→K−π+ and D0→K−π+π0 and via an inclusive technique. The product of |Vcb| and the decay form factor of the B̄0→D∗+ℓ−ν̄ transition at zero recoil F(1) was measured to be F(1)|Vcb|=(37.1±1.0±2.0)×10−3, where the uncertainties are statistical and systematic respectively. By using Heavy Quark Effective Theory calculations for F(1), a value of|Vcb|=(40.7±1.1±2.2±1.6)×10−3was obtained, where the third error is due to theoretical uncertainties in the value of F(1). The branching ratio Br(B̄0→D∗+ℓ−ν̄) was also measured to be (5.26±0.20±0.46)%.

This letter describes the first observation of W boson pair production at a centre-of-mass energy s=161GeV in the OPAL detector at LEP. The analysis is sensitive to all expected W+W− decay channels. A total of 28 events have been selected for an integrated luminosity of 9.89±0.06 pb−1. This is consistent with the Standard Model expectation, including signal and background contributions. The W pair production cross-section is measured to be σWW = 3.62−0.82+0.93±0.16 pb. An analysis of the predicted MW dependence of the accepted cross-section, taking into account interference in the four-fermion production processes, yields MW = 80.40−0.41−0.10+0.44+0.09±0.10 GeV, where the first and second uncertainties are statistical and systematic, respectively, and the third arises form the beam energy uncertainty.

Gluon jets with about 39 GeV energy are identified in hadronic Z0 decays by tagging two jets in the same hemisphere of an event as quark jets. Identifying the gluon jet to be all the particles observed in the hemisphere opposite to that containing the two tagged jets yields an inclusive gluon jet definition corresponding to that used in analytic calculations, allowing the first direct test of those calculations. In particular, this jet definition yields results which are only weakly dependent on a jet finding algorithm. We find rch.=1.552±0.0041 (stat) ±0.061 (syst.) for the ratio of the mean charged particle multiplicity in gluon jets to that in light quark uds jets, where the uds jets are identified using an inclusive jet definition similar to that used for the gluon jets. Our result is in general agreement with the prediction of a recent analytic calculation which incorporates energy conservation into the parton shower branching processes, but is considerably smaller than analytic predictions which do not incorporate energy conservation.

From a data sample of 183 pb−1 recorded at a center-of-mass energy of s=189 GeV with the OPAL detector at LEP, 3068 W-pair candidate events are selected. Assuming Standard Model W boson decay branching fractions, the W-pair production cross section is measured to be σWW=16.30±0.34(stat.)±0.18(syst.) pb. When combined with previous OPAL measurements, the W boson branching fraction to hadrons is determined to be 68.32±0.61(stat.)±0.28(syst.)% assuming lepton universality. These results are consistent with Standard Model expectations.

We have analysed the data collected by OPAL at centre-of-mass energies between 189 and 209 GeV searching for Higgs boson candidates from the process $\mathrm {e^+e^-} \to \mathrm {h^0}\mathrm {Z}^0$ followed by the decay of $\mathrm {h^0} \to \mathrm {A^0} \mathrm {A^0}$ where $\mathrm {A^0}$ is the CP-odd Higgs boson. The search is done in the region where the $\mathrm {A^0}$ mass, $m_{\mathrm{A}}$ , is below the production threshold for $\mathrm {b \bar b}$ , and the CP-even Higgs boson mass $m_{\mathrm{h}}$ is within the range 45-86 GeV/c 2 . In this kinematic range, the decay of $\mathrm {h^0} \to \mathrm {A^0} \mathrm {A^0}$ may be dominant and previous Higgs boson searches have very small sensitivities. This search can be interpreted within any model that predicts the existence of at least one scalar and one pseudoscalar Higgs boson. No excess of events is observed above the expected Standard Model backgrounds. Model-independent limits on the cross-section for the process $\mathrm {e^+e^-}\rightarrow \mathrm {h^0} \mathrm {Z}^0$ are derived assuming 100% decays of the $\mathrm {h^0}$ into $\mathrm {A^0} \mathrm {A^0}$ and 100% decays of the $\mathrm {A^0} \mathrm {A^0}$ into each of the following final states: $\mathrm {c \bar c} \mathrm {c \bar c}$ , $\mathrm {gggg}$ , $\tau^+ \tau^- \tau^+ \tau^-$ , $\mathrm {c \bar c} \mathrm {gg}$ , $\mathrm {gg} \tau^+ \tau^-$ and $\mathrm {c \bar c} \tau^+ \tau^-$ . The results are also interpreted in the CP-conserving no-mixing MSSM scenario, where the region $45 \le m_{\mathrm{h}} \le 85 {\mathrm{GeV}}/c^2$ and $2\le m_{\mathrm{A}} \le 9.5 {\mathrm{GeV}}/c^2$ is excluded.

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=206.3GeV. 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 45GeV/c2<mM<102GeV/c2. The average limit is σ<0.05pb 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.

Abstract 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 .

Cross-sections for hadronic and leptonic two-fermion events, and leptonic forward-backward asymmetries, have been measured in e+e− collisions at a centre-of-mass energy of 161 GeV, using the OPAL detector at LEP. Results are presented both including and excluding the dominant production of radiative γZ0 events. We have measured Rb, the ratio of the number of bb to all multihadronic events at 161 GeV, and compared it to the result obtained at 130–136 GeV. All results agree well with the Standard Model expectations. In a model-independent fit to the Z0 lineshape, the data presented here give an improved precision on the γZ0-interference term. The data have also been used to obtain new limits on extensions of the Standard Model described by effective four-fermion contact interactions.

We present the results of experimental studies of hadron showers in a copper/quartz-fiber calorimeter, based on the detection of Cherenkov light. These studies show that there are very significant differences between the signals from protons and pions at the same energies. In the energy range between 200 and 375 GeV, where these studies were performed, the calorimeter's response to protons was typically 10% smaller than the response to pions. On the other hand, the energy resolution was about 25% better for protons. In addition, the protons had a Gaussian line shape, whereas the pion response curve was asymmetric. These differences can be understood from the requirements of baryon number conservation in the shower development. They are expected to be present in any non-compensating calorimeter, to a degree determined by the e/h value.

A search was performed for charginos with masses close to the mass of the lightest neutralino in e+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 570pb-1. Upper limits were derived on the chargino pair-productin 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.

The photon structure function $F_2^\gamma(x,Q^2)$ has been measured using data taken by the OPAL detector at $\rm e^+e^-$ centre-of-mass energies of 91 GeV, 183 GeVand 189 \GeV, in $Q^2$ ranges of 1.5–30.0 GeV $^2$ (LEP1), and 7.0–30.0 GeV $^2$ (LEP2), probing lower values of x than ever before. Since previous OPAL analyses, new Monte Carlo models and new methods, such as multi-variable unfolding, have been introduced, reducing significantly the model dependent systematic errors in the measurement. The results do not conclusively prove, but are completely consistent with, the presence of a rise in $F_2^\gamma$ at low-x as expected from QCD.

Non-commutative QED would lead to deviations from the Standard Model depending on a new energy scale ΛNC and a unique direction in space defined by two angles η and ξ. In this analysis, η is defined as the angle between the unique direction and the rotation axis of the earth. The predictions of a tree level calculation for the process e+e−→γγ are evaluated for the specific orientation of the OPAL detector and compared to the measurements. Distributions of the polar and azimuthal photon angles are used to extract limits on the energy scale ΛNC depending on the model parameter η. It is shown that the time dependence of the total cross-section could be used to determine the model parameter ξ if there were a detectable signal. This is the first experimental study of non-commutative QED at an e+e− collider.

Flavour inclusive, udsc and b fragmentation functions in unbiased jets, and flavour inclusive, udsc, b and gluon fragmentation functions in biased jets are measured in e+e- annihilations from data collected at centre-of-mass energies of 91.2, and 183-209 GeV with the OPAL detector at LEP. The unbiased jets are defined by hemispheres of inclusive hadronic events, while the biased jet measurements are based on three-jet events selected with jet algorithms. Several methods are employed to extract the fragmentation functions over a wide range of scales. Possible biases are studied in the results are obtained. The fragmentation functions are compared to results from lower energy e+e- experiments and with earlier LEP measurements and are found to be consistent. Scaling violations are observed and are found to be stronger for the fragmentation functions of gluon jets than for those of quarks. The measured fragmentation functions are compared to three recent theoretical next-to-leading order calculations and to the predictions of three Monte Carlo event generators. While the Monte Carlo models are in good agreement with the data, the theoretical predictions fail to describe the full set of results, in particular the b and gluon jet measurements.

Within a Two-Higgs-Doublet Model (2HDM) a search for a light Higgs boson in the mass range of 4–12 GeV has been performed in the Yukawa process $\mathrme^+ \mathrme^-\rightarrow \mathrm{b}\bar{\mathrm{b}}\mathrm{A}/\mathrm{h}\rightarrow\mathrm{b} \bar{\mathrm{b}}\tau^+\tau^-$ , using the data collected by the OPAL detector at LEP between 1992 and 1995 in $\mathrme^+ \mathrme^-$ collisions at about 91 GeV centre-of-mass energy. A likelihood selection is applied to separate background and signal. The number of observed events is in good agreement with the expected background. Within a CP-conserving 2HDM type II model the cross-section for Yukawa production depends on $\xi^{\mathrm{A}}_d=|\tan\beta|$ and $\xi^{\mathrm{h}}_d=|\sin\alpha/\cos\beta|$ for the production of the CP-odd A and the CP-even h, respectively, where $\tan\beta$ is the ratio of the vacuum expectation values of the Higgs doublets and $\alpha$ is the mixing angle between the neutral CP-even Higgs bosons. From our data 95% C.L. upper limits are derived for $\xi^{\mathrm{A}}_d$ within the range of 8.5 to 13.6 and for $\xi^{\mathrm{h}}_d$ between 8.2 to 13.7, depending on the mass of the Higgs boson, assuming a branching fraction into $\tau^+\tau^-$ of 100%. An interpretation of the limits within a 2HDM type II model with Standard Model particle content is given. These results impose constraints on several models that have been proposed to explain the recent BNL measurement of the muon anomalous magnetic moment.

Searches were performed for topologies predicted by gauge-mediated supersymmetry breaking models (GMSB). All possible lifetimes of the next-to-lightest SUSY particle (NLSP), either the lightest neutralino or slepton, decaying into the lightest SUSY particle, the gravitino, were considered. No evidence for GMSB signatures was found in the OPAL data sample collected at centre-of-mass energies up to \(\sqrt{s}=209 \mathrm{GeV}\) at LEP. Limits on the product of the production cross-sections and branching fractions are presented for all search topologies. To test the impact of the searches, a complete scan over the parameters of the minimal model of GMSB was performed. NLSP masses below \(53.5 \mathrm{GeV}/c^2\) in the neutralino NLSP scenario, below \(87.4 \mathrm{GeV}/c^2\) in the stau NLSP scenario and below \(91.9 \mathrm{GeV}/c^2\) in the slepton co-NLSP scenario are excluded at 95% confidence level for all NLSP lifetimes. The scan determines constraints on the universal SUSY mass scale Λ from the direct SUSY particle searches of Λ>40, 27, 21, 17, \(15 \mathrm{GeV}/c^2\) for messenger indices N=1,2,3,4,5 for all NLSP lifetimes.

The International Linear Collider (ILC) is a 200-500 GeV center-of-mass high-luminosity linear electron-positron collider, based on 1.3 GHz superconducting radio-frequency (SCRF) accelerating cavities. The ILC has a total footprint of about 31 km and is designed for a peak luminosity of 2x10^34 cm^-2 s^-1. The complex includes a polarized electron source, an undulator-based positron source, two 6.7 km circumference damping rings, two-stage bunch compressors, two 11 km long main linacs and a 4.5 km long beam delivery system. This report is Volume III (Accelerator) of the four volume Reference Design Report, which describes the design and cost of the ILC.

A search is performed for Higgs bosons decaying into invisible final states, produced in association with a Z0 boson in e+e− collisions at energies between 183 and 209 GeV. The search is based on data samples collected by the OPAL detector at LEP corresponding to an integrated luminosity of about 660 pb−1. The analysis aims to select events containing the hadronic decay products of the Z0 boson and large missing momentum, as expected from Higgs boson decay into a pair of stable weakly interacting neutral particles, such as the lightest neutralino in the Minimal Supersymmetric Standard Model. The same analysis is applied to a search for nearly invisible Higgs boson cascade decays into stable weakly interacting neutral particles. No excess over the expected background from Standard Model processes is observed. Limits on the production of invisibly decaying Higgs bosons produced in association with a Z0 boson are derived. Assuming a branching ratio BR(h0→invisible)=1, a lower limit of 108.2 GeV is placed on the Higgs boson mass at the 95% confidence level. Limits on the production of nearly invisibly decaying Higgs bosons are also obtained.

A search for charginos and neutralinos, predicted by supersymmetric theories, has been performed using a data sample of 2.6 pb−1 at a centre-of-mass energy of S=130 GeV and 2.6 pb−1 at 136 GeV collected with the OPAL detector at LEP during November 1995. No candidate events were observed. The 95% C.L. lower limit on the lightest chargino mass in the Minimal Supersymmetric Standard Model is 65.4 GeV if the universal scalar mass m0 is greater than 1 TeV, and 58.7 GeV for the smallest m0 compatible with slepton and sneutrino mass limits obtained at centre-of-mass energies near the Z peak. These limits were obtained under the conditions that the lightest chargino is heavier than the lightest neutralino by more than 10 GeV and tan β is larger than 1.5. The results of a model independent search for charginos and neutralinos are also given.

.A search for pair-produced leptoquarks is performed using \(\mbox{e}^ + \mbox{e}^-\) collision events collected by the OPAL detector at LEP at centre-of-mass energies between 189 and 209 GeV. The data sample corresponds to a total integrated luminosity of 596 pb-1. The leptoquarks are assumed to be produced via couplings to the photon and the Z0. For a given search channel only leptoquark decays involving a single lepton generation are considered. No evidence for leptoquark pair production is observed. Lower limits on masses for scalar and vector leptoquarks are calculated. The results improve most of the LEP limits derived from previous searches for the pair production process by 10-25 GeV, depending on the leptoquark quantum numbers.

Charged particle momentum distributions are studied in the reaction e+e- -&gt; hadrons, using data collected with the OPAL detector at centre-of-mass energies from 192 GeV to 209 GeV. The data correspond to an average centre-of- mass energy of 201.7 GeV and a total integrated luminosity of 433 pb-1. The measured distributions and derived quantities, in combination with corresponding results obtained at lower centre-of-mass energies, are compared to QCD predictions in various theoretical approaches to study the energy dependence of the strong interaction and to test QCD as the theory describing it. In general, a good agreement is found between the measurements and the corresponding QCD predictions.

A study of W+W− events accompanied by hard photon radiation, Eγ>2.5 GeV, produced in e+e− collisions at LEP is presented. Events consistent with being two on-shell W-bosons and an isolated photon are selected from 681 pb−1 of data recorded at 180 GeV<s<209 GeV. From the sample of 187 selected W+W−γ candidates with photon energies greater than 2.5 GeV, the W+W−γ cross-section is determined at five values of s. The results are consistent with the Standard Model expectation. Averaging over all energies, the ratio of the observed cross-section to the Standard Model expectation is R(data/SM)=0.99±0.09±0.04, where the errors represent the statistical and systematic uncertainties respectively. These data provide constraints on the related O(α) systematic uncertainties on the measurement of the W-boson mass at LEP. Finally, the data are used to derive 95% confidence level upper limits on possible anomalous contributions to the W+W−γγ and W+W−Z0γ vertices: −0.020 GeV−2<a0Λ2<0.020 GeV−2,−0.053 GeV−2<acΛ2<0.037 GeV−2,−0.16 GeV−2<anΛ2<0.15 GeV−2, where Λ represents the energy scale for new physics and a0, ac and an are dimensionless coupling constants.

The energy stored in the nominal LHC beams is two times 362 MJ, 100 times the energy of the Tevatron. As little as 1 mJ/cm3 deposited energy quenches a magnet at 7 TeV and 1 J/cm3 causes magnet damage. The beam dumps are the only places to safely dispose of this beam. One of the key systems for machine protection is the beam loss monitoring (BLM) system. About 3600 ionization chambers are installed at likely or critical loss locations around the LHC ring. The losses are integrated in 12 time intervals ranging from 40 μs to 84 s and compared to threshold values defined in 32 energy ranges. A beam abort is requested when potentially dangerous losses are detected or when any of the numerous internal system validation tests fails. In addition, loss data are used for machine set-up and operational verifications. The collimation system for example uses the loss data for set-up and regular performance verification. Commissioning and operational experience of the BLM are presented: The machine protection functionality of the BLM system has been fully reliable; the LHC availability has not been compromised by false beam aborts.

A study of W+W− events accompanied by hard photon radiation produced in e+e− collisions at LEP is presented. Events consistent with two on-shell W-bosons and an isolated photon are selected from 183 pb−1 of data recorded at s=189 GeV. From these data, 17 W+W−γ candidates are selected with photon energy greater than 10 GeV, consistent with the Standard Model expectation. These events are used to measure the e+e−→W+W−γ cross-section within a set of geometric and kinematic cuts, σ̂WWγ=136±37±8 fb, where the first error is statistical and the second systematic. The photon energy spectrum is used to set the first direct, albeit weak, limits on possible anomalous contributions to the W+W−γγ and W+W−γZ0 vertices:−0.070GeV−2<a0/Λ2<0.070GeV−2,−0.13GeV−2<ac/Λ2<0.19GeV−2,−0.61GeV−2<an/Λ2<0.57GeV−2,where Λ represents the energy scale for new physics.

Bose-Einstein correlations between like charged track pairs have been studied using a sample of approximately 3.6 million multihadronic Z0 decays collected with the OPAL detector at LEP. The radius of the emitting sourceR and the chaoticity parameterλ were studied using two parametrisations, the Goldhaber (G) parametrisation and the one-dimensional Kopylov-Podgoretskii (KP) parametrisation. The radiiR G andR KP are found to increase linearly with the average observed charged multiplicityn ch , with changes with respect to a unit increase inn ch of $$\begin{gathered} \frac{1}{{\langle R_G \rangle }}\frac{{\Delta R_G }}{{\Delta n_{ch} }} = (3.6 \pm 0.6) \cdot 10^{ - 3} and \hfill \\ \frac{1}{{\langle R_{KP} \rangle }}\frac{{\Delta R_{KP} }}{{\Delta n_{ch} }} = (3.4 \pm 1.0) \cdot 10^{ - 3} . \hfill \\ \end{gathered} $$ where the 〈R〉 are the radius values measured in the inclusive event sample. The chaoticity parametersλ G andλ KP decrease with increasing charged multiplicity. It is shown that the increase ofR with multiplicity may be connected with differences between two- and three-jet events.

Measurements of the $\tau$ lepton polarization and forward-backward polarization asymmetry near the Z $^0$ resonance using the OPAL detector are described. The measurements are based on analyses of $\tau \rightarrow{\rm e} \nu_e\nu_{\tau}, \tau\rightarrow \mu\nu_{\mu}\nu_{\tau}, \tau\rightarrow \pi\nu_{\tau}, \tau \rightarrow \rho\nu_{\tau}$ and $\tau\rightarrow{\rm a}_1\nu_{\tau}$ decays from a sample of 144,810 $\rm e^+e^-\rightarrow \tau^+\tau^-$ candidates corresponding to an integrated luminosity of 151 pb $^{-1}$ . Assuming that the $\tau$ lepton decays according to V–A theory, we measure the average $\tau$ polarization near $\sqrt{s} ={\rm M}_{\mathrm{Z}}$ to be $\langle P_{\tau}\rangle= (-14.10 \pm 0.73 \pm 0.55)\%$ and the $\tau$ polarization forward-backward asymmetry to be $\rm A_{\mathrm{pol}}^{\mathrm{FB}} = (-10.55 \pm 0.76 \pm 0.25)\%$ , where the first error is statistical and the second systematic. Taking into account the small effects of the photon propagator, photon-Z $^0$ interference and photonic radiative corrections, these results can be expressed in terms of the lepton neutral current asymmetry parameters: \begin{eqnarray} {\cal A}_{\tau} & = & 0.1456 \pm 0.0076 \pm 0.0057, \nonumber {\cal A}_{\mathrm e}& = & 0.1454 \pm 0.0108 \pm 0.0036. \nonumber \end{eqnarray} These measurements are consistent with the hypothesis of lepton universality and combine to give ${\cal A}_{\ell} = 0.1455 \pm 0.0073$ . Within the context of the Standard Model this combined result corresponds to $=0.23172 \pm 0.00092$ . Combing these results with those from the other OPAL neutral current measurements yields a value of $=0.23211 \pm 0.00068$ .

The interaction of virtual photons is investigated using the reaction ${\rm e^+e^-} \rightarrow{\rm e^+e^-}$ hadrons based on data taken by the OPAL experiment at ${\rm e^+e^-}$ centre-of-mass energies $\sqrt{s_{\rm ee}}=189-209$ GeV, for $W>5$ GeV and at an average $Q^{2}$ of 17.9 GeV $^2$ . The measured cross-sections are compared to predictions of the Quark Parton Model (QPM), to the Leading Order QCD Monte Carlo model PHOJET, to the NLO prediction for the reaction ${\rm e^+e^-}\rightarrow{\rm e^+e^-} q\bar{q}$ , and to BFKL calculations. PHOJET, NLO ${\rm e^+e^-}\rightarrow{\rm e^+e^-} q\bar{q}$ , and QPM describe the data reasonably well, whereas the cross-section predicted by a Leading Order BFKL calculation is too large.

.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.

A search is performed for production of short-lived particles in e+e−→XY, with X→γγ and Y→ff̄, for scalar X and scalar or vector Y. Model-independent limits in the range of 25–60 femtobarns are presented on σ(e+e−→XY)×B(X→γγ)×B(Y→ff̄) for centre-of-mass energies in the range 205–207 GeV. The data from all LEP centre-of-mass energies 88–209 GeV are also interpreted in the context of fermiophobic Higgs boson models, for which a lower mass limit of 105.5 GeV is obtained for a “benchmark” fermiophobic Higgs boson.

The spin composition of ΛΛ, ΛΛ and ΛΛ pairs at low invariant mass values has been measured for the first time in multihadronic Z0 decays with the OPAL detector at LEP. No single spin state has been observed in the ΛΛ sample, verifying that the low mass enhancement in this sample, attributed to local baryon number compensation, is not a resonance state. The fraction of the spin 1 contribution to the ΛΛ pairs was found to be consistent with the value 0.75, as expected from a statistical spin mixture. This may be the net effect of many different QCD processes which contribute to the hyperon anti-hyperon pair production. The spin composition of the identical ΛΛ and ΛΛ pairs, well above threshold, is found to be similar to that of the ΛΛ sample. A ΛΛ emitter dimension is estimated from the data assuming the onset of the Pauli exclusion principle near threshold.

We compared the multiplicities of $\pi^0, \eta, \rm K^0 $ and of charged particles in quark and gluon jets in 3-jet events, as measured by the OPAL experiment at LEP. The comparisons were performed for distributions unfolded to 100% pure quark and gluon jets, at an effective scale $\rm Q_{jet} $ which took into account topological dependences of the 3-jet environment. The ratio of particle multiplicity in gluon jets to that in quark jets as a function of $\rm Q_{jet}$ for $ \pi^0, \eta$ or $\rm K^0 $ was found to be independent of the particle species. This is consistent with the QCD prediction that the observed enhancement in the mean particle rate in gluon jets with respect to quark jets should be independent of particle species. In contrast to some theoretical predictions and previous observations, we observed no evidence for an enhancement of $ \eta $ meson production in gluon jets with respect to quark jets, beyond that observed for charged particles. We measured the ratio of the slope of the average charged particle multiplicity in gluon jets to that in quark jets, C, and we compared it to a next-to-next-to-next-to leading order calculation. Our result, $\rm C=2.27\pm 0.20(stat.+syst.),$ is about one standard deviation higher than the perturbative prediction.

Using about 3.9 million hadronic Z decays from e+e− collisions recorded by the OPAL detector at LEP at centre-of-mass energies s≈MZ, the branching ratio for the decay D−s→τ−ν̄τ has been measured to be BR(D−s→τ−ν̄τ)=(7.0±2.1(stat)±2.0(syst))%. This result can be used to derive the decay constant of the D−s meson: fDs=(286±44(stat)±41(syst))MeV.

Elements of the spin density matrix for W bosons in e+e−→W+W−→qq̄′ℓνℓ events are measured from data recorded by the OPAL detector at LEP. This information is used to calculate polarised differential cross-sections and to search for CP-violating effects. Results are presented for W bosons produced in e+e− Collisions with centre-of-mass energies between 183 GeV and 209 GeV. The average fraction of W bosons that are longitudinally polarised is found to be (23.9±2.1±1.1)% compared to a Standard Model prediction of (23.9±0.1)%. All results are consistent with CP conservation.

Di-jet production is studied in collisions of quasi-real photons at e + e -centre-of-mass energies √ s ee from 189 to 209 GeV at LEP.The data were collected with the OPAL detector.Jets are reconstructed using an inclusive k ⊥ -clustering algorithm for all cross-section measurements presented.A cone jet algorithm is used in addition to study the different structure of the jets resulting from either of the algorithms.The inclusive di-jet cross-section is measured as a function of the mean transverse energy Ējet T of the two leading jets, and as a function of the estimated fraction of the photon momentum carried by the parton entering the hard sub-process, x γ , for different regions of Ējet T .Angular distributions in di-jet events are measured and used to demonstrate the dominance of quark and gluon initiated processes in different regions of phase space.Furthermore the inclusive di-jet cross-section as a function of |η jet | and |∆η jet | is presented, where η jet is the jet pseudo-rapidity.Different regions of the x + γ -x - γ -space are explored to study and control the influence of an underlying event.The results are compared to next-to-leading order perturbative QCD calculations and to the predictions of the leading order Monte Carlo generator PYTHIA.

Protecting agricultural fields, like crops, vineyards, and husbandry areas, has been a difficult challenge since historical times. Classical methods to prevent intrusion are often destructive to wild and domestic animals alike. Even more current nondestructive systems, like camera-based systems are attributed to specific problems related to environmental or technological issues. Furthermore, verifying the effectiveness of installed systems is difficult, as the triggering situations are unmanageable and typically occur unsupervised. This paper presents a complex vision-based intrusion detection system to overcome these problems and further proposes more extensive control and flexibility on the development process. The solution provides a workflow integrating Digital Reality methods into the system development by creating a digital twin of the drone and its surrounding environment in a general-purpose robotic simulator. With this simulation, the triggering events and environmental effects can be easily emulated, such as a wild animal entering the area of interest. The solution also focuses on incorporating new 5G info-communication networks on handling communication between the intrusion detection system and the base station in a distributed manner.

We search for lepton flavour violating events (eμ, eτ and μτ) that could be directly produced in e+e− annihilations, using the full available data sample collected with the OPAL detector at centre-of-mass energies between 189 GeV and 209 GeV. In general, the Standard Model expectations describe the data well for all the channels and at each s. A single eμ event is observed where according to our Monte Carlo simulations only 0.019 events are expected from Standard Model processes. We obtain the first limits on the cross-sections σ(e+e−→eμ, eτ and μτ) as a function of s at LEP2 energies.

A search for charged excited leptons decaying into a lepton and photon has been performed using approximately 680 pb-1 of e+e- collision data collected by the OPAL detector at LEP at centre-of-mass energies between 183 GeV and 209 GeV. No evidence for their existence was found. Upper limits on the product of the cross-section and the branching fraction are inferred. Using results from the search for singly produced excited leptons, upper limits on the ratio of the excited lepton coupling constant to the compositeness scale are calculated. From pair production searches, 95% confidence level lower limits on the masses of excited electrons, muons and taus are determined to be 103.2 GeV.

We present the first experimental results based on the jet boost algorithm, a technique to select unbiased samples of gluon jets in ${e}^{+}{e}^{\ensuremath{-}}$ annihilations, i.e. gluon jets free of biases introduced by event selection or jet finding criteria. Our results are derived from hadronic ${Z}^{0}$ decays observed with the OPAL detector at the LEP ${e}^{+}{e}^{\ensuremath{-}}$ collider at CERN. First, we test the boost algorithm through studies with HERWIG Monte Carlo events and find that it provides accurate measurements of the charged particle multiplicity distributions of unbiased gluon jets for jet energies larger than about 5 GeV, and of the jet particle energy spectra (fragmentation functions) for jet energies larger than about 14 GeV. Second, we apply the boost algorithm to our data to derive unbiased measurements of the gluon jet multiplicity distribution for energies between about 5 and 18 GeV, and of the gluon jet fragmentation function at 14 and 18 GeV. In conjunction with our earlier results at 40 GeV, we then test QCD calculations for the energy evolution of the distributions, specifically the mean and first two nontrivial normalized factorial moments of the multiplicity distribution, and the fragmentation function. The theoretical results are found to be in global agreement with the data, although the factorial moments are not well described for jet energies below about 14 GeV.

The hadronic structure function of the photon F2γ(x,Q2) is measured as a function of Bjorken x and of the photon virtuality Q2 using deep-inelastic scattering data taken by the OPAL detector at LEP at e+e− centre-of-mass energies from 183 to 209 GeV. Previous OPAL measurements of the x dependence of F2γ are extended to an average Q2 of 〈Q2〉=780 GeV2 using data in the kinematic range 0.15<x<0.98. The Q2 evolution of F2γ is studied for 12.1<〈Q2〉<780 GeV2 using three ranges of x. As predicted by QCD, the data show positive scaling violations in F2γ with F2γ(Q2)/α=(0.08±0.02+0.05−0.03)+(0.13±0.01+0.01−0.01)lnQ2, where Q2 is in GeV2, for the central x region 0.10–0.60. Several parameterisations of F2γ are in qualitative agreement with the measurements whereas the quark-parton model prediction fails to describe the data.

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.).

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.

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 αs 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 αs is studied. Our results are consistent with the running of αs 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 αs(MZ)=0.1182±0.0015(stat.)±0.0101(syst.).

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.

Charginos and neutralinos, predicted by supersymmetric theories, have been searched for in e+e− collisions with an intergrated luminosity of 10.0 pb−1 at a centre-of-mass energy of s= 161GeV with the OPAL detector at LEP. Two candidate events are selected; consistent with the total background estimate of 0.7 ± 0.2 events. The 95% C.L. lower limit on the lightest chargino mass in the Minimal Supersymmetric Standard Model is 78.5 GeV if the universal scalar mass m0 is greater than 1 TeV, and 62.0 GeV for the smallest m0 compatible with slepton and sneutrino mass limits obtained at centre-of-mass energies near the Z peak. These limits were obtained under the conditions that the lightest chargino is heavier than the lightest neutralino by more than 10 GeV and tan β = 1.5. The new exclusion limits significantly improve on the results obtained at s= 130 and 136 GeV.

Data collected with the OPAL detector during 1990–1994 are used to measure the time dependence of $$B_d^0 \leftrightarrow \bar B_d^0 $$ mixing. A sample of 348 D*± candidates with a lepton in the opposite hemisphere are reconstructed, of which 167±25 are expected to be from B d 0 decays. The B d 0 oscillation frequency is measured to be $$\Delta m_d = 0.567 \pm 0.089(stat)_{ - 0.023}^{ + 0.029} (syst) ps^{ - 1} .$$ . A previously published analysis ofΔm d using D*± and lepton candidates in the same hemisphere and jet charge is also updated with a larger data sample. From 1200 D*±ℓ∓ candidates, of which 778±84 are expected to be from B d 0 decays, we find a value of: $$\Delta m_d = 0.539 \pm 0.060(stat) \pm 0.024(syst) ps^{ - 1} .$$ . The combined result of these two analyses is $$\Delta m_d = 0.548 \pm 0.050(stat)_{ - 0.019}^{ + 0.023} (syst) ps^{ - 1} .$$ .

A measurement of triple gauge boson couplings is presented, based on W-pair data recorded by the OPAL detector at LEP during 1998 at a centre-of-mass energy of 189 GeV with an integrated luminosity of 183 pb $^{-1}$ . After combining with our previous measurements at centre-of-mass energies of 161–183 GeV we obtain $\kappa=0.97_{-0.16}^{+0.20}$ , $g^{\mathrm{z}}_1=0.991^{+0.060}_{-0.057}$ and $\lambda=-0.110_{-0.055}^{+0.058}$ , where the errors include both statistical and systematic uncertainties and each coupling is determined by setting the other two couplings to their Standard Model values. These results are consistent with the Standard Model expectations.

The magnitude of the CKM matrix element Vub is determined by measuring the inclusive charmless semileptonic branching fraction of beauty hadrons at OPAL based on b -> Xu l nu event topology and kinematics. This analysis uses OPAL data collected between 1991 and 1995, which correspond to about four million hadronic Z decays. We measure Br(b -> Xu l) to be (1.63 +/- 0.53 +0.55/-0.62) x 10^(-3). The first uncertainty is the statistical error and the second is the systematic error. From this analysis, Vub is determined to be: |Vub| = (4.00 +/- 0.65(stat) +0.67/-0.76(sys) +/- 0.19(HQE)) x 10^(-3). The last error represents the theoretical uncertainties related to the extraction of |Vub| from Br(b -> Xu l) using the Heavy Quark Expansion.

The mass and width of the W boson are determined in e+e− collisions at LEP using 183 pb−1 of data recorded at a centre-of-mass energy s=189 GeV with the OPAL detector. The invariant mass distributions from 970 W+W−→qqqq and 1118 W+W−→qqℓνℓ candidate events are used to measure the mass of the W boson, MW=80.451±0.076 (stat.)±0.049 (syst.) GeV. A direct measurement of the width of the W boson gives ΓW=2.09±0.18 (stat.)±0.09 (syst.) GeV. The results are combined with previous OPAL results from 78 pb−1 of data recorded with s from 161 to 183 GeV, to obtain: MW=80.432±0.066 (stat.)±0.045 (syst.) GeV, ΓW=2.04±0.16 (stat.)±0.09 (syst.) GeV. The consistency of the direct measurement of MW with that inferred from other measurements of electroweak parameters provides an important test of the Standard Model of electroweak interactions.

Searches for first generation scalar and vector leptoquarks, and for squarks in R-parity violating SUSY models with the direct decay of the squark into Standard Model particles, have been performed using e+e− collisions collected with the OPAL detector at LEP at e+e− centre-of-mass energies between 189 and 209 GeV. No excess of events is found over the expectation from Standard Model background processes. Limits are computed on the leptoquark couplings for different values of the branching ratio to electron–quark final states.

The lifetime and oscillation frequency of the B0 meson has been measured using B̄0→D∗+ℓ−ν̄ decays recorded on the Z0 peak with the OPAL detector at LEP. The D∗+→D0π+ decays were reconstructed using an inclusive technique and the production flavour of the B0 mesons was determined using a combination of tags from the rest of the event. The results τB0=1.541±0.028±0.023ps,Δmd=0.497±0.024±0.025ps−1 were obtained, where in each case the first error is statistical and the second is systematic.

A search for the decay B±→K±K±π∓ was performed using data collected by the OPAL detector at LEP. These decays are strongly suppressed in the Standard Model but could occur with a higher branching ratio in supersymmetric models, especially in those with R-parity violating couplings. No evidence for a signal was observed and a 90% confidence level upper limit of 1.29×10−4 was set for the branching ratio.

The rates are measured per hadronic $\mathrm{Z}^0$ decay for gluon splitting to $\mathrm{b\overline{b}}$ quark pairs, $g_{\mathrm{b\overline{b}}}$ , and of events containing two $\mathrm{b\overline{b}}$ quark pairs, $g_{\mathrm{4b}}$ , using a sample of four-jet events selected from data collected with the OPAL detector. Events with an enhanced signal of gluon splitting to $\mathrm{b\overline{b}}$ quarks are selected if two of the jets are close in phase-space and contain detached secondary vertices. For the event sample containing two $\mathrm{b\overline{b}}$ quark pairs, three of the four jets are required to have a significantly detached secondary vertex. Information from the event topology is combined in a likelihood fit to extract the values of $g_{\mathrm{b\overline{b}}}$ and $g_{\mathrm{4b}}$ , namely \begin{eqnarray*} g_{\mathrm{b\overline{b}}} & = & (3.07 \pm 0.53 \mathrm{(stat)} \pm 0.97\mathrm{(syst)})\times 10^{-3},\\ g_{\mathrm{4b}} & = & (0.36\pm 0.17 \mathrm{(stat)} \pm 0.27\mathrm{(syst)})\times 10^{-3}. \end{eqnarray*}

The charged particle multiplicities of two- and three-jet events from the reaction e $^+$ e $^-$ $\rightarrow$ Z $^0\rightarrow hadrons$ are measured for Z $^0$ decays to light quark (uds) flavors. Using recent theoretical expressions to account for biases from event selection, results corresponding to unbiased gluon jets are extracted over a range of jet energies from about 11 to 30 GeV. We find consistency between these results and direct measurements of unbiased gluon jet multiplicity from $\Upsilon$ and Z $^0$ decays. The unbiased gluon jet data including the direct measurements are compared to corresponding results for quark jets. We perform fits based on analytic expressions for particle multiplicity in jets to determine the ratio $r\equiv\mathrm{N}_{g}/\mathrm{N}_{q}$ of multiplicities between gluon and quark jets as a function of energy. We also determine the ratio of slopes, $r^{(1)}\equiv(\mathrm{d}\mathrm{N}_{g} /\mathrm{d}y) /(\mathrm{d}\mathrm{N}_{q} /\mathrm{d}y)$ , and of curvatures, $r^{(2)}\equiv(\mathrm{d}^2\mathrm{N}_{g} /\mathrm{d}y^2) /(\mathrm{d}^2\mathrm{N}_{q} /\mathrm{d}y^2)$ , where y specifies the energy scale. At 30 GeV, we find $r=1.422\pm0.051,r^{(1)}=1.761\pm0.071$ and $r^{(2)}=1.98\pm0.13$ , where the uncertainties are the statistical and systematic terms added in quadrature. These results are in general agreement with theoretical predictions. In addition, we use the measurements of the energy dependence of ${\mathrm{N}}_{g}$ and ${\mathrm{N}}_{q}$ to determine an effective value of the ratio of QCD color factors, $C_{\mathrm A}/C_{\mathrm F}$ . Our result, $C_{\mathrm A}/C_{\mathrm F}=2.23\pm0.14 $ (total), is consistent with the QCD value of 2.25.

The τ−→μ−ν̄μντ branching ratio has been measured using data collected from 1990 to 1995 by the OPAL detector at the LEP collider. The resulting value of B(τ−→μ−ν̄μντ)=0.1734±0.0009(stat)±0.0006(syst) has been used in conjunction with other OPAL measurements to test lepton universality, yielding the coupling constant ratios gμ/ge=1.0005±0.0044 and gτ/ge=1.0031±0.0048, in good agreement with the Standard Model prediction of unity. A value for the Michel parameter η=0.004±0.037 has also been determined and used to find a limit for the mass of the charged Higgs boson, mH±>1.28tanβ, in the Minimal Supersymmetric Standard Model.

We present a determination of the LEP beam energy using “radiative return” fermion-pair events recorded at centre-of-mass energies from 183 to 209 GeV. We find no evidence of a disagreement between the OPAL data and the LEP Energy Working Group's standard calibration. Including the energy-averaged 11 MeV uncertainty in the standard determination, the beam energy we obtain from the OPAL data is higher than that obtained from the LEP calibration by 0±34(stat.)±27(syst.)MeV.

A search for the Higgsstrahlung process e+e−→hZ is described, where the neutral Higgs boson h is assumed to decay into hadronic final states. In order to be sensitive to a broad range of models, the search is performed independent of the flavour content of the Higgs boson decay. The analysis is based on e+e− collision data collected by the OPAL detector at energies between 192 and 209 GeV. The search does not reveal any significant excess over the Standard Model background prediction. Results are combined with previous searches at energies around 91 and at 189 GeV. A limit is set on the product of the cross-section and the hadronic branching ratio of the Higgs boson, as a function of the Higgs boson mass. Assuming the hZ coupling predicted by the Standard Model, and a Higgs boson decaying only into hadronic final states, a lower bound of 104 GeV/c2 is set on the mass at the 95% confidence level.

Data from e+e- annihilation into hadrons at centre-of-mass energies between 91 GeV and 209 GeV collected with the OPAL detector at LEP, are used to study the four-jet rate as a function of the Durham algorithm resolution parameter ycut. The four-jet rate is compared to next-to-leading order calculations that include the resummation of large logarithms. The strong coupling measured from the four-jet rate is alphas(Mz0)= 0.1182+-0.0003(stat.)+-0.0015(exp.)+-0.0011(had.)+-0.0012(scale)+-0.0013(mass) in agreement with the world average. Next-to-leading order fits to the D-parameter and thrust minor event-shape observables are also performed for the first time. We find consistent results, but with significantly larger theoretical uncertainties.

The differential cross-sections for e+e− → e+e−, e+e− → μ+μ− and e+e− → τ+τ−, and the total cross-section for e+e− → qq̄ at centre-of-mass energies of 130–140 GeV were studied using about 5 pb−1 of data collected with the OPAL detector at LEP in October and November 1995. The results are in agreement with the Standard Model predictions. Four-fermion contact interaction models were fitted to the data and lower limits were obtained on the energy scale Λ at the 95% confidence level.

The process e+e−→γγ(γ) is studied using data recorded with the OPAL detector at LEP. The data sample taken at a centre-of-mass energy of 189 GeV corresponds to a total integrated luminosity of 178 pb−1. The measured cross-section agrees well with the expectation from QED. A fit to the angular distribution is used to obtain improved limits at 95% CL on the QED cut-off parameters: Λ+> 304 GeV and Λ−> 295 GeV as well as a mass limit for an excited electron, Me∗> 306 GeV assuming equal e∗eγ and eeγ couplings. Graviton exchange in the context of theories with higher dimensions is excluded for scales G+< 660 GeV and G−< 634 GeV. No evidence for resonance production is found in the invariant mass spectrum of photon pairs. Limits are obtained for the cross-section times branching ratio for a resonance decaying into two photons and produced in association with another photon.

A search is described for the generic process e+e−→XY, where X is a neutral heavy scalar boson decaying into a pair of photons, and Y is a neutral heavy boson (scalar or vector) decaying into a fermion pair. The search is motivated mainly by the cases where either X, or both X and Y, are Higgs bosons. In particular, we investigate the case where X is the Standard Model Higgs boson and Y the Z0 boson. Other models with enhanced Higgs boson decay couplings to photon pairs are also considered. The present search combines the data set collected by the OPAL collaboration at 189 GeV collider energy, having an integrated luminosity of 182.6 pb−1, with data samples collected at lower energies. The search results have been used to put 95% confidence level bounds, as functions of the mass MX, on the product of the cross-section and the relevant branching ratios, both in a model independent manner and for the particular models considered.

A study of Z boson pair production in e+e− annihilation at center-of-mass energies near 183 GeV and 189 GeV is reported. Final states containing only leptons, (ℓ+ℓ−ℓ+ℓ− and ℓ+ℓ−νν), quark and lepton pairs, (qqℓ+ℓ−, qqνν) and the all-hadronic final state (qqqq) are considered. In all states with at least one Z boson decaying hadronically, qq and bb final states are considered separately using lifetime and event-shape tags, thereby improving the cross-section measurement. At s=189 GeV the Z-pair cross section was measured to be 0.80+0.14−0.13(stat.)+0.06−0.05(syst.)pb, consistent with the Standard Model prediction. At s=183 GeV the 95% C.L. upper limit is 0.55pb. Limits on anomalous ZZγ and ZZZ couplings are derived.

The inclusive branching ratio for the process b→τ−ν̄τX has been measured using hadronic Z decays collected by the OPAL experiment at LEP in the years 1992–2000. The result is: BR(b→τ−ν̄τX)=(2.78±0.18±0.51)%. This measurement is consistent with the Standard Model expectation and puts a constraint of tanβ/MH±<0.53 GeV−1 at the 95% confidence level on Type II Two Higgs Doublet Models.

The production of charm quarks is studied in deep-inelastic electron–photon scattering using data recorded by the OPAL detector at LEP at nominal e+e− centre-of-mass energies from 183 to 209 GeV. The charm quarks have been identified by full reconstruction of charged D★ mesons using their decays into D0π with the D0 observed in two decay modes with charged particle final states, Kπ and Kπππ. The cross-section σD★ for production of charged D★ in the reaction e+e−→e+e−D★X is measured in a restricted kinematical region using two bins in Bjorken x, 0.0014<x<0.1 and 0.1<x<0.87. From σD★ the charm production cross-section σ(e+e−→e+e−cc̄X) and the charm structure function of the photon F2,cγ are determined in the region 0.0014<x<0.87 and 5<Q2<100 GeV2 . For x>0.1 the perturbative QCD calculation at next-to-leading order agrees perfectly with the measured cross-section. For x<0.1 the measured cross-section is 43.8±14.3±6.3±2.8 pb with a next-to-leading order prediction of 17.0+2.9−2.3 pb.

We observe Bose–Einstein correlations in π0 pairs using back-to-back two jet hadronic events from Z0 decays in the data sample collected by the OPAL detector at LEP 1 from 1991 to 1995. Using a static Gaussian picture for the pion emitter source, we obtain the chaoticity parameter λ=0.55±0.10±0.10 and the source radius R=(0.59±0.08±0.05) fm. According to the JETSET and HERWIG Monte Carlo models, the Bose–Einstein correlations in our data sample largely connect π0s originating from the decays of different hadrons. Prompt pions formed at string break-ups or cluster decays only form a small fraction of the sample.

A novel method of determining the mass of the W boson in the ${\rm W^+ W^-} \to \ell \nu \ell^\prime \nu^\prime$ channel is presented and applied to 667 pb-1 of data recorded at center-of-mass energies in the range 183-207 GeV with the OPAL detector at LEP. The measured energies of charged leptons and the results of a new procedure based on an approximate kinematic reconstruction of the events are combined to give: M_W = 80.41\pm 0.41\pm 0.13 GeV, where the first error is statistical and the second is systematic. The systematic error is dominated by the uncertainty on the lepton energy, which is calibrated using data, and the parameterization of the variables used in the fitting, which is obtained using Monte Carlo events. Both of these are limited by statistics.

A measurement of the forward–backward asymmetries of e+e−→bb̄ and e+e−→cc̄ events using electrons and muons produced in semileptonic decays of bottom and charm hadrons is presented. The outputs of two neural networks designed to identify b→ℓ− and c→ℓ+ decays are used in a maximum likelihood fit to a sample of events containing one or two identified leptons. The b and c quark forward–backward asymmetries at three centre-of-mass energies s and the average B mixing parameter χ̄ are determined simultaneously in the fit. Using all data collected by OPAL near the Z resonance, the asymmetries are measured to be: AFBbb̄=(4.7±1.8±0.1)%,AFBcc̄=(−6.8±2.5±0.9)%at〈s〉=89.51 GeV,AFBbb̄=(9.72±0.42±0.15)%,AFBcc̄=(5.68±0.54±0.39)%at〈s〉=91.25 GeV,AFBbb̄=(10.3±1.5±0.2)%,AFBcc̄=(14.6±2.0±0.8)%at〈s〉=92.95 GeV. For the average B mixing parameter, a value of: χ̄=(13.12±0.49±0.42)% is obtained. In each case the first uncertainty is statistical and the second systematic. These results are combined with other OPAL measurements of the b and c forward–backward asymmetries, and used to derive a value for the effective electroweak mixing angle for leptons sin2θeffℓ of 0.23238±0.00052.

The inclusive production of charged hadrons in the collisions of quasi-real photons (e+e−→e+e−+X) has been measured using the OPAL detector at LEP. The data were taken at e+e− centre-of-mass energies from 183 to 209GeV. The differential cross-sections as a function of the transverse momentum and the pseudorapidity of the hadrons are compared to theoretical calculations of up to next-to-leading order (NLO) in the strong coupling constant αs. The data are also compared to a measurement by the L3 Collaboration, in which a large deviation from the NLO predictions is observed.

Inclusive jet production (e+e- -> e+e- +jet+X) is studied in collisions of quasi-real photons radiated by the LEP beams at e+e- centre-of-mass energies sqrt see from 189 to 209 GeV. Jets are reconstructed using the kp jet algorithm. The inclusive differential cross-section is measured as a function of the jet transverse momentum, ptjet, in the range 5 <ptjet < 40 GeV for pseudo-rapidities, etaj, in the range -1.5 < etaj < 1.5. The results are compared to predictions of perturbative QCD in next-to-leading order in the strong coupling constant.

Cross-sections and angular distributions for the production of events with single and multiple photons are measured from data recorded with the OPAL detector at the recently upgraded LEP collider. The measured cross-sections are generally consistent with Standard Model expectations for the e+e−→ νvγ(γ) and e+e− → γγ(γ) processes. Six events with an acoplanar photon pair and large missing mass are found. The observed number of events is larger than expected from e+e−→ ννγγ; however, the missing mass distribution is compatible with the Z0 resonance. Deviations from QED are constrained by the data on e+e− → γγ(γ). Lower limits are set at 95% confidence level on the QED cut-off parameters Λ+ and Λ− of 152 GeV and 142 GeV, respectively, and also on the mass of an excited electron of 147 GeV.

The b quark forward–backward asymmetry has been measured using hadronic Z0 decays collected by the OPAL experiment at LEP. Z0→bb̄ decays were selected using a combination of secondary vertex and lepton tags, and the sign of the b quark charge was determined using an inclusive tag based on jet, vertex and kaon charges. The results, corrected to the quark level, are: AFBb=0.0582±0.0153±0.0012ats=89.50GeV,AFBb=0.0977±0.0036±0.0018ats=91.26GeV,AFBb=0.1221±0.0123±0.0025ats=92.91 GeV, where the first error is statistical and the second systematic in each case. Within the framework of the Standard Model, the result is interpreted as a measurement of the effective weak mixing angle for electrons of sin2θeff,eW=0.23205±0.00068.

From about 4 million hadronic $\rm Z^0$ decays recorded by the OPAL detector on and near to the $\rm Z^0$ resonance, we select a sample of more than 570 000 inclusively reconstructed B mesons. Orbitally-excited mesons ${\rm B}^*_J$ are reconstructed using ${\rm B}\pi^\pm$ combinations. Independently, ${\rm B}^*$ mesons are reconstructed using the decay ${\rm B}^*\rightarrow{\rm B}{\gamma} $. The selected $ {\rm B}^*$ candidates are used to obtain samples enriched or depleted in the decay ${\rm B}^*_J \rightarrow{\rm B}^* \pi^{\pm}(X)$ , where (X) refers to decay modes with or without additional accompanying decay particles. From the number of signal candidates in the ${\rm B}\pi^\pm$ mass spectra of these two samples, we perform the first measurement of the branching ratio of orbitally-excited B mesons decaying into ${\rm B}^*\pi(X)$ : $$ {\rm BR}( {\rm B}^*_J \rightarrow{\rm B}^* \pi (X)) = 0.85^{ +0.26}_{ -0.27} \pm 0.12 ,$$ where the first error is statistical and the second systematic.

The exclusive production of proton-antiproton pairs in the collisions of two quasi-real photons has been studied using data taken at $\sqrt{s_{ee}} = 183 $ GeV and 189 GeV with the OPAL detector at LEP. Results are presented for $p\bar{p}$ invariant masses, W, in the range 2.15<W<3.95 GeV. The cross-section measurements are compared with previous data and with recent analytic calculations based on t he quark-diquark model.

Evidence is presented for the production of prompt Jψ mesons (not originating in b-hadron decays) in hadronic Z0 decays. Using a sample of 3.6 million hadronic events, 24 prompt Jψ candidates are identified from their decays into e+e− and μ+μ− pairs. The background is estimated to be 10.2 ± 2.0 events. The following branching ratio for prompt Jψ production is obtained: Br(Z0→ promptJψ+ X) = (1.9 +- 0.7 ± 0.5 ± 0.5) · 10−4, where the first error is statistical, the second systematic and the third error accounts for uncertainties in the prompt Jψ production mechanism.

We have measured the mean charged particle multiplicities separately for bb̄, cc̄ and light quark (uū,dd̄,ss̄) initiated events produced in e+e− annihilations at LEP. The data were recorded with the OPAL detector at eleven different energies above the Z0 peak, corresponding to the full statistics collected at LEP1.5 and LEP2. The difference in mean charged particle multiplicities for bb̄ and light quark events, δbl, measured over this energy range is consistent with an energy independent behaviour, as predicted by QCD, but is inconsistent with the prediction of a more phenomenological approach which assumes that the multiplicity accompanying the decay of a heavy quark is independent of the quark mass itself. Our results, which can be combined into the single measurement δbl=3.44±0.40(stat)±0.89(syst) at a luminosity weighted average centre-of-mass energy of 195 GeV, are also consistent with an energy independent behaviour as extrapolated from lower energy data.

From data collected around the Z resonance by the OPAL detector at LEP, a sample of B $_s^0$ decays was obtained using D $_s^-}\ell^+$ combinations, where the $D_s^-$ was fully reconstructed in the $\phi\pi^-, \mathrm{K}^{*0}\mathrm{K}^-$ and $\mathrm{K}^0_{\mathrm{S}}\mathrm{K}^-$ decay channels or partially reconstructed in the $\phi\ell^-\bar{\nu}(\mathrm{X})$ decay channel. These events were used to study $\mathrm{B_s^0}$ oscillation. The flavor (b or $\bar{b}$ ) at decay was determined from the lepton charge while the flavor at production was determined from a combination of techniques. The expected sensitivity of the experiment is 4.1 ps $^{-1}$ . The experiment was not able to resolve the oscillatory behavior, and we deduced that the B $_s^0$ oscillation frequency $\Delta m_s > 1.0$ ps $^{-1}$ at the 95% confidence level.

This letter describes a search for the Standard Model Higgs boson using data from e+e− collisions collected at a centre-of-mass energy of 161 GeV by the OPAL detector at LEP. The ata correspond to an integrated luminosity of 10.0 pb−1. The search is sensitive to the main final states from the process where the Higgs boson is produced in association with a fermion anti-fermion pair, namely four jets, two jets with missing energy, and two jets produced together with a pair of electrons, muons or tau leptons. Two candidate events are observed, in agreement with Standard Model background expectations. In combination with previous OPAL searches at centre-of-mass energies close to the Z0 resonance, we derive a lower limit of 65.0 GeV for the mass of the Standard Model Higgs boson, at the 95% confidence level.