M. Cuffiani

We report an experimental determination of the cross section for e+e− → hadrons from a scan around the Z0 pole. On the basis of 4350 hadronic events collected over seven energy points between 89.26 GeV and 93.26 GeV we obtain a mass of mz=91.01±0.05±0.05 GeV, and a total decay width of Γz=2.60±0.13 GeV. In the context of the standard model t these results imply 3.1 ± 0.4 neutrino generations.

We present measurements of global event shape distributions in the hadronic decays of theZ 0. The data sample, corresponding to an integrated luminosity of about 1.3 pb−1, was collected with the OPAL detector at LEP. Most of the experimental distributions we present are unfolded for the finite acceptance and resolution of the OPAL detector. Through comparison with our unfolded data, we tune the parameter values of several Monte Carlo computer programs which simulate perturbative QCD and the hadronization of partons. Jetset version 7.2, Herwig version 3.4 and Ariadne version 3.1 all provide good descriptions of the experimental distributions. They in addition describe lower energy data with the parameter values adjusted at theZ 0 energy. A complete second order matrix element Monte Carlo program with a modified perturbation scale is also compared to our 91 GeV data and its parameter values are adjusted. We obtained an unfolded value for the mean charged multiplicity of 21.28±0.04±0.84, where the first error is statistical and the second is systematic.

We study the inclusive momentum distribution of charged particles in multihadronic events produced in e+e− annihilations at ECM∼M(Z0). We find agreement with the analytical formulae for gluon production that include the phenomena of soft gluon interference. Using data from CM energies between 14 and 91 GeV, we study the dependence of the inclusive momentum distribution on the centre of momentum energy. We find that the analytical formulae describe the data over the entire energy range. Both the momentum distribution at a fixed energy and the change with energy are described by QCD shower Monte Carlo's which include either coherent gluon branchings or string fragmentation. Simple incoherent models with independent fragmentation fail to reproduce the energy dependence and momentum spectra.

An event sample enriched in semileptonic decays of b hadrons is selected using an inclusive lepton selection from approximately 3.0 million hadronic Z0 decays collected with the OPAL detector. This sample is used to investigate B meson oscillations by reconstructing a proper decay time for the parent of each lepton, using a jet charge method to estimate the production flavour of this parent, and using the lepton charge to tag the decay flavour. We measure the mass difference between the two B0 d mass eigenstates

Theoretical predictions as well as experiments performed at storage rings have shown that the lifetimes of β-radionuclides can change significantly as a function of the ionization state. In this paper we describe an innovative approach, based on the use of a compact plasma trap to emulate selected stellar-like conditions. It has been proposed within the PANDORA project (Plasmas for Astrophysics, Nuclear Decay Observation and Radiation for Archaeometry) with the aim to measure, for the first time in plasma, nuclear β-decay rates of radionuclides involved in nuclear-astrophysics processes. To achieve this task, a compact magnetic plasma trap has been designed to reach the needed plasma densities, temperatures, and charge-states distributions. A multi-diagnostic setup will monitor, on-line, the plasma parameters, which will be correlated with the decay rate of the radionuclides. The latter will be measured through the detection of the γ-rays emitted by the excited daughter nuclei following the β-decay. An array of 14 HPGe detectors placed around the trap will be used to detect the emitted γ-rays. For the first experimental campaign three isotopes, 176Lu, 134Cs, and 94Nb, were selected as possible physics cases. The newly designed plasma trap will also represent a tool of choice to measure the plasma opacities in a broad spectrum of plasma conditions, experimentally poorly known but that have a great impact on the energy transport and spectroscopic observations of many astrophysical objects. Status and perspectives of the project will be highlighted in the paper.

.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.

The longitudinal polarization, the transverse polarization, and the forward-backward asymmetry of $\Lambda$ baryons, have been measured using a sample of 4.34 million hadronic $\mathrm{Z}^0$ decays collected with the OPAL detector at LEP between 1990 and 1995. These results are important as an aid to the understanding of hadronization mechanisms. Significant longitudinal polarization has been observed at intermediate and high momentum. For $x_E$ ( $\equiv 2 E_{\Lambda}/\sqrt{s}) >$ 0.3, the longitudinal polarization has been measured to be $-32.9 \pm 5.5 $ (stat) $\pm$ 5.2 (syst)%. We have observed no transverse polarization. A significant forward-backward asymmetry has been measured and can be described by a JETSET model.

A search for the minimal standard model Higgs boson (H0) has been performed with data from e+e− collisions in the OPAL detector at LEP. The analysis is based on approximately 8 pb−1 of data taken at centre-of-mass energies between 88.2 and 95.0 GeV. The search concentrated on the reaction e+e−→(e+e−, μ+μ−, vv or τ+τ−)H0, H0→(qq or τ+τ−) for Higgs boson masses above 25 GeV/c2. No Higgs boson candidates have been observed. The present study, combined with previous OPAL publications, excludes the existence of a standard model Higgs boson with mass in the range 3<mH0<44GeV/c2 at the 95% confidence level.

Relative production rates of multijet hadronic final states of Z0 boson decays, observed in e+e− annihilation around 91 GeV centre of mass energy, are presented. The data can be well described by analytic O(αs2) QCD calculations and by QCD shower model calaculations with parameters as determined at lower energies. A first judgement of ΛMS and of the renormalization scale μ2 in O(αs2) QCD results in values similar to those obtained in the continuum of e+e− annihilations. Significant scaling violations are observed when the 3-jet fractions are compared to the corresponding results from smaller centre of mass energies. They can be interpreted as being entirely due tot the energy dependence of αs, as proposed by the nonabelian nature of QCD, The possibility of an energy independent coupling constant can be excluded with a significance of 5.7 standard deviations.

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)

We have searched for Z0 decays into acoplanar pairs of leptons or jets. The data were recorded with the OPAL detector during an energy scan around the Z0 peak and correspond to about 17000 produced Z0,s. We have determined model independent limits on branching ratios for the Z0 to decay into pairs of heavy particles with subsequent decays leading to the above topologies. In the context of supersymmetric models, mass bounds close to the kinematic limit were obtained for the scalar leptons, ẽ, g̃m, g̃t, and for the chargino, X̃±.

Photonic events with large missing energy have been observed in $\rm e^+e^-$ collisions at a centre-of-mass energy of 189 GeV using the OPAL detector at LEP. Results are presented for event topologies consistent with a single photon or with an acoplanar photon pair. Cross-section measurements are performed within the kinematic acceptance of each selection, and the number of light neutrino species is measured. Cross-section results are compared with the expectations from the Standard Model process $\mathrme^+\mathrme^-\to \nu\overline{\nu}$ + photon(s). No evidence is observed for new physics contributions to these final states. Upper limits on $\sigma(\mathrme^+\mathrme^-\to\mathrm{X}\mathrm{Y})\cdot\mathrm{BR}(\mathrm{X}\to\mathrm{Y}\gamma)$ and $\sigma(\mathrme^+\mathrme^-\to\mathrm{XX})\cdot\mathrm{BR}^2(\mathrm{X}\to\mathrm{Y}\gamma)$ are derived for the case of stable and invisible $\mathrm{Y}$ . These limits apply to single and pair production of excited neutrinos ( $\mathrm{X} = \nu^*, \mathrm{Y} = \nu$ ), to neutralino production ( $\mathrm{X}={{{\tilde{\chi}}^{0}}_{2}}, \mathrm{Y}={{{\tilde{\chi}}^{0}}_{1}}$ ) and to supersymmetric models in which $\mathrm{X} ={{{\tilde{\chi}}^{0}}_{1}}$ and $\mathrm{Y}={\tilde{\mathrm{G}}}$ is a light gravitino. The case of macroscopic decay lengths of particle X is considered for $\mathrme^+\mathrme^- \to \mathrm{XX}$ , $\rm X \to Y \gamma$ , when $M_{\mathrm Y}\approx 0$ . The single-photon results are also used to place upper limits on superlight gravitino pair production as well as graviton-photon production in the context of theories with additional space dimensions.

We have searched for lepton flavour violating Z0→eμ, Z0→eτ and Z0→μτ decays in a sample of 4.0×106 visible Z0 decays collected with the OPAL detector at LEP during 1991 to 1994. No candidates are found for Z0→eμ. The samples of selected Z0→eτ and Z0→μτ candidates are consistent with the expected background. The following limits are set at 95% confidence level: $$\begin{gathered} BR(Z^0 \to e\mu ){\text{ }}< {\text{ 1}}{\text{.7 }} \times {\text{ 10}}^{ - {\text{6}}} \hfill \\ BR(Z^0 \to e\tau ){\text{ }}< {\text{ 9}}{\text{.8 }} \times {\text{ 10}}^{ - {\text{6}}} \hfill \\ BR(Z^0 \to \mu \tau ){\text{ }}< {\text{ 17}}{\text{. }} \times {\text{ 10}}^{ - {\text{6}}} . \hfill \\ \end{gathered}$$

Quark and gluon jets with equal energies are identified in three-jet hadronicZ 0 events, using reconstructed secondary vertices from heavy quark decay in conjunction with energy ordering of the jets to anti-tag the gluon jets. Selection of jets from a symmetric event topology allows their properties to be compared in a simple and direct manner. The jets under study have an energy of about 24 GeV. It is observed that gluon jets have a larger angular width than quark jets and yield a softer particle energy spectrum. Correspondingly, the mean particle multiplicity is found to be larger for gluon than for quark jets. Correcting the distributions for residual misidentification of the quark and gluon jets, the ratio of mean particle multiplicty of gluon relative to quark jets is measured to be $$\frac{{\left\langle n \right\rangle _{gluon} }}{{\left\langle n \right\rangle _{quark} }} = 1.27 \pm 0.04(stat.) \pm 0.06(syst.),$$ where the jets are defined using thek ⊥ jet finder. The numerical value of this ratio is found to be sensitive to the choice of the jet algorithm. The experimental results are compared to Monte Carlo calculations which incorporate perturbative QCD along with different assumptions about the hadronization process.

Evidence is presented for kinematic and charge correlations of B mesons with charged pions and kaons. Using a new technique, a sample of over 80 000 partially reconstructed B mesons is obtained in 3.5·106 hadronic Z0 decays recorded using the OPAL detector at LEP. The invariant mass distributions of B+π− and B+K− combinations show enhancements consistent with the decays of P-wave resonances of a b antiquark and a light quark. We observe an excess of 1738±195 B+π− pairs with invariant masses in the range 5.60-5.85 GeV and an excess of 149±31 B+K− pairs with invariant masses in the range 5.80–6.00 GeV. Labeling the observed enhancements generically as B** we find $$\begin{gathered} \frac{{BR(Z^0 \to \bar b \to B^{**0} \to B^{(*) + } \pi ^ - )}}{{BR(Z^0 \to \bar b \to B^ + )}} = 0.18 \pm 0.04, \hfill \\ \frac{{BR(Z^0 \to \bar b \to B_s^{**0} \to B^{(*) + } K^ - )}}{{BR(Z^0 \to \bar b \to B^ + )}} = 0.026 \pm 0.008, \hfill \\ \end{gathered} $$ where B(*)+ indicates the sum of B+ and B*+ and the errors include statistical and systematic contributions. From a study of π-B charge-flavor correlations we conclude that the production flavor of a B meson can be tagged with the charge of a pion in an appropriately chosen kinematic region, and that the performance of this flavor tag compares favorably in the $$Z^0 \to q\bar q$$ environment with lepton-based tags.

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.

A search for the minimal standard model Higgs boson has been performed with data from e+e− collisions in the OPAL detector at LEP. The analysis is based on 825 nb−1 of data taken at centre-of-mass energies between 88.3 and 95.0 GeV. The search concentrated on the reactions e+e−→(e+e− or μ+μ− or vv)H0,H0→(qq or τ+τ−), for Higgs masses above 3 GeV/c2. No Higgs boson candidates have been observed. The present study excludes the existence of a standard model H0 with mass in the range 3.0⩽mH⩽19.3 GeV/c2 at the 95% confidence level.

From a sample of 3308 e+e− → τ+τ− events with an estimated background of 1.9%, we find 964 τ → eνν, 903 τ → μνν, and 309 τ → π(K)ν candidates. Efficiency and background estimates determined from both Monte Carlo and control sample studies yield the following branching ratios: B(τ → eνν) = 17.4 ± 0.5 (stat) ± 0.4 (sys)%, B(τ → μνν) = 16.8 ± 0.5 ± 0.4%, and B(τ → π(K)ν) = 12.1 ± 0.7 ± 0.5%. These values are in good agreement with previous measurements. The measured lepton branching ratios, when combined with the world-average measured value for the τ lifetime, yield a ratio of the τ Fermi coupling constant to that of the lighter leptons given by Gτ/Ge,μ = 0.92 ± 0.04, where it is assumed Ge = Gμ ≡ Ge,μ. The average τ polarization at the Z° resonance is measured to be −0.01±0.09 from an analysis of the momentum spectra of the electron, muon, and pion candidates, implying that the ratio of vector to axial vector couplings of the τ to the Z° is ντ/ατ = 0.01 ± 0.04. The measurements of the average polarizations in the forward and backward hemispheres lead to the efficiency-corrected, forward-backward polarization asymmetry AFBpol = −0.22 ± 0.10, implying for the electron couplings to the Z° the ratio νe/αe = 0.15±0.07. Since these values for the tau and electron couplings are consistent with one another, we assume lepton universality to derive ν/α = 0.05 ± 0.04 and a value for the weak mixing angle of sin2θW = 0.237 ± 0.009, with no ambiguity introduced by the relative signs of ν and α.

We report on a measurement of the mass of the Z0 boson, its total width, and its partial decay widths into hadrons and leptons. On the basis of 25 801 hadronic decays and 1999 decays into electrons, muons or taus, selected over eleven energy points between 88.28 GeV and 95.04 GeV, we obtain from a combined fit to hadrons and leptons a mass of Mz = 91.154 ± 0.021 (exp) ± 0.030 (LEP) GeV, and a total width of Γz = 2.536 ± 0.045 GeV. The errors on Mz have been separated into the experimental error and the uncertainty due to the LEP beam energy. The measured leptonic partial widths are Γee = 81.2 ± 2.6 MeV, Γμμ = 82.6 ± 5.8 MeV, and Γττ = 85.7 ± 7.1 MeV, consistent with lepton universality. From a fit assuming lepton universality we obtain Γℓ+ ℓ− = 81.9 ± 2.0 MeV. The hadronic partial width is Γhad = 1838 ± 46 MeV. From the measured total and partial widths a model independent value for the invisible width is calculated to be Γinv = 453 ± 44 MeV. The errors quoted include both the statistical and the systematic uncertainties.

We describe a cone-based jet finding algorithm (similar to that used in $$\bar p$$ p experiments), which we have applied to hadronic events recorded using the OPAL detector at LEP. Comparisons are made between jets defined with the cone algorithm and jets found by the “JADE” and “Durham” jet finders usually used ine + e − experiments. Measured jet rates, as a function of the cone size and as a function of the minimum jet energy, have been compared with O(α 2 ) calculations, from which two complementary measurements $$\alpha _s \left( {M_{Z^0 } } \right)$$ have been made. The results are $$\alpha _s \left( {M_{Z^0 } } \right)$$ =0.116±0.008 and $$\alpha _s \left( {M_{Z^0 } } \right)$$ =0.119±0.008 respectively, where the errors include both experimental and theoretical uncertainties. Measurements are presented of the energy flow inside jets defined using the cone algorithm, and compared with equivalent data from $$\bar p$$ p interactions, reported by the CDF collaboration. We find that the jets ine + e − are significantly narrower than those observed in $$\bar p$$ p. The main contribution to this effect appears to arise from differences between quark- and gluon-induced jets.

Jet production rates in hadronicZ 0 decays are studied using four different recombination schemes to define resolvable jets. The strong coupling constant α s ( $$M_{Z^0 } $$ ) is determined in fits of the correspondingO(α 2 ) QCD calculations to the differential 2-jet distributionsD 2(y). Hadronisation corrections and renormalisation scale uncertainties are found to be different for each recombination scheme. Within their overall systematic uncertainties, the four schemes yield consistent values of α s ( $$M_{Z^0 } $$ ), leading to a final result of $$\alpha _s (M_{Z^0 } ) = 0.118 \pm 0.008.$$ . The error includes the experimental uncertainties (±0.003), uncertainties of hadronisation corrections and of the degree of parton virtualities to which the data are corrected, as well as the uncertainty of choosing the renormalisation scale.

Quark and gluon jets in e+e− three-jet events at LEP are identified using lepton tagging of quark jets, through observation of semi-leptonic charm and bottom quark decays. Events with a symmetry under transposition of the energies and directions of a quark and gluon jet are selected: these quark and gluon jets have essentially the same energy and event environment and as a consequence their properties can be compared directly. The energy of the jets which are studied is about 24.5 GeV. In the cores of the jets, gluon jets are found to yield a softer particle energy spectrum than quark jets. Gluon jets are observed to be broader than quark jets, as seen from the shape of their particle momentum spectra both in and out of the three-jet event plane. The greater width of gluon jets relative to quark jets is also visible from the shapes of their multiplicity distributions. Little difference is observed, however, between the mean value of particle multiplicity for the two jet types.

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.

Bose-Einstein correlations between like-sign charged track pairs have been studied in e+e− annihilation hadronic events at centre-of-mass energies around the Z0 peak as a function of Q, the four-momentum difference of the pair. The measurement was performed with the OPAL detector at LEP. Assuming the charged tracks to be pions, the observed Bose-Einstein enhancement was used to extract the values of the strength of the effect and the radius of the pion emitting source, which were found to be λ=0.866±0.032±0.140 and R0=0.928±0.019±0.150 fm, respectively, where the first error is statistical and the second is systematic. The results do not show significant variation in comparison to e+e− annihilation measurements at lower centre-of-mass energies. If non-pion track contamination is taken into account, the value of the strength λ becomes consistent with unity.

The production rates of ${\rm D}^{*\pm}$ mesons in charm and bottom events at centre-of-mass energies of about 91 GeV and the partial width of primary ${\rm c\bar c}$ pairs in hadronic ${\rm Z}^0$ decays have been measured at LEP using almost 4.4 million hadronic ${\rm Z}^0$ decays collected with the OPAL detector between 1990 and 1995. Using a combination of several charm quark tagging methods based on fully and partially reconstructed ${\rm D}^{*\pm}$ mesons, and a bottom tag based on identified muons and electrons, the hadronisation fractions of charm and bottom quarks into ${\rm D}^{*\pm}$ mesons have been found to be \[ {\rm f(b \to D^{*+}}X) = 0.173 \pm 0.016 \pm 0.012\quad {\rm and }\quad {\rm f (c \to D^{*+}}X) = 0.222 \pm 0.014 \pm 0.014\; . \] The fraction of ${\rm c\bar c}$ events in hadronic ${\rm Z}^0$ decays, $\Gamma_{\rm c\bar c}/\Gamma_{\rm had}=\Gamma({\rm Z}^0\to{\rm c\bar c}) / \Gamma({\rm Z}^0\to\rm hadrons)$ , is determined to be \[ \Gamma_{\rm c\bar c}/\Gamma_{\rm had} = 0.180\pm 0.011 \pm 0.012 \pm 0.006 \ . \] In all cases the first error is statistical, and the second one systematic. The last error quoted for $\Gamma_{\rm c\bar c}/\Gamma_{\rm had}$ is due to external branching ratios.

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.

The strong coupling constant, αs, has been determined in hadronic decays of theZ 0 resonance, using measurements of seven observables relating to global event shapes, energy correlations and jet rates. The data have been compared with resummed QCD calculations, which are combined with theO( s 2 ) theory. The seven measurements agree to about 10% and the final results, based on a weighted average, is: $$\alpha _s (M_{Z^0 } ) = 0.120 \pm 0.006,$$ where the error includes both experimental and theoretical uncertainties. This value corresponds to renormalization scale $$\mu = M_{Z^0 } $$ and the error includes the uncertainty in this choice of scale. The present measurement complements previous determinations using theO( s 2 ) QCD matrix elements alone, and yields a compatible result, with comparable errors.

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.

The cross section of the pure QED process e+e−→γγ has been measured using data accumulated during the 1989 and 1990 scans of the Z0 resonance at LEP. Both the energy dependence and the angular distribution are in good agreement with the QED prediction. Upper limits on the branching ratios of Z0→γγ, Z0→π0γ and Z0→ηγ have been set at 1.4×10 −4, 1.4×10−4 and 2.0×10−4 respectively. Lower limits on the cutoff parameters of the modified electron propagator have been found to be Λ+ > 117 GeV and Λ− > 110 GeV. The reaction e+e− → γγγ has also been studied and was found to be consistent with the QED prediction. An upper limit on the branching ratio of Z0→γγγ has been set at 6.6 × 10−5. All the limits are given at 95% confidence level.

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.

From a sample of about 450 000 hadronicZ 0 decays, measurements of the average $$B^0 - \bar B^0 $$ mixing parameter, χ, the branching fraction ofZ 0 bosons into hadrons containing bottom quarks, $${{\Gamma (Z^0 \to b\bar b)} \mathord{\left/ {\vphantom {{\Gamma (Z^0 \to b\bar b)} {\Gamma (Z^0 \to hadrons)}}} \right. \kern-\nulldelimiterspace} {\Gamma (Z^0 \to hadrons)}}$$ , the average semileptonic branching ratios for such hadrons,B(b→l) andB(b→c→l), and the mean scaled energy of these hadrons, 〈xE〉, are presented. The measurements were obtained using a simultaneous fit to single-lepton and dilepton events collected with the OPAL detector at LEP, including both electrons and muons. The results are where the errors are statistical and systematic, respectively, in each case. The result forB)b→c→l) excludes decays of the typeb→→ℓ.

A search has been performed for unstable charged scalar particles (S±) such as technipions or charged Higgs bosons, pair-produced in e+e− annihilation at energies near the Z0 pole. No evidence for such particles was observed in the decay modes e+e−→S+S−→(cs)(τv), (τv) (τv)and(cs)(cs). A lower limit (at the 95% CL ) of 35 Gev/c2 is obtained for the mass of the charged scalar particles, independent of the branching ratio.

We report on a search for new quarks in hadronic Z° decays. From the event shape analysis of a data sample containing 2185 multihadronic annihilation events, we observe no evidence for the top or b' quarks. We derive limits for the top and b' quark masses under the assumption of various possible standard model and non-standard model decay schemes. Our search is sensitive to quark masses larger than 23 GeV/c2; it yields the following lower limits at a 95% confidence level: 44.5 GeV/c2 for the top quark mass and 45.2 GeV/c2 for the b′ quark mass.

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.

Using radiative Z0→ τ+τ−γevents collected with the OPAL detector at LEP at s=MZ during 1990–95, a direct study of the electromagnetic current at the τγ vertex has been performed in terms of the anomalous magnetic form factor F2 of the τ lepton. The analysis is based on a data sample of 1429 e+e−→τ+τ−γevents which are examined for a deviation from the expectation with F2=0. From the non-observation of anomalous τ+τ−γproduction a limit of−0.068<F2<0.065is obtained. This can also be interpreted as a limit on the electric dipole form factor F3 as|eF3|<3.7×10−16ecm.The above ranges are valid at the 95% confidence level.

Using muon candidates in 133 000 hadronic decays of the Z0, recorded by the OPAL detector at LEP, we have measured the fraction of events containing semi-leptonic decays of b flavoured hadrons. An analysis based on fitting the shape of the momentum and transverse momentum distributions of the muon candidates gave the result: (Γ(Z0→bb)Γ(Z0→hadrons))×Br(b→μ)=0.0226±0.0007±0.0013, and also yielded: (Γ(Z0→cc)Γ(Z0→hadrons))×Br(c→μ)=0.0176±0.0025±0.0042. Using the charge of the muon and the angle of the event thrust axis with respect to the electron beam, in a b-enriched event sample, we measured the forward-backward asymmetry for Z0 → bb decays. Without correction for B0B0 mixing, the asymmetry was found to be AbFB = 0.072±0.042±0.010.

We have studied the production of D*± mesons in a sample of 1.25 million multihadronic decays of the Z0, in which 1969 candidates have been identified. We have determined the total multiplicity of charged D* mesons in multihadronic Z0 decays to be

The production of the octet and decuplet baryons Λ, Ξ−, Σ(1385)±, Ξ(1530)0 and Ω− and the corresponding antibaryons has been measured in a sample of 485 000 hadronic Z0 decays. Results on differential and integrated cross sections are presented. The differential cross section of Λ baryons is found to be softer than the one predicted by the Jetset and Herwig Monte Carlo generators. The measured decuplet yields are found to disagree with the simple diquark picture where only one tuning parameter for spin 1 diquarks is used. Comparisons of the momentum spectra for Λ and Ξ− with the predictions of an analytical QCD formula are also presented.

The observation and measurement of the time-dependence of B0d↔B0d mixing are described. The B0d meson is reconstructed in final states that contain a D∗− and an ℓ+, where the b flavour of the B0d at decay time is tagged by the electric charge of the lepton. A new and efficient method, using a jet charge technique, is developed for identifying the b flavour of the produced B0d. From a sample of 556 D∗±ℓ∓ candidates reconstructed in the OPAL data collected during 1990–1993, the B0d↔B0d oscillation frequency is measured to be Δmd = 0.508 ± 0.075 (stat) ±0.025(syst) ps−1 giving an oscillation parameter of χd = 0.73 ± 0.11 (stat) ±0.08 (syst), where 0.076 of the systematic error on χd arises from the uncertainty on the B0d lifetime.

Using a sample of 5558 Z0 → τ+τ− decays produced at LEP a direct test of CP-invariance in the neutral current reaction e+e− → τ+τ− is performed. Samples of events where eachy τ decays into a single particle have been isolated for the construction of CP-odd observables. Three different event classes are considered: lepton-lepton, lepton-hadron, and hadron-hadron. No evidence for a non-zero expectation value of the considered CP-observables and hence for CP-violation is observed. Quantitatively, we deduce from this null result an estimate on the weak dipole moment d̃τ(m2Z = (−4.5 ± 5.3 ± 1.4) × 10−17 e cm for the lepton-lepton signature and d̃τ(m2Z = (1.4 ± 3.7 ± 1.3) × 10−17 e cm for the hadron-hadron signature. Combining these results we place a limit with 95% confidence of |d̃τ|⩽7.0 × 10−17 e cm.

A search for scalar leptoquarks has been performed with data from the OPAL detector at the e+e− storage ring LEP. In a data sample corresponding to an integrated luminosity of 6.3 pb−1 no evidence for leptoquark production was observed where the leptoquark decays into a quark and either a charged lepton (e, μ, τ) or a neutrino. An upper limit of 1.7 pb on the production cross section for leptoquarks is obtainedassuming a branching ratio of 50% for the decay of the leptoquark into the channels with a charged lepton. Lower limits on the leptoquark mass between 41.4 and 46.4 GeV/c2 at 95% CL are obtained, depending on the effecte SU(2) × U(1) invariant couplings assigned to the leptoquark.

Data taken with the OPAL detector at LEP during a scan of the Z0 resonance were searched for evidence of neutral heavy leptons that decay via mixing. Four different decay modes of the neutral heavy lepton are considered: L0→eW∗, L0→μW∗, L0→τW∗, and L0→vZ∗. No evidence is seen of a neutral heavy lepton signal; branching fraction limits in the range of 10−3–10−4 are set for Z0→L0L0 and for Z0→vL0 (orvL0) relative to Z0→hadrons.

We observe evidence for the production of b-flavoured baryons in decays of the Z0 boson with the OPAL detector at LEP. We find 68 Λl−, Λl+ candidates in 458 583 hadronic Z0 decays. We interpret this as a signal of 55 ± 9+0.3−3.1 events from the semi-leptonic decays of b baryons. Assuming weakly decaying b baryons produced in Z0 decays are mostly Λb particles, we measure the product branching ratio (Γbb/Γhad) f (b→Λb) B (Λb→Λl−vX), averaged over the electron and muon channels, to be (6.2±1.0±1.5)×10−4.

The production of K0 mesons in e+e− interactions at center of mass energies in the region of the Z0 mass has been investigated with the OPAL detector at LEP. The rate is found to be 2.10±0.02±0.14 K0, Z0 per hadronic event. The predictions from the JETSET and HERWIG generators agree very well with both the rate and the scale invariant cross section (1/σhadβ) (dσ/dxE) for K0 production. Comparisons of the inclusive momentum spectrum with predictions of an analytical QCD formula and with data from lower center of mass energies are presented.

The production dynamics of baryon-antibaryon pairs are investigated using hadronic Z0 decays, recorded with the OPAL detector, which contain at least two identified Λ baryons. The rapidly difference for ΛΛ pairs shows the correlations expected from models with a chain-like production of baryon-antibaryon pairs. If the baryon number of a Λ is compensated by a Λ, the Λ is found with a probability of 53% in an interval of ±0.6 around the Λ rapidity. This correlation strength is weaker than predicted by the Herwig Monte Carlo and the Jetset Monte Carlo with a production chain of baryon-antibaryon, and stronger than predicted by the UCLA model. The observed rapidity correlations can be described by the Jetset Monte Carlo with a dominant production chain of baryon-meson-antibaryon, the popcorn mechanism. In addition to the short range correlations, one finds an indication of a correlation of ΛΛ pairs in opposite hemispheres if both the Λ and the Λ have large rapidities. Such long range correlations are expected if the primary quark flavours are compensated in opposite hemispheres and if these quarks are found in energetic baryons. Rates for simultaneous baryon and strangeness number compensation for ΛΛ, Ξ−Ξ+ and Ξ−Λ (Λ+ Λ) are measured and compared with different Monte Carlo models.

From the observed yield and properties of isolated energetic photons in the reaction e+e−→Z0→hadrons+γ measured with the OPAL detector at LEP, evidence for final state radiation from primary quarks is obtained. Combined with the measurement of the total hadronic width of the Z0, the observed rate allows the extraction of the electroweak coupling constants of up and down type quarks: ν132 + a132 = 1.24±0.47 and ν232 + a232 = 1.72±0.70 No evidence for additional photon production from anomalous decays of the Z0 or from decays of new particles is found. This measurement limits the contribution to the total Z0 width from such sources to be less than 8.2 MeV at the 95% confidence level.

From an analysis of multi-hadron events from Z0 decays, values of the strong coupling constant αs(M2Z0)=0.131±0.006 (exp)±0.002(theor.) and αs(Mz02) = −0.009+0.007(exp.)−0.002+0.006(theor.) are derived from the energy-energy correlation distribution and its asymmetry, respectively, assuming the QCD renormalization scale μ=MZ0. The theoretical error accounts for differences between O(α2s) calculations. A two parameter fit ΛMS and the renormalization scale μ leads to Λ MS=216±85 MeV and μ2s=0.027±0.013 or to αs(M2Z0)=0.117+0.006−0.008(exp.) for the energy-energy correlation distribution. The energy-energy correlation asymmetry distribution is insensitive to a scale change: thus the αs value quoted above for this variable includes the theoretical uncertainty associated with the renormalization scale.

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.

Quark and gluon in 3-jet events from hadronic Z0 decays are identified through use of the semi-leptonic decays of charm and bottom quarks. This tagging method allows geometrically symmetric or transposed configurations of the quark and gluon jets to be selected, permitting a study of the asymmetry in the population of particles in the regions between jets, commonly called the string effect, which avoids restrictions present in previous studies. In particular, our demonstration of a population asymmetry is performed without use of a model. Our results imply that dynamical differences exist between quarks and gluons or between quark-antiquark and quark-gluon jet systems with respect to their particle production properties.

The average transverse momentum and the size of the particle emitting source (measured via Bose-Einstein correlations) have been studied as functions of the charged particle density in the central region inp-p interactions at $$\sqrt s $$ =62, 44 and 31 GeV. Both the average transverse momentum and the source size increase with increasing density at all three energies. This effect, very weak at $$\sqrt s $$ =31 GeV, becomes stronger with increasing energy.

We present evidence for the existence of the strange b-flavoured meson Bs0 in a data sample of 470 628 hadronic Z0 decays recorded with the OPAL detector at LEP. A signal of 18.3±5.2 (stat.)±0.9(syst.) Ds−l+ pairs (or charge conjugate, l = e or l = μ) is found after background subtraction, where the Ds meson is detected in the øπ and K∗0 K final states. Interpreting this signal as coming from the semileptonic decay Bs0→Ds−l+vX and combining t he two decay modes of the Ds, we find f (b→Bs0)B(Bs0→Ds−l+vX)BDs−→φ π−) = [3.9±1.1 (stat.)±0.8(syst.)]×10−4, where f (b→Bs0) is the fraction of b quarks that results in a B0s meson in Z0 decays. This signal for the Bs0 is supported by our observation of an excess of 147 ± 48 inclusive Ds mesons in the φπ and K∗0K modes above the number expected from B0 and B+ decays and from the fragmentation of primary c quarks. In addition, a search is made for the exclusive decay Bs0→J/ψφ. Based on one candidate event the 90% confidence level upper limit is determined to be f (b → Bs0) B (Bs0 → J/ψφ) < 0.22%.

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.

We report on a measurement of the processes e+e−→e+e−, e+e−→μ+μ−, and e+e−→τ+τ− near the Z0 pole. On the basis of 163 e+e−, 101 μ+μ− and 87 τ+τ− events we obtain Γee=89±4±4 MeV, Γμμ=85±9±6 MeV and Γττ=87±10±8 MeV, compatible with the standard model. Combining these with our previous results on hadronic Z0 decays, we find a hadronic width Γhad=1787±81±90 MeV and an invisible width Γinv=552±85±71 MeV.

.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.

Searches for a scalar top quark and a scalar bottom quark have been performed using a data sample of 182 pb−1 at a centre-of-mass energy of s=189 GeV collected with the OPAL detector at LEP. No evidence for a signal was found. The 95% confidence level (C.L.) lower limit on the scalar top quark mass is 90.3 GeV if the mixing angle between the supersymmetric partners of the left- and right-handed states of the top quark is zero. In the worst case, when the scalar top quark decouples from the Z0 boson, the lower limit is 87.2 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. From a search for the scalar bottom quark, a mass limit of 88.6 GeV was obtained if the mass difference between the scalar bottom quark and the lightest neutralino is larger than 7 GeV. These limits significantly improve the previous OPAL limits.

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)%.

We present an update with increased statistics to our published analysis of hadronic and leptonic cross sections and of the leptonic forward-backward asymmetries ine + e − collisions. The published results were based on a total 454 000 hadronic and 58 000 leptonic events. This analysis adds 733 000 hadronic and 88 000 leptonic events recorded at theZ 0 peak in 1992 by the OPAL experiment at LEP. A model independent analysis ofZ 0 parameters based on an extension of the improved Born approximation leads to tests of lepton universality and gives an interpretation of the results within the Standard Model framework. We also present a model independent test for new physics.

Results are reported of a study of neutral vector meson production in multihadronicZ 0 decays in the OPAL experiment at LEP. Pions and kaons have been identified by specific ionisation energy loss andK ± π ∓ andK + K − mass spectra have been fitted, in bins of the scaled momentum variablex p , to combinations of resonance signals and non-resonant backgrounds. Rates are given forK *(892)° and ø(1020), and production cross sections are compared to the predictions of Monte Carlo models. Overall multiplicities have been determined as 0.76±0.07±0.06K *(892)° and 0.086±0.015±0.010 ø(1020) per hadronicZ 0 decay (the quoted errors are respectively statistical and systematic). Momentum dependent distortions of the ππ mass spectra, possibly associated indirectly with Bose-Einstein effects, have prevented reliable measurement of the ρ(770)° cross section in this study.

A study of b quark fragmentation at LEP is presented using a sample of semileptonic B decays containing a fully reconstructed charm meson. The data are compared to several theoretical models for heavy quark fragmentation; the free parameters in these models are fitted and the sensitivity of the model parameters to the rate of P-wave B meson production is studied. The mean scaled energy fraction of B0 and B+ mesons has been determined to be <xE> = 0.695 ± 0.006 ± 0.003 ± 0.007, where the errors are statistical, systematic and model dependence respectively. This result is consistent with previous, less direct measurements from inclusive leptonic B decays. Also presented is a model independent fit to the shape of the energy distribution of weakly decaying B mesons at LEP.

Searches for the neutral Higgs bosons predicted by the Standard Model (SM) and the Minimal Supersymmetric extension of the Standard Model (MSSM) have been performed with the OPAL detector at LEP. Approximately 170 pb $^{-1}$ of $\mathrm{e}^+ \mathrm{e}^-$ collision data collected at $\sqrt{s} \approx 189$ GeV were used to search for Higgs boson production in the SM process ${\mathrm{e}}^+{\mathrm{e}}^- \rightarrow \mathrm{H}^{0}\mathrm{Z}^{0}$ and the MSSM processes ${\mathrm{e}}^+{\mathrm{e}}^- \rightarrow \mathrm{h}^{0} \mathrm{Z}^{0}$ and ${\mathrm{e}}^+ {\mathrm{e}}^- \rightarrow \mathrm{A}^{0} \mathrm{h}^{0}$ . The searches are sensitive to the $\mathrm{b}\bar{\mathrm{b}}$ and $\tau^+\tau^-$ decay modes of the Higgs bosons, and also to the MSSM decay mode $\mathrm{h}^{0} \rightarrow \mathrm{A}^{0} \mathrm{A}^{0}$ . OPAL search results at lower centre-of-mass energies have been incorporated in the limits, which are valid at the 95% confidence level. For the SM Higgs boson, a lower mass bound of 91.0 GeV is obtained. In the MSSM, the limits are $m_{\mathrm{H}} >74.8$ GeV and $m_{\mathrm{A}} >76.5$ GeV, assuming $\tan\beta > 1$ , that the mixing of the scalar top quarks is either zero or maximal, and that the soft SUSY-breaking masses are 1 TeV. For the case of zero scalar top mixing, the values of $\tan\beta$ between 0.72 and 2.19 are excluded.

The fraction of $$b\bar b$$ events in hadronicZ 0 decays has been measured from the yield of leptons in the data samples collected by OPAL in 1990 and 1991. A sample enriched in events containing $${\rm Z}^0 \to b\bar b$$ decays was obtained by requiring the presence of an electron or muon with high momentum and high momentum component transverse to the associated hadronic jet. After accounting for backgrounds and acceptances, a value of $$\frac{{\Gamma ({\rm Z}^0 \to b\bar b)}}{{\Gamma ({\rm Z}^0 \to hadrons)}} = 0.220 \pm 0.002 \pm 0.0006 \pm 0.011$$ was obtained. The first two errors reflect the data statistics and the systematic uncertainties arising from detector modelling uncertainties, respectively. The third error includes systematic effects fromb andc fragmentation and decay uncertainties.

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.

Using data from e+e− collisions collected with the OPAL detector during the 1990 LEP run, a search was mode for a low mass Higgs boson (H0) with arbitrary decay mode. The existence of a minimal standard model H0 with mass in the range 0 ⩽ mH ⩽ 11.3 GeV/c2 is excluded at the 95% confidence level for all possible decay modes of the H0. Limits on the production of Higgs bosons predicted by the minimal supersymmetric standard model and new scalar particles that couple to the Z0 are presented as a function of the scalar mass.

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 .

The Compact Muon Solenoid (CMS) experiment prepares its Phase-2 upgrade for the high-luminosity era of the LHC operation (HL-LHC). Due to the increase of occupancy, trigger latency and rates, the full electronics of the CMS Drift Tube (DT) chambers will need to be replaced. In the new design, the time bin for the digitization of the chamber signals will be of around 1 ns, and the totality of the signals will be forwarded asynchronously to the service cavern at full resolution. The new backend system will be in charge of building the trigger primitives of each chamber. These trigger primitives contain the information at chamber level about the muon candidates position, direction, and collision time, and are used as input in the L1 CMS trigger. The added functionalities will improve the robustness of the system against ageing. An algorithm based on analytical solutions for reconstructing the DT trigger primitives, called Analytical Method, has been implemented both as a software C++ emulator and in firmware. Its performance has been estimated using the software emulator with simulated and real data samples, and through hardware implementation tests. Measured efficiencies are 96 to 98% for all qualities and time and spatial resolutions are close to the ultimate performance of the DT chambers. A prototype chain of the HL-LHC electronics using the Analytical Method for trigger primitive generation has been installed during Long Shutdown 2 of the LHC and operated in CMS cosmic data taking campaigns in 2020 and 2021. Results from this validation step, the so-called Slice Test, are presented.

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.

New measurements are presented of the photon structure function F2γ(x,Q2) at four values of Q2 between 9 and 59 GeV2 based on data collected with the OPAL detector at centre-of-mass energies of 161–172 GeV, with a total integrated luminosity of 18.1 pb−1. The evolution of F2γ with Q2 in bins of x is determined in the Q2 range from 1.86 to 135 GeV2 using data taken at centre-of-mass energies of 91 GeV and 161–172 GeV. F2γ is observed to increase with Q2 with a slope of α−1dF2γdlnQ2 = 0.10−0.03+0.05 measured in the range 0.1 < x < 0.6.

Using the OPAL data accumulated in 1991–1993 amounting to 74 pb−1 of integrated luminosity, corresponding to 1.64×106 selected multi-hadronic events, a search has been performed for charged particles with unusual mass or unusual charge. The mass was determined from a combination of momentum and ionization energy loss measurements. No isolation criteria were applied to the tracks examined, so that both isolated particles and particles produced in jets were valid candidates. For particles with chargeQ/e=−1, one candidate with a mass of approximately 4.2 GeV/c2 was found, which is compatible with the background rate expected according to a Monte Carlo simulation. The implications of this search for the mass limits of a conjectured stable or quasi-stable charged gluino composite $$(\tilde gq\bar q')^ \pm $$ are discussed. Limits are also presented for the production of fractionally-charged particles withQ/e=±2/3 and ±4/3 as well as for particles withQ/e=±2.

Three jet events are selected from hadronic Z0 decays with a symmetry such that the two lower energy jets are produced with the same energy and in the same jet environment. In some of the events, a displaced secondary vertex is reconstructed in one of the two lower energy jets, which permits the other lower energy jet to be identified as a gluon jet, with an estimated purity of about 93%. Comparing these gluon jets to the inclusive sample of lower energy jets from the symmetric data set yields direct, model independent measurements of quark and gluon jet properties, which have essentially no bias except from the jet definition. Results are reported using both thek ⊥ and cone jet definitions. For thek ⊥ algorithm, we find $$\frac{{\left\langle {n_{k_ \bot }^{ch.} } \right\rangle gluon}}{{\left\langle {n_{k_ \bot }^{ch.} } \right\rangle quark}} = 1.25 \pm 0.02(stat.) \pm 0.03(syst.)$$ for the ratio of the mean charged particle multiplicity of gluon to quark jets, while for the cone algorithm, we find $$\frac{{\left\langle {n_{cone}^{ch.} } \right\rangle gluon}}{{\left\langle {n_{cone}^{ch.} } \right\rangle quark}} = 1.10 \pm 0.02(stat.) \pm 0.02(syst.)$$ using a cone size of 30°. We also report measurements of the angular distributions of particle energy and multiplicity around the jet directions, and of the fragmentation functions of the jets. Gluon jets are found to be substantially broader and to have a markedly softer fragmentation function than quark jets, in agreement with our earlier observations.

This paper presents updated measurements of the lifetimes of the Bs0 meson and the Λb0 baryon using 4.4 million hadronic Z0 decays recorded by the OPAL detector at LEP from 1990 to 1995. A sample of Bs0 decays is obtained using Ds−ℓ+ combinations, where the Ds− is fully reconstructed in the φπ−, K∗0K− and K−K0S decay channels and partially reconstructed in the φℓ−ν̄X decay mode. A sample of Λb0 decays is obtained using Λc+ℓ− combinations, where the Λc+ is fully reconstructed in its decay to a pK−π+ final state and partially reconstructed in the Λℓ+νX decay channel. From 172±28 Ds−ℓ+ combinations attributed to Bs0 decays, the measured lifetime is τ(Bs0)=1.50+0.16−0.15±0.04ps, where the errors are statistical and systematic, respectively. From the 129±25 Λc+ℓ− combinations attributed to Λb0 decays, the measured lifetime is τ(Λb0)=1.29+0.24−0.22±0.06ps, where the errors are statistical and systematic, respectively.

Helicity density matrix elements for inclusive K∗(892)0 mesons from hadronic Z0 decays have been measured over the full range of K∗ 0 momentum using data taken with the OPAL experiment at LEP. A preference for occupation of the helicity zero state is observed at all scaled momentum xp values above 0.3, with the matrix element ϱ00 rising to 0.66 ± 0.11 for xp > 0.7. The values of the real part of the off-diagonal element ϱ1 - 1 are negative at large xp, with a weighted average value of −0.09 ± 0.03 for xp > 0.3, in agreement with new theoretical predictions based on Standard Model parameters and coherent fragmentation of the qq system from the Z0 decay. All other helicity density matrix elements measured are consistent with zero over the entire xp range. The K∗ 0 fragmentation function has also been measured and the total rate determined to be 0.74 ± 0.02 ± 0.02 K∗(892)0 mesons per hadronic Z0 decay.

A factorial moment analysis has been performed on the differential multiplicity distributions of hadronic final states of the Z0 recorded with the OPAL detector at LEP. The moments of the one-dimensional rapidity and the two-dimensional rapidity versus azimuthal angle distributions are found to exhibit “intermittent” behaviour attributable to the jet structure of the events. The moments are reproduced by both parton shower and matrix element QCD based hadronisation models. No evidence for fluctuations beyond those attributable to jet structure is observed.

A search for decays of the Bc meson was performed using data collected from 1990–1995 with the OPAL detector on or near the Z0 peak at LEP. The decay channels Bc+→J/ψπ+, Bc+→J/ψa1+ and Bc+→J/ψℓ+ν were investigated, where ℓ denotes an electron or a muon. Two candidates are observed in the mode Bc+→J/ψπ+, with an estimated background of (0.63±0.20) events. The weighted mean of the masses of the two candidates is (6.32±0.06) GeV/c2, which is consistent with the predicted mass of the Bc meson. One candidate event is observed in the mode Bc+→J/ψℓ+ν, with an estimated background of (0.82±0.19) events. No candidate events are observed in the Bc+→J/ψa1+ decay mode, with an estimated background of (1.10±0.22) events. Upper bounds at the 90% confidence level are set on the production rates for these processes.

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.

Results are presented from a search for a new charged heavy lepton in e+e− annihilation. The data were taken with the OPAL detector at LEP during a scan of the Z0 resonance. Two independent search techniques were used, one looking for events with large missing energy and missing momentum transverse to the beam, and the other for events with isolated energetic leptons. Two candidate events, consistent with expected background, were found in the first search; none was found in the second. These results allow the exclusion at the 95% confidence level of a charged heavy lepton of mass less than 44.3 GeV/c2 if it is assumed to have a massless neutrino partner. Limits are also presented for the case of a massive neutrino.

In this paper an investigation of the production of D∗± mesons produced in e+e− collisions at energies around the Z0 pole is presented. Based on 115D∗± mesons with xD∗ 2ED∗/Ecm> 0.2 the properties of D∗ mesons produced in the reaction Z0→ cc are studied. Fixing the yield and the fragmentation function of bottom quarks to the values obtained at LEP using lepton tags, and average energy fraction of the D∗± mesons from primary charmed quarks of 〈xc→D∗〉 = 0.52 ± 0.03 +- 0.01 is found and Γz0→cc= (323 ± 61 ± 35)MeV is determined. The first error is the combined statistical and systematic error from this experiment, and the second the total error from other sources.

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.

A search has been performed for the production of neutralinos (χ, χ′) in e+e− annihilation at energies near the Z0 pole. No evidence for these particles was found either in searches for events with two acoplanar jets, low visible energy, and missing pt (sensitive to Z0→χχ′→χχff) or in searches for single-photon events (sensitive to Z0→χχ′→χχγ). Model independent upper limits (at the 95% CL) on the branching ratio for the decay mode Z0 →χχ′ of a few 10−4 are obtained for most of the range of neutralino masses that is kinematically accessible at LEP energies. Upper limits on the mixing factor of neutralinos are also placed as a function of the neutralino masses.

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.

Data are presented for the exclusive reaction pp → pp π+ π− at $$\sqrt s = 62GeV$$ with two leading protons at large Feynman-x and a centrally produced π+;π− system. In this kinematical configuration one expects a substantial contribution from Double Pomeron Exchange, which is a potential source of glueballs. The experiment was performed at the CERN ISR using the Split Field Magnet spectrometer. In the mass range between 1,000 and 1,700 MeV/c2 the invariant mass distribution for the central π+;π− system exhibits a very significant signal for thef 0(1270) and no other obvious resonant states.

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 scalar particles and neutralinos such as those predicted by supersymmetric models has been performed using a data sample of 4.4 million hadronic Z0 decays recorded by the OPAL detector at the e+e− collider LEP. The production of such particles typically leads to event topologies consisting of an acoplanar pair of jets, or of a mono-jet, accompanied by sizeable missing energy owing to neutrinos and other undetectable neutral particles. Limits are obtained, at the 95% confidence level, on the masses and production rates of scalar particles produced in association with the Z0. Limits are also placed on neutralino production, for which an additional possible signature is also studied, events containing a single observed photon. Within the Minimal Supersymmetric Standard Model, the mass of the lightest neutralino is found to be larger than 12.5 GeV/c2 at 95% C.L., provided that tan β is larger than 1.5.

A new measurement of the τ lifetime is presented. It uses data collected with the Opal detector during 1994, which almost doubles the size of the Opal τ sample. Two statistically independent techniques are used: an impact parameter analysis of one-prong decay tracks and a fit to the decay length distribution of three-prong decays. The lifetime obtained from the 1994 data by combining the results of these methods is ττ = 289.7 ± 2.5 (stat)± 1.5 (sys) fs. When combined with the previous Opal τ lifetime measurement the improved τ lifetime is ττ = 289.2 ± 1.7 (stat.) ± 1.2 (sys.) fs.

The results of a study of charm meson production in semileptonic B hadron decays are presented. Based on a sample of 1.72 million hadronic Z0 decays the following product branching ratios (averaged over electrons and muons) are obtained: $$\begin{gathered} B(b \to D^ + \ell X) \cdot B(D^ + \to K^ - \pi ^ + \pi ^ + ) \hfill \\ = (1.82 \pm 0.20 \pm 0.12) \times 10^{ - 3} , \hfill \\ B(b \to D^0 \ell X) \cdot B(D^0 \to K^ - \pi ^ + ) \hfill \\ = (2.52 \pm 0.14 \pm 0.17) \times 10^{ - 3} \hfill \\ B(b \to D^* \ell X) \cdot B(D^{* + } \to D^0 \pi ^ + ) \cdot B(D^0 \to K^ - \pi ^ + ) \hfill \\ = (7.53 \pm 0.47 \pm 0.56) \times 10^{ - 4} \hfill \\ \end{gathered} $$ where the first error is statistical and the second systematic. The results of a search for the narrow P-wave charmed mesons, (D J ), in semileptonic B decays are also reported. Using the decay channels D 0 →D+π−, D 0 →D*+π− and D + →D0π+ we observe a total signal of 44±8 −7 +3 D 0 and 48±10 −6 +3 D + events. This is the first evidence for semileptonic B decays to charged P-wave charm states. The observed signals also provide evidence for the production of both pseudovector and tensor P-wave charmed mesons in semileptonic B decay. Together these modes are estimated to make up 34±7% of charmed semileptonic B0 and B+ decays.

Measurements of helicity density matrix elements have been made for the φ(1020), D*± and B* vector mesons in multihadronic Z0 decays in the OPAL experiment at LEP. Results for inclusive φ produced with high energy show evidence for production preferentially in the helicity zero state, with ρ00 = 0.54 ± 0.08, compared to the value of 1/3 expected for no spin alignment. The corresponding element for the D*± has a value of 0.40 ± 0.02, also suggesting a deviation from 1/3. The B* result, with ρ00 = 0.36 ± 0.09, is consistent with no spin alignment. Off-diagonal elements have been measured for the f and D* mesons; for the D* the element Re ρ1−1 is non-zero, indicating non-independent fragmentation of the primary quarks.

We report a study of forward-backward multiplicity correlations and a measurement of the dependence on charged multiplicity of the mean transverse momentum of charged hadrons, measured with respect to the thrust axis. The study was performed on a high statistics sample of Z0 decays to multihadronic final states collected by the OPAL Collaboration at LEP. The positive forward-backward multiplicity correlation observed in our inclusive sample can be understood in terms of a superposition of distinct event topologies characterized by a different amount of hard gluon radiation (2-, 3- and 4-jet events) and with different mean multiplicities. The residual positive correlation that we see in a clean 2-jet sample can be interpreted in terms of fragmentation properties of different quark flavours and of the production and decay of resonances. We have compared the observed effects with the predictions of QCD-based parton shower models. The data are well described by the Jetset 7.3 Monte Carlo, while Herwig 5.5 does not satisfactorily reproduce the measured correlations. Hard gluon radiation is also shown to be responsible for the observed increase of about 40% in the mean transverse momentum of produced charged hadrons in the multiplicity range from 10 to 30. The comparison with the results obtained in an analysis of a sample enriched in Z0 → bb̄ events, shows that the presence of heavy flavours does not contribute significantly to the observed effect.

The fragmentation function for the process e+e−→h+X, whereh represents a hadron, may be decomposed into transverse, longitudinal and asymmetric contributions by analysis of the distribution of polar production angles. A number of new tests of QCD have been proposed using these fragmentation functions, but so far no data have been published on the separate components. We have performed such a separation using data on charged particles from hadronic Z0 decays atOpal, and have compared the results with the predictions of QCD. By integrating the fragmentation functions, we determine the average charged particle multiplicity to be $$\overline {n_{ch} } = 21.05 \pm 0.20$$ . The longitudinal to total cross-section ratio is determined to be σ L /σ tot =0.057±0.005. From the longitudinal fragmentation function we are able to extract the gluon fragmentation function. The connection between the asymmetry fragmentation function and electroweak asymmetrics is discussed.

A search for events with photon pairs of large invariant mass is described based on a data sample of 43 pb−1 collected with the OPAL detector. This search is motivated by the L3 observation of four events of the type e+e− → l+l−γγ with an invariant mass of the two photons, mγγ, clustering around 60 GeV. In the OPAL data, four ℓ+ℓ−γγ events are found with mγγ above 40 GeV. The number of observed events is consistent with the QED expectation and no obvious resonance structure is seen. Three candidate qq̄γγ events with mγγ above 40 GeV are found and no νν&#x0304;nγγ candidate with mγγ > 5 GeV is found. From these searches an upper limit is set on any anomalous production of ℓ+ℓ−γγ (ℓ = e, μ, τ) events, where the ℓ+ℓ− pair comes from a virtual Z0 and mγγ is near 60 GeV , of B(Z0 → Z∗0γγ → ℓ+ℓ−γγ) < 6.0 × 10−7 at the 95% CL. In a similar search in the reaction e+e− → γγγ, seven events are observed with a photon pairing of mass near 60 GeV compared to an expected background of 2.7 ± 0.4 from QED. In a search for the production of a 60 GeV resonance X in two-photon collisions, an upper limit is set on ΓXB2(X → γγ) of 2.6 MeV at the 95% CL. A search is also performed for ℓ+ℓ−X (ℓ = e, μ) events in non-γγ decay modes of X. No indication of a 60 GeV resonance is seen. Combining these two searches, ΓX values greater than 110 MeV are excluded at the 95% CL for ℓ+ℓ−γγ (ℓ = e, μ) cross-sections greater than 0.05 pb.