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  • 1
    Electronic Resource
    Electronic Resource
    QCD studies with ${\rm e^+e^-}$ annihilation data at 172-189 GeV (2000)
    Springer
    The European physical journal 16 (2000), S. 185-210 
    Details
    ISSN: 1434-6052
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. We have studied hadronic events from $\mathrm{e^+e^-}$ annihilation data at centre-of-mass energies of $\sqrt{s}=$ 172, 183 and 189 GeV. The total integrated luminosity of the three samples, measured with the OPAL detector, corresponds to 250 pb $^{-1}$ . We present distributions of event shape variables, charged particle multiplicity and momentum, measured separately in the three data samples. From these we extract measurements of the strong coupling $\alpha_s $ , the mean charged particle multiplicity $\langle n_{\mathrm{ch}}\rangle$ and the peak position $\xi_0$ in the $\xi_p=\ln(1/x_p)$ distribution. In general the data are described well by analytic QCD calculations and Monte Carlo models. Our measured values of $\alpha_s $ , $\langle n_{\mathrm{ch}}\rangle$ and $\xi_0$ are consistent with previous determinations at $\sqrt{s}=M_{\mathrm{Z^0}}$ .
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s100520050015
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  • 2
    Electronic Resource
    Electronic Resource
    Transverse and longitudinal Bose-Einstein correlations in hadronic Z $^0$ decays (2000)
    Springer
    The European physical journal 16 (2000), S. 423-433 
    Details
    ISSN: 1434-6052
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. Bose-Einstein correlations in pairs of identical charged pions produced in a sample of 4.3 million Z $^0$ hadronic decays are studied as a function of the three components of the momentum difference, transverse (“out” and “side”) and longitudinal with respect to the thrust direction of the event. A significant difference between the transverse, r $_{t_{side}}$ , and longitudinal, r $_l$ , dimensions is observed, indicating that the emitting source of identical pions, as observed in the Longitudinally CoMoving System, has an elongated shape. This is observed with a variety of selection techniques. Specifically, the values of the parameters obtained by fitting the extended Goldhaber parametrisation to the correlation function ${\mathrm C'} ={\mathrm C^{DATA}}/{\mathrm C^{MC}}$ for two-jet events, selected with the Durham algorithm and resolution parameter y $_{cut}$ = 0.04, are r $_{t_{side}}$ = (0.809 $\pm$ 0.009 (stat) $^{+0.019}_{-0.032}$ (syst)) fm, r $_l$ = (0.989 $\pm$ 0.011 (stat) $^{+0.030}_{-0.015}$ ({\it syst})) fm and r $_l$ /r $_{t_{side}}$ = 1.222 $\pm$ 0.027 (stat) $^{+0.075}_{-0.012}$ (syst). The results are discussed in the context of a recent model of Bose-Einstein correlations based on string fragmentation. The results of a unidimensional analysis are also presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s100520000407
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  • 3
    Electronic Resource
    Electronic Resource
    Search for trilinear neutral gauge boson couplings in Z $\gamma$ production at $\sqrt{s}=189$ GeV at LEP (2000)
    Springer
    The European physical journal 17 (2000), S. 553-566 
    Details
    ISSN: 1434-6052
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. The data recorded at a centre-of-mass energy of 189 \GeV by the OPAL detector at LEP are used to search for trilinear couplings of the neutral gauge bosons in the process ${\rm e}^+{\rm e}^-\rightarrow{\rm Z\gamma}$ . The cross-sections for multihadronic events with an energetic isolated photon, and for events with a high energy photon accompanied by missing energy are measured. These cross-sections and the photon energy, polar angle and isolation angle distributions are compared to the Standard Model predictions and to the theoretical expectations of a model allowing for ${\rm Z\gamma Z}$ and ${\rm Z\gamma\gamma}$ vertices. Since no significant deviations with respect to the Standard Model expectations are found, constraints are derived on the strength of neutral trilinear gauge couplings.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s100520000490
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  • 4
    Electronic Resource
    Electronic Resource
    Photonic events with missing energy in $\mathrme^+\mathrme^-$ collisions at $\sqrt{s}$ = 189 GeV (2000)
    Springer
    The European physical journal 18 (2000), S. 253-272 
    Details
    ISSN: 1434-6052
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s100520000522
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  • 5
    Electronic Resource
    Electronic Resource
    Production rates of $\mathrm{b\overline{b}}$ quark pairs from gluons and $\mathrm{b\overline{b}b\overline{b}}$ events in hadronic $\mathrm{Z}^0$ decays (2001)
    Springer
    The European physical journal 18 (2001), S. 447-460 
    Details
    ISSN: 1434-6052
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. 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*}
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s100520100580
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  • 6
    Electronic Resource
    Electronic Resource
    A study of B $_s^0$ meson oscillation using D $_{\rm s}^-$ -lepton correlations (2001)
    Springer
    The European physical journal 19 (2001), S. 241-256 
    Details
    ISSN: 1434-6052
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s100520100591
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  • 7
    Electronic Resource
    Electronic Resource
    Tests of the Standard Model and constraints on new physics from measurements of fermion-pair production at 183 GeV at LEP (1999)
    Springer
    The European physical journal 6 (1999), S. 1-18 
    Details
    ISSN: 1434-6052
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. Cross-sections for hadronic, $\rm b\bar{b}$ and lepton pair final states in $\rm e^+e^-$ collisions at $\sqrt{s}$ =183 GeV, measured with the OPAL detector at LEP, are presented and compared with the predictions of the Standard Model. Forward-backward asymmetries for the leptonic final states have also been measured. Cross-sections and asymmetries are also presented for data recorded in 1997 at $\sqrt{s}$ =130 and 136 GeV. The results are used to measure the energy dependence of the electromagnetic coupling constant $\alpha_{\rm em}$ , and to place limits on new physics as described by four-fermion contact interactions or by the exchange of a new heavy particle such as a leptoquark, or of a squark or sneutrino in supersymmetric theories with R-parity violation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s100529801027
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  • 8
    Electronic Resource
    Electronic Resource
    Search for chargino and neutralino production at $\sqrt{s} = 181$ –184 GeV at LEP (1999)
    Springer
    The European physical journal 8 (1999), S. 255-272 
    Details
    ISSN: 1434-6052
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. A search for charginos and neutralinos, predicted by supersymmetric theories, has been performed using a data sample of 57 pb $^{-1}$ at centre-of-mass energies of 181–184 GeV taken with the OPAL detector at LEP. No evidence for chargino or neutralino production has been found. Upper limits on chargino and neutralino pair production ( $\tilde\chi^+_1 \tilde\chi^-_1$ , $\tilde\chi^0_1 \tilde\chi^0_2$ ) cross-sections are obtained as a function of the chargino mass ( $m_{\tilde\chi^\pm_1}$ ), the lightest neutralino mass ( $m_{\tilde\chi^0_1}$ ) and the second lightest neutralino mass ( $m_{\tilde\chi^0_2}$ ). For large chargino masses the limits have been improved with respect to the previous analyses at lower centre-of-mass energies. Exclusion regions at 95% confidence level (C.L.) of parameters of the Constrained Minimal Supersymmetric Standard Model are determined for the case of a large universal scalar mass, $m_0$ , implying heavy scalar fermions, and for the case of a small $m_0$ resulting in light scalar fermions and giving the worst-case limits. Within this framework and for $m_{\tilde\chi^\pm_1} - m_{\tilde\chi^0_1} \geq 5$ GeV the 95% C.L. lower limits on $m_{\tilde\chi^\pm_1}$ for $m_0=500$ GeV are 90.0 and 90.2 GeV for $\tan \beta = 1.5$ and 35 respectively. These limits for all $m_0$ (the worst-case) are 69.1 and 65.2 GeV for $\tan \beta = 1.5$ and 35 respectively. Exclusion regions are also presented for neutralino masses, including an absolute lower limit at 95% C.L. for the mass of the lightest neutralino of 30.1 GeV for $m_0 = 500$ GeV (24.2 GeV for all $m_0$ ), with implications for experimental searches for the lightest neutralino as a dark matter candidate.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s100529901059
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  • 9
    Electronic Resource
    Electronic Resource
    A measurement of $R_{\rm b}$ using a double tagging method (1999)
    Springer
    The European physical journal 8 (1999), S. 217-239 
    Details
    ISSN: 1434-6052
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. The fraction of ${\rm Z^0 \to b\overline b}$ events in hadronic ${\rm Z^0}$ decays has been measured by the OPAL experiment using the data collected at LEP between 1992 and 1995. The ${\rm Z^0 \to b\overline b}$ decays were tagged using displaced secondary vertices, and high momentum electrons and muons. Systematic uncertainties were reduced by measuring the b-tagging efficiency using a double tagging technique. Efficiency correlations between opposite hemispheres of an event are small, and are well understood through comparisons between real and simulated data samples. A value of \[ R_{\rm b} \equiv \frac{\sigma (\rm e^+e^- \rightarrow{\rm b\overline b} )} {\sigma (\rm e^+e^- \rightarrow hadrons)} = 0.2178 \pm 0.0011 \pm 0.0013 \] was obtained, where the first error is statistical and the second systematic. The uncertainty on $R_{\rm c}$ , the fraction of ${\rm Z^0 \to c \overline c}$ events in hadronic ${\rm Z^0}$ decays, is not included in the errors. The dependence on $R_{\rm c}$ is \[ \frac{\Delta R_{\rm b}}{R_{\rm b}} = -0.056 \frac{\Delta R_{\rm c}}{R_{\rm c}} , \] where $\Delta R_{\rm c}$ is the deviation of $R_{\rm c}$ from the value 0.172 predicted by the Standard Model. The result for $R_{\rm b}$ agrees with the value of $0.2155 \pm 0.0003$ predicted by the Standard Model.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s100529901087
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  • 10
    Electronic Resource
    Electronic Resource
    $\mathrm{W^+W^-}$ production and triple gauge boson couplings at LEP energies up to 183 GeV (1999)
    Springer
    The European physical journal 8 (1999), S. 191-215 
    Details
    ISSN: 1434-6052
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. A study of W-pair production in ${\rm e^+e^-}$ annihilations at LEP is presented, based on $877\,{\rm W^+W^-}$ candidates corresponding to an integrated luminosity of 57 pb $^{-1}$ at $\sqrt s=183$ GeV. Assuming that the angular distributions of the W-pair production and decay, as well as their branching fractions, are described by the Standard Model, the W-pair production cross-section is measured to be $15.43\pm 0.61 ({\rm stat.}) \pm 0.26 ({\rm syst.})$ pb. Assuming lepton universality and combining with our results from lower centre-of-mass energies, the W branching fraction to hadrons is determined to be $67.9 \pm 1.2 ({\rm stat.})\pm 0.5 ({\rm syst.})$ %. The number of W-pair candidates and the angular distributions for each final state ( ${\rm q \bar q} \bar \ell \nu_\ell$ , ${\rm q \bar q q \bar q}, \bar \ell \nu_\ell \ell^\prime \bar \nu_{\ell^\prime })$ are used to determine the triple gauge boson couplings. After combining these values with our results from lower centre-of-mass energies we obtain $\Delta \kappa_\gamma = 0.11^{+0.52}_{-0.37}$ , $\Delta g^{\rm z}_1 = 0.01^{+ 0.13}_{-0.12}$ and $\lambda = -0.10^{+0.13}_{-0.12}$ , where the errors include both statistical and systematic uncertainties and each coupling is determined by setting the other two couplings to the Standard Model value. The fraction of W bosons produced with a longitudinal polarisation is measured to be $0.242 \pm 0.091({\rm stat.}) \pm 0.023 ({\rm syst.})$ . All these measurements are consistent with the Standard Model expectations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s100529901106
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