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  • 1995-1999  (67)
  • 1975-1979
  • 1998  (67)
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  • 1995-1999  (67)
  • 1975-1979
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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 5 (1998), S. 1-17 
    ISSN: 1434-6052
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. In ${\rm e}^+{\rm e}^-$ collisions at centre-of-mass energies around 91 GeV, ${\rm D}^{*0}$ mesons have been reconstructed using data collected with the OPAL detector at LEP. The hadronisation fraction has been measured to be \[ \begin{array}{r@{ = }l} f({\rm c}\to{\rm D}^{*0}) & 0.218 \pm 0.054 \pm 0.045 \pm 0.007 \,, \end{array} \] where the errors correspond to the statistical and systematic errors specific to this analysis, and to systematic uncertainties from externally measured branching fractions, respectively. Together with previous OPAL measurements of the hadronisation fractions of other charmed mesons, this value is used to investigate the relative production of observed vector and pseudoscalar charmed mesons in ${\rm Z}^0\to{\rm c\bar c}$ decays. The production ratio is determined to be \[ P^{\rm eff}_V = V/(V\!+\!P) = 0.57 \pm 0.05\; . . \] The relative primary production of vector and pseudoscalar mesons, $P^{\rm prim}_V$ , is studied in the context of the production and decay of orbitally excited charmed resonances. The first measurement of the inclusive ${\rm D}^{*+}_{\rm s}$ production rate in hadronic ${\rm Z}^0$ decays is presented.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 5 (1998), S. 19-40 
    ISSN: 1434-6052
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. A search is described for the neutral Higgs bosons ${\rm h}^0$ and ${\rm A}^0$ predicted by models with two scalar field doublets and, in particular, the Minimal Supersymmetric Standard Model (MSSM). The search in the ${\rm Z^0 h^0}$ and ${\rm h^0A^0}$ production channels is based on data corresponding to an integrated luminosity of 25 pb $^{-1}$ from ${\rm e^+e^-}$ collisions at centre-of-mass energies between 130 and 172 GeV collected with the OPAL detector at LEP. The observation of a number of candidates consistent with Standard Model background expectations is used in combination with earlier results from data collected at the ${\rm Z}^0$ resonance to set limits on $m_{\rm h}$ and $m_{\rm A}$ in general models with two scalar field doublets and in the MSSM. For example, in the MSSM, for $\tan \beta b〉1$ , minimal and maximal scalar top quark mixing and soft SUSY-breaking masses of 1 TeV, the 95% confidence level limits $m_{\rm h}〉59.0$ GeV and $m_{\rm A}〉59.5$ GeV are obtained. For the first time, the MSSM parameter space is explored in a detailed scan.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 5 (1998), S. 41-56 
    ISSN: 1434-6052
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. Differential dijet cross sections have been measured with the ZEUS detector for photoproduction events in which the hadronic final state containing the jets is separated with respect to the outgoing proton direction by a large rapidity gap. The cross section has been measured as a function of the fraction of the photon ( $x^{OBS}_\gamma $ ) and pomeron ( $\beta^{OBS}$ ) momentum participating in the production of the dijet system. The observed $x^{OBS}_\gamma $ dependence shows evidence for the presence of a resolved- as well as a direct-photon component. The measured cross section $d\sigma d\beta^{OBS}$ increases as $\beta^{OBS}$ increases indicating that there is a sizeable contribution to dijet production from those events in which a large fraction of the pomeron momentum participates in the hard scattering. These cross sections and the ZEUS measurements of the diffractive structure function can be described by calculations based on parton densities in the pomeron which evolve according to the QCD evolution equations and include a substantial hard momentum component of gluons in the pomeron.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 5 (1998), S. 195-203 
    ISSN: 1434-6052
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. Time-dependent ${{\rm B^{0}}\mbox{-}\bar{{\rm B}}^{0}}$ mixing is studied using about two million hadronic Z decays registered by L3 in 1994 and 1995. For this study three techniques are used. Tagging of the b-quark charge at decay time is performed by identifying leptons from semileptonic B decays. The flavour of the b quark at production time is determined from the charge of the lepton in the opposite hemisphere or by using a jet-charge technique. The proper time of the B-particle decay is obtained by reconstructing the production and decay vertices or by a measurement of the lepton impact parameter. The combined result for the frequency of ${{\rm B_d^0}}$ meson oscillations is \[ \Delta m_d = 0.444 \pm 0.040 {\rm ps}^{-1}. \]
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 5 (1998), S. 255-267 
    ISSN: 1434-6052
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. New results on transverse mass spectra of neutral pions measured at central rapidity are presented for impact parameter selected 200 $A\cdot$ GeV S + S and S + Au collisions. The distributions cover more than 8 orders of magnitude in cross section over the range $0.3 \mathrm{GeV}/c \le p_{T} \le 4.0 \mathrm{GeV}/c$ . Detailed comparisons to results from pp collisions are made. The spectra from all systems show a clear power-law like shape with similar curvature. Collisions of S + Au exhibit a larger mean transverse momentum than pp increasing with centrality. Predictions of string models and by hydrodynamic approaches including collective expansion and decays of short lived resonances are compared to the data and the implications are discussed.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 5 (1998), S. 239-248 
    ISSN: 1434-6052
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. Bose-Einstein Correlations (BEC) of three identical charged pions were studied in $4 \times 10^6$ hadronic ${\rm Z}^0$ decays recorded with the OPAL detector at LEP. The genuine three-pion correlations, corrected for the Coulomb effect, were separated from the known two-pion correlations by a new subtraction procedure. A significant genuine three-pion BEC enhancement near threshold was observed having an emitter source radius of $r_3 = 0.580 \pm 0.004 \mbox{ (stat.)} \pm 0.029 \mbox{ (syst.)} \mbox{ fm}$ and a strength of $\lambda_3 = 0.504 \pm 0.010 \mbox{ (stat.)}\pm 0.041 \mbox{ (syst.)}$ . The Coulomb correction was found to increase the $\lambda_3$ value by $\sim $ 9% and to reduce $r_3$ by $\sim $ 6%. The measured $\lambda_3$ corresponds to a value of $0.707 \pm 0.014 \mbox{ (stat.)} \pm 0.078 \mbox{ (syst.)}$ when one takes into account the three-pion sample purity. A relation between the two-pion and the three-pion source parameters is discussed.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 5 (1998), S. 575-584 
    ISSN: 1434-6052
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. A search for events with an imbalance in transverse momentum and with isolated high energy leptons has been carried out at the positron-proton collider HERA. One event with an $e^-$ and five events with a $\mu^{\pm}$ are found together with evidence for undetected particles carrying transverse momentum. Within the Standard Model the dominant origin of events with this kind of topology is the production of W bosons with subsequent leptonic decay. Three of the six events are within measurement errors found in a region of phase space likely to be populated by this process, while the remaining events show kinematic properties which are atypical of all Standard Model processes considered.
    Type of Medium: Electronic Resource
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  • 8
    ISSN: 1434-6052
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. The DELPHI experiment at LEP uses Ring Imaging Cherenkov detectors for particle identification. The good understanding of the RICH detectors allows the identification of charged pions, kaons and protons, covering the full momentum range from $0.7 〈 p 〈 45.6$ GeV/c. The $\pi^{\pm}$ , ${\rm K}^{\pm}$ , p and $\overline{\rm p}$ normalised production rates, their differential cross sections, multiplicities and the maxima $\xi_{p}^{*}$ of the $\xi_{p}=\ln(1/X_{p})$ distributions are measured for three event samples ${\rm Z}^0 \rightarrow q\overline{q}$ , ${\rm Z}^0 \rightarrow b\overline{b}$ , ${\rm Z}^0 \rightarrow u\overlineu$ , $d\overline{d}$ , $s\overline{s}$ , selected from $\sim 1\,400\,000\,\rm Z^{0}$ decays collected by DELPHI in 1994. The results are compared to the predictions of the JETSET string fragmentation model and the HERWIG cluster fragmentation model. The Modified Leading Logarithm Approximation with Local Parton-Hadron Duality is tested. The $\xi_{p}^{*}$ dependence on the primary quark flavour is investigated and quantified for the different particle distributions. The $\pi^{\pm}$ , ${\rm K}^{\pm}$ , p and $\overline{\rm p}$ multiplicities are measured with precisions from $\pm4$ % to $\pm6$ %. For the ${\rm Z}^0 \rightarrow q\overline{q}$ , and ${\rm Z}^0 \rightarrow b\overline{b}$ , event samples, these improve on previous measurements. The $\pi^{\pm}$ , ${\rm K}^{\pm}$ , p and $\overline{\rm p}$ multiplicities for ${\rm Z}^0 \rightarrow u\overlineu$ , $d\overline{d}$ , $s\overline{s}$ are presented for the first time.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 5 (1998), S. 625-639 
    ISSN: 1434-6052
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. Events with a (2+1) jet topology in deep–inelastic scattering at HERA are studied in the kinematic range 200 $〈$ Q $^2〈$ 10 000 GeV $^2$ . The rate of (2+1) jet events has been determined with the modified JADE jet algorithm as a function of the jet resolution parameter and is compared with the predictions of Monte Carlo models. In addition, the event rate is corrected for both hadronization and detector effects and is compared with next–to–leading order QCD calculations. A value of the strong coupling constant of $\alpha_s(M_Z^2)=0.118 \pm 0.002\,(stat.)\,^{+0.007}_{-0.008}\, (syst.)\,^{+0.007}_{-0.006}\,(theory)$ is extracted. The systematic error includes uncertainties in the calorimeter energy calibration, in the description of the data by current Monte Carlo models, and in the knowledge of the parton densities. The theoretical error is dominated by the renormalization scale ambiguity.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 2 (1998), S. 407-416 
    ISSN: 1434-6052
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. We report a measurement of the $\mathrm{B_s^0}$ meson lifetime from $\mathrm{B_s^0} \to \mathrm{ D_s^-} \rm{X}$ decays, where $\mathrm{ D_s^-}$ mesons are reconstructed in the $\mathrm{ D_s^-} \to \mathrm{\phi\pi^-}$ and $\mathrm{ D_s^-}\to\mathrm{K^{*0}K^-}$ decay channels. From approximately 3.7 million hadronic $\mathrm{Z}^0$ decays recorded by the OPAL detector at LEP a sample is selected containing 911 $\pm$ 83 candidates, of which $519 \pm 136$ are estimated to be from $\mathrm{B_s^0}$ meson decays. Fitting the distribution of the distance from the beam spot to the decay vertex of the $\mathrm{ D_s^-}$ candidates with an unbinned likelihood function we measure \[ \tau ({\rm B^0_{\rm s}} = 1.72^{+0.20+0.18}_{-0.19-0.17}\,{\rm ps}\; , \] where the errors are statistical and systematic, respectively.
    Type of Medium: Electronic Resource
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