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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 17 (2000), S. 19-51 
    ISSN: 1434-6052
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. We employ data taken by the JADE and OPAL experiments for an integrated QCD study in hadronic e $^+$ e $^-$ annihilations at c.m.s. energies ranging from 35 GeV through 189 GeV. The study is based on jet-multiplicity related observables. The observables are obtained to high jet resolution scales with the JADE, Durham, Cambridge and cone jet finders, and compared with the predictions of various QCD and Monte Carlo models. The strong coupling strength, $\alpha_s$ , is determined at each energy by fits of ${\cal O}(\alpha_s^2)$ calculations, as well as matched ${\cal O}(\alpha_s^2)$ and NLLA predictions, to the data. Matching schemes are compared, and the dependence of the results on the choice of the renormalization scale is investigated. The combination of the results using matched predictions gives \[\alpha_s(M_{Z^0})=0.1187^{0.0034}_{0.0019}.\] The strong coupling is also obtained, at lower precision, from ${\cal O}(\alpha_s^2)$ fits of the c.m.s. energy evolution of some of the observables. A qualitative comparison is made between the data and a recent MLLA prediction for mean jet multiplicities.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Der Unfallchirurg 101 (1998), S. 1-1 
    ISSN: 1433-044X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Science Ltd
    ISSN: 1365-2389
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 1 (1998), S. 21-30 
    ISSN: 1434-6052
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. The process ${\rm e^+e^-}\to \gamma \gamma (\gamma)$ is studied using data recorded with the OPAL detector at LEP. The data sample corresponds to a total integrated luminosity of 25.38 pb $^{-1}$ taken at centre-of-mass energies of 130–172 GeV. The measured cross-sections agree well with the expectation from QED. In a combined fit using data from all centre-of-mass energies, the angular distribution is used to obtain improved limits on the cut-off parameters: $\Lambda_+ 〉 195$ GeV and $\Lambda_- 〉 210$ GeV (95% CL). In addition, limits on non-standard ${\rm e^+e^-}\gamma$ couplings and contact interactions, as well as a 95% CL mass limit for an excited electron, $M_{\mathrm{e^{\ast}}} 〉 194$ GeV for an ${\rm e^+e^-}\gamma$ coupling $\kappa = 1$ , are determined.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 1 (1998), S. 1-20 
    ISSN: 1434-6052
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. An update of the searches for charginos and neutralinos in DELPHI is presented, based mainly on recent data collected at centre-of-mass energies of 161 GeV and 172 GeV. No signal is found. For a sneutrino with mass above 300 GeV/ $c^2$ and a mass difference between the chargino and the lightest neutralino above 10 GeV/ $c^2$ , the lower limit at 95% confidence level on the chargino mass ranges from 84.3 GeV/ $c^2$ to the kinematical limit (86.0 GeV/ $c^2$ ), depending on the mixing parameters. The limit decreases for lower chargino-neutralino mass differences. The limit in the case of a light sneutrino is 67.6 GeV/ $c^2$ , provided that that there is no light sneutrino with a mass within 10 GeV $/c^2$ below the chargino mass. Upper limits on neutralino pair production cross-sections of about a picobarn are derived. The $(\mu,M_2)$ domain excluded in the MSSM-GUT scenario is determined by combining the neutralino and chargino searches. These results imply a limit on the mass of the lightest neutralino which, for a heavy sneutrino, is constrained to be above 24.9 GeV/ $c^2$ for tan $\beta \ge 1$ . The search has also been extended to the case where the lightest neutralino is unstable and decays into a photon and a gravitino.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 1 (1998), S. 31-43 
    ISSN: 1434-6052
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. A search for the resonant production of high mass photon pairs associated with a leptonic or hadronic system has been performed using a total data sample of 25.7 pb $^{-1}$ taken at centre-of-mass energies between 130 GeV and 172 GeV with the OPAL detector at LEP. The observed number of events is consistent with the expected number from Standard Model processes. The observed candidates are combined with search results from $\sqrt{s} \approx M_{\mathrm Z}$ to place limits on $B({\mathrm H}^{0} \to \gamma\gamma$ ) within the Standard Model for Higgs boson masses up to 77 GeV, and on the production cross section of any scalar resonance decaying into di-photons. Upper limits on $B({\mathrm H}^{0} \to \gamma\gamma)\times\sigma({\mathrm e}^+{\mathrm e}^-\to{\mathrm H}^{0}{\mathrm Z}^{0})$ of 290–830 fb are obtained over $40 〈 M_{\mathrm H} 〈 160$ GeV. Higgs scalars which couple only to gauge bosons with Standard Model strength are ruled out up to a mass of 76.5 GeV at the 95% confidence level.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 1 (1998), S. 45-64 
    ISSN: 1434-6052
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. We have searched for unstable neutral and charged heavy leptons, N and ${\rm L^\pm}$ , and for excited states of neutral and charged leptons, $\nu^*$ , ${\rm e}^*$ , $\mu^*$ and $\tau^*$ , in ${\rm e^+e^-}$ collisions at centre-of-mass energies of 170 and 172 GeV using the OPAL detector at LEP. No evidence for their existence was found. From the analysis of charged-current decays of pair-produced unstable heavy leptons, and of charged-current and photonic decays of pair-produced excited leptons, lower limits on their masses are derived. From the analysis of charged-current and photonic decays of singly-produced excited leptons, upper limits on the ratio of the coupling to the compositeness scale, $f/\Lambda$ , are determined for masses up to the kinematic limit.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 1 (1998), S. 65-79 
    ISSN: 1434-6052
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. Final states with charged kaons in three-prong $\tau$ decays are studied by exploiting the particle identification from the dE/dx measurement. The results are based on a sample of about $1.6\times 10^{5}$ detected $\tau$ pairs collected with the ALEPH detector between 1991 and 1995 around the Z peak. The following branching ratios have been measured: $B(\tau^{-} \rightarrow K^{-}K^{+}\pi^{-}\nu_{\tau}) = (1.63 \pm 0.21 \pm 0.17) \times 10^{-3}$ , $B(\tau^{-} \rightarrow K^{-}\pi^{+}\pi^{-}\nu_{\tau}) = (2.14 \pm 0.37 \pm 0.29) \times 10^{-3}$ , $B(\tau^{-} \rightarrow K^{-}K^{+}\pi^{-}\pi^{0} \nu_{\tau}) = (0.75\pm 0.29\pm 0.15)\times 10^{-3}$ , and $B(\tau^{-}\rightarrow K^{-}\pi^{+}\pi^{-}\pi^{0}\nu_{\tau}) =(0.61\pm 0.39\pm 0.18)\times 10^{-3}$ . The first two measurements are more precise than the current world averages, while the last two channels are investigated for the first time. The 95% C.L. upper limit on the branching ratio for the decay $\tau^{-}\rightarrow K^{-}K^{+}K^{-}\nu_{\tau}$ is $0.19 \times 10^{-3}$ . A study of intermediate states occurring in the $K^{-}K^{+}\pi^{-}\nu_{\tau}$ and $K^{-}\pi^{+}\pi^{-}\nu_{\tau}$ decays is also presented.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 1 (1998), S. 97-107 
    ISSN: 1434-6052
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. The double-differential inclusive di-jet cross section in photoproduction processes is measured with the H1 detector at HERA. The cross section is determined as a function of the average transverse jet energy & E_T^{\rm jets}& for ranges of the fractional energy & x_\gamma^{\rm jets}& of the parton from the photon side. An effective leading order parton distribution in the photon is determined at large parton fractional energies for scales between & 80 〈 p_T^2 〈 1250\,{\rm GeV}^2$ . The measurement is compatible with the logarithmic scale dependence that is predicted by perturbative QCD.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 1 (1998), S. 81-96 
    ISSN: 1434-6052
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. This paper presents the first analysis of diffractive photon dissociation events in deep inelastic positron-proton scattering at HERA in which the proton in the final state is detected and its momentum measured. The events are selected by requiring a scattered proton in the ZEUS leading proton spectrometer (LPS) with $x_L〉0.97$ , where $x_L$ is the fraction of the incoming proton beam momentum carried by the scattered proton. The use of the LPS significantly reduces the contamination from events with diffractive dissociation of the proton into low mass states and allows a direct measurement of $t$ , the square of the four-momentum exchanged at the proton vertex. The dependence of the cross section on $t$ is measured in the interval $0.073 〈 |t| 〈 0.4~{\rm GeV}^2$ and is found to be described by an exponential shape with the slope parameter $b=7.2 \pm 1.1({\rm stat.})^{+0.7}_{-0.9}({\rm syst.})\, {\rm GeV}^2$ . The diffractive structure function $F^{D(4)}_2$ is presented as a function of $x_{{\Bbb P}} \simeq 1-x_L$ and $\beta$ , the momentum fraction of the struck quark with respect to $x_{\Bbb P}$ , and averaged over the $t$ interval $0.073〈 |t| 〈 0.4~{\rm GeV}^2$ and the photon virtuality range $5 〈 Q^2 〈 20~{\rm GeV}^2$ . In the kinematic range $4 \times 10^{-4} 〈 x_{\Bbb P} 〈 0.03$ and $0.015 〈 \beta 〈 0.5$ , the $x_{\Bbb P}$ dependence of $F_2^{D(4)}$ is fitted with a form $(1/x_{\Bbb P})^a$ , yielding $a= 1.00 \pm 0.09({\rm stat.})^{+0.11}_{-0.05}({\rm syst.})$ . Upon integration over $t$ , the structure function $F_2^{D(3)}$ is determined in a kinematic range extending to higher $x_{\Bbb P}$ and lower $\beta$ compared to our previous analysis; the results are discussed within the framework of Regge theory.
    Type of Medium: Electronic Resource
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