1,659 research outputs found

    QCD measurements in photon-photon collisions at LEP

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    An overview of the latest results of the LEP collaborations on QCD measurements in photon-photon collisions is presented, including measurements of the total hadronic cross-section, the production of heavy quarks and dijets and the structure functions of real and virtual photons.Comment: 6 pages, Talk presented at DPF 2000, Columbus, Ohio, August 200

    Hint for axial-vector contact interactions in the data on e+e- -> e+e-(gamma) at centre-of-mass energies 192-208 GeV

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    For the first time the experiments ALEPH, DELPHI, L3 and OPAL have presented preliminary results for fermion-pair production in e+e- collisions on the full data set above the Z pole. A combined analysis of the Bhabha scattering measurements is performed to search for effects of contact interactions. In the case of two axial-vector (AA) currents the best fit to the data is 2.6 standard deviations away from the Standard Model expectation, corresponding to an energy scale őõ=10.3‚ąí1.6+2.8\Lambda = 10.3^{+2.8}_{-1.6} TeV for contact interactions. For other models no statistically significant deviations are observed, and the data are used to set lower limits at 95 % confidence level on the contact interaction scales ranging from 8.2 to 21.3 TeV, depending on the helicity structure.Comment: 6 pages, 1 table, 3 figure

    The Scalar Sector in 331 Models

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    We calculate the exact tree-level scalar mass matrices resulting from symmetry breaking using the most general gauge-invariant scalar potential of the 331 model, both with and without the condition that lepton number is conserved. Physical masses are also obtained in some cases, as well as couplings to standard and exotic gauge bosons.Comment: LaTex, 15 page

    Search for TeV Strings and New Phenomena in Bhabha Scattering at LEP2

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    A combined analysis of the data on Bhabha scattering at centre-of-mass energies 183 and 189 GeV from the LEP experiments ALEPH, L3 and OPAL is performed to search for effects of TeV strings in quantum gravity models with large extra dimensions. No statistically significant deviations from the Standard Model expectations are observed and lower limit on the string scale M_S = 0.631 TeV at 95 % confidence level is derived. The data are used to set lower limits on the scale of contact interactions ranging from 4.2 to 16.2 TeV depending on the model. In a complementary analysis we derive an upper limit on the electron size of 2.8 x 10^{-19} m at 95 % confidence level.Comment: 10 pages, 1 table, 3 figure

    A Combination of Preliminary Electroweak Measurements and Constraints on the Standard Model, 1999

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    This note presents a combination of published and preliminary electroweak results from the four LEP collaborations and the SLD collaboration which were prepared for the 1998 summer conferences. Averages are derived for hadronic and leptonic cross-sections, the leptonic forward-backward asymmetries, the *polarisation asymmetries, the bb and cc partial widths and forward-backward asymmetries and the qq charge asymmetry. The major changes with respect to results presented in summer 1997 are updates to the measurements of the Z lineshape, tau polarisation, W mass and triple-gaugeboson couplings from LEP, and ALR from SLD. The results are compared with precise electroweak measurements from other experiments. A signifi- cant update here is a new measurement of the mixing angle from the NuTeV Collaboration. The parameters of the Standard Model are evaluated, first using the combined LEP electroweak measurements, and then using the full set of electroweak results

    B Mixing

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    The neutral BB mesons, B0B^0 and BsB_s, can oscillate between their particle and antiparticle states owing to flavor-changing weak interactions. In recent years, techniques to detect these oscillations as a function of the meson's decay time have been developed. In this article the physics of flavor oscillations is reviewed and theoretical predictions are summarized. The many observations that demonstrate the time-dependence of B^0-\B0bar oscillations are presented along with a combined measurement of its frequency, őīmd\delta m_d. The attempts to measure the BsB_s oscillation frequency, both directly and indirectly, are then summarized. Finally, values for the CKM elements ‚ą£Vtd‚ą£|V_{td}| and ‚ą£Vts/Vtd‚ą£|V_{ts}/V_{td}| are extracted
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