6,853 research outputs found

    Running of the QED coupling in small-angle Bhabha scattering at LEP

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    Using the OPAL detector at LEP, the running of the effective QED coupling alpha(t) is measured for space-like momentum transfer, 2 \leq -t \leq 6 GeV^2, from the angular distribution of small-angle Bhabha scattering. This is currently the most significant direct observation of the running of the QED coupling in a single experiment and the first clear evidence of the hadronic contribution to the running in the space-like region. Our result is in good agreement with standard evaluations of alpha(t), based on data in the time-like region.Comment: Talks given at the 19th Rencontres De Physique De La Vallee D'Aoste (27.2-5.3.05), La Thuile, Italy and at the 40th Rencontres De Moriond On Electroweak Interactions And Unified Theories (5.3-12.3.05), La Thuile, Italy. Added 1 figure and related text wrt vers.

    Fermion pair production at LEP2 and interpretations

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    Preliminary results on e+effˉe^+ e^- \to f \bar{f}, f=e,μ,τ,qf=e, \mu, \tau, q, including all LEP2 data are discussed. Good agreement is found with the Standard Model up to the highest energies. Limits on possible new physics are extracted.Comment: 8 pages, 4 figures, to appear in the Proceedings of the New Trends in High-Energy Physics, Yalta, Ukraine, September 22 - 29, 200

    A Framework to Simultaneously Explain Tiny Neutrino Mass and Huge Missing Mass Problem of the Universe

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    Recently a minimalistic scenario has been developed to explain dark matter and tiny but nonzero neutrino masses. In this scenario, a new scalar called SLIM plays the role of the dark matter. Neutrinos achieve Majorana mass through a one-loop diagram. This scenario can be realized for both real and complex SLIM. Simultaneously explaining the neutrino mass and dark matter abundance constrains the scenario. In particular for real SLIM, an upper bound of a few MeV on the masses of the new particles and a lower bound on their coupling are obtained which make the scenario testable. The low energy scenario can be embedded within various SU(2)×U(1)SU(2)\times U(1) symmetric models. I shall briefly review the scenario and a specific model that embeds the scenario, with special emphasis on the effects in the charged Kaon decay which might be observable at the KLOE and NA62 experiments.Comment: 7 pages, 1 figur

    Quark mixing and CP violation - the CKM matrix

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    I present the status of the elements and parameters of the Cabibbo-Kobayashi-Maskawa (CKM) matrix and summarise the related theoretical progress since "Lepton-Photon 2003". One finds |V_{us}| = 0.2227 +/- 0.0017 from K and tau decays and |V_{cb}| = (41.6 +/- 0.5) * 10^{-3} from inclusive semileptonic B decays. The unitarity triangle can now be determined from tree-level quantities alone and the result agrees well with the global fit including flavour-changing neutral current (FCNC) processes, which are sensitive to new physics. From the global fit one finds the three CKM angles theta_{12}=12.9 +/- 0.1 degrees, theta_{23}= 2.38 +/- 0.03 degrees and theta_{13}= 0.223 +/- 0.007 degrees in the standard PDG convention. The CP phase equals delta_{13} = gamma = (58.8 +5.3/-5.8) degrees at 1 sigma CL and gamma=(58.8 +11.2/-15.4) degrees at 2 sigma CL. A major progress are first results from fully unquenched lattice QCD computations for the hadronic quantities entering the UT fit. I further present the calculation of three-loop QCD corrections to the charm contribution in K+ -> pi+ nu nu-bar decays, which removes the last relevant theoretical uncertainty from the K -> pi nu nu-bar system. Finally I discuss mixing-induced CP asymmetries in b -> s q-bar q penguin decays, whose naive average is below its Standard Model value by 3 sigma.Comment: 14 pages, 12 figures, plenary talk at "Lepton-Photon 2005

    Performance of the OPAL Si-W luminometer at LEP I-II

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    A pair of compact Silicon-Tungsten calorimeters was operated in the OPAL experiment at LEP to measure the integrated luminosity from detection of Bhabha electrons scattered at angles between 25 and 58 mrad from the beam line. In the eight years from 1993 to 2000 the detector worked first at the Z mass peak and then at center of mass energies up to 209 GeV. The fine radial and longitudinal segmentation (2.5mm x 1X0) allowed the radial position of electron and photon showers to be measured with a resolution of 130-170 microns and a residual radial bias as small as 7 microns. Reducing the bias in the definition of the inner acceptance radius was the key element in obtaining an experimental systematic error on the integrated luminosity of only 3.4 10^-4. The performance of the detector at both LEP-I and LEP-II is reviewed. Energy resolution, sensitivity to overlapping electromagnetic showers and sensitivity to minimum ionizing particles are discussed.Comment: 9 pages, 7 figures, 10th International Conference on Calorimetry in High Energy Physics. http://3w.hep.caltech.edu/calor02