3,274 research outputs found

    Two-fermion and Two-photon Final States at LEP2 and Search for Extra Dimensions

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    For the first time the experiments ALEPH, DELPHI, L3 and OPAL have presented preliminary results for fermion-pair and photon-pair production in e+e- collisions on the full LEP2 data set. The details of the experimental measurements and results from their LEP-wide combination are presented. No statistically significant deviations from the Standard Model expectations are observed and lower limits, some obtained in dedicated analyses, for new physics phenomena at 95 % confidence level are derived. The scales of contact interactions are constrained to lie above 10-20 TeV, depending on the helicity structure. The Standard Model has thus been tested at LEP2 down to distances 10^{-19} - 10^{-20} m. This has many implications, one example being the interpretation of the new result on the anomalous magnetic moment of the muon as coming from muon substructure. For gamma-gamma final states the QED cut-offs are Lambda_+ > 0.44 TeV and Lambda_- > 0.37 TeV. In a combined analysis, using e+e- and gamma-gamma final states, the most stringent lower limits to date, M_s^+ > 1.13 TeV (lambda = +1) and M_s^- > 1.39 TeV (lambda = -1), on the low gravity effective Planck scale are set. Constrains on the scales of string models like TeV strings and D-branes are derived. In the last case the lower limit is 1.5 - 4 TeV, depending on the coupling strength.Comment: 20 pages, 12 figures, 7 tables; Invited LEP2 review talk, Les Rencontres de Physique de la Vallee d'Aoste, La Thuile, Aosta Valley, Italy, 4-10 March 2001; v2 - minor typo correcte

    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 Λ=10.31.6+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

    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

    Brane world models need low string scale

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    Models with large extra dimensions offer the possibility of the Planck scale being of order the electroweak scale, thus alleviating the gauge hierarchy problem. We show that these models suffer from a breakdown of unitarity at around three quarters of the low effective Planck scale. An obvious candidate to fix the unitarity problem is string theory. We therefore argue that it is necessary for the string scale to appear below the effective Planck scale and that the first signature of such models would be string resonances. We further translate experimental bounds on the string scale into bounds on the effective Planck scale

    Wide Area Network Access to CMS Data Using the Lustre Filesystem

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    In this paper, we explore the use of the Lustre cluster filesystem over the wide area network to access Compact Muon Solenoid (CMS) data stored on physical devices located hundreds of kilometres away. We describe the experimental testbed and report on the I/O performance of applications writing and reading data on the distributed Lustre filesystem established across the WAN. We compare the I/O performance of a CMS application to the performance obtained with IOzone, a standard benchmark tool. We then examine the I/O performance of the CMS application running multiple processes on a single server. And compare the Lustre results to results obtained on data stored on local filesystems. Our measurements reveal that the IOzone benchmark tool, accessing data sequentially, can saturate the Gbps network link that connects our Lustre client in Miami Florida to the Lustre storage located in Gainesville, Florida. We also find that the I/O rates of the CMS application is significantly less than what can be obtained with IOzone for sequential access to data

    Two-Fermion Production in Electron-Positron Collisions

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    This report summarizes the results of the two-fermion working group of the LEP2-MC workshop, held at CERN from 1999 to 2000. Recent developments in the theoretical calculations of the two fermion production process in the electron-positron collision at LEP2 center of the mass energies are reported. The Bhabha process and the production of muon, tau, neutrino and quark pairs is covered. On the basis of comparison of various calculations, theoretical uncertainties are estimated and compared with those needed for the final LEP2 data analysis. The subjects for the further studies are identified.Comment: 2-fermion working group report of the LEP2 Monte Carlo Workshop 1999/2000, 113 pages, 24 figures, 35 table

    Beam Spot Position Measurement at the LEP Collider

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    A precise knowledge of the beam spot position is required for many physics topics at LEP2. The movement of the beam spot is studied at LEP1 using beam orbit monitors close to the interaction points and compared with measurements from tracks produced in e+e- collisions. The beam orbit monitors are found to follow the beam spot position well, particularly when corrected for movements of nearby quadrupole magnets. Data from the LEP high energy run of November 1995 are also analysed, and projections made for the prospects at LEP2
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