3,111 research outputs found

    Search for the Standard Model Higgs boson in the WW⋆ decay mode with ATLAS

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    A search for the Standard Model Higgs boson in the H → WW(*) → lÎœlÎœ (l = e, ÎŒ) decay modes is presented. The search is performed using protonproton collision data corresponding to an integrated luminosity of 4.7 fb−1 at a centre-of-mass energy of 7TeV collected during 2011 with the ATLAS detector at the Large Hadron Collider. No significant excess of events over the expected background is observed. An upper bound is placed on the Higgs boson production cross section as a function of its mass. A Standard Model Higgs boson with mass in the range between 133 GeV and 261 GeV is excluded at 95% confidence level, while the expected exclusion range is from 127 GeV to 233 GeV

    Neutralino spin measurement with ATLAS detector at LHC

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    Minimal Supergravity (mSUGRA) [1] Supersimmetry breaking mechanism is a leading candidate for yielding new physics beyond the Standard Model (SM). Within mSUGRA framework masses, mixings and decays of all SUSY and Higgs particles are determined in terms of four input parameters and a sign: the common mass m 0 of scalar particles at the grand unification scale, the common fermion mass m 1/2, the common trilinear coupling A 0, the ratio of the Higgs vacuum expectation values tan ÎČ and the sign of the supersymmetric Higgs mass parameter ÎŒ. Once a signal of a physics beyond the Standard Model is seen at LHC, it will be fundamental to measure properties of new particles, like spin, in order to prove that they are indeed supersymmetric partners. The present work [2] is based on the spin analysis method proposed in [3] and allows the discrimination of different hypotheses for spin assignments. Some studies [4, 5] show that this method can also be used for the discrimination of SUSY from an Universal Extra Dimensions model which can mimick low energy SUSY at hadron colliders. In this report two selected points inside stau-coannihilation and bulk regions of the allowed mSUGRA parameter space are considered. Fast simulation [6] of the ATLAS detector was performed in order to investigate the feasibility of supersymmetric particles’ spin measurement

    Study of second lightest neutralino spin measurement with ATLAS detector at LHC

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    One of the goals of the ATLAS experiment at the CERN Large Hadron Collider is to search for evidence of Supersymmetry (SUSY) signals. If SUSY would be discovered, it will be fundamental to measure the spin of the new particles in order to prove that they are indeed supersymmetric partners. Left-handed squark cascade decay to second lightest neutralino which further decays to slepton can represent a good opportunity for SUSY particles' spin measurement. Assuming the neutralino spin to be 1/2, the invariant mass distributions of some detectable final products of the reactions have to be charge asymmetric. In the present work the detectability of this charge asymmetry is analysed in the stau-coannihilation region and in the bulk region of the minimal Supergravity parameter space allowed by the latest experimental constraints. The criteria used to isolate the decay chain of interest and to reject the background, coming from both Standard Model and different SUSY decay channels, are described as obtained by suitable optimizations on Monte Carlo samples produced with the ATLAS fast simulation. The estimates of the residual contributions to background and of the applied cut efficiencies are presented. Results on charge asymmetry are then shown and discussed

    Ageing test of the ATLAS RPCs at X5-GIF

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    An ageing test of three ATLAS production RPC stations is in course at X5-GIF, the CERN irradiation facility. The chamber efficiencies are monitored using cosmic rays triggered by a scintillator hodoscope. Higher statistics measurements are made when the X5 muon beam is available. We report here the measurements of the efficiency versus operating voltage at different source intensities, up to a maximum counting rate of about 700Hz/cm^2. We describe the performance of the chambers during the test up to an overall ageing of 4 ATLAS equivalent years corresponding to an integrated charge of 0.12C/cm^2, including a safety factor of 5.Comment: 4 pages. Presented at the VII Workshop on Resistive Plate Chambers and Related Detectors; Clermont-Ferrand October 20th-22nd, 200

    Study of Spin and Decay-Plane Correlations of W Bosons in the e+e- -> W+W- Process at LEP

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    Data collected at LEP at centre-of-mass energies \sqrt(s) = 189 - 209 GeV are used to study correlations of the spin of W bosons using e+e- -> W+W- -> lnqq~ events. Spin correlations are favoured by data, and found to agree with the Standard Model predictions. In addition, correlations between the W-boson decay planes are studied in e+e- -> W+W- -> lnqq~ and e+e- -> W+W- -> qq~qq~ events. Decay-plane correlations, consistent with zero and with the Standard Model predictions, are measured

    Search for Branons at LEP

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    We search, in the context of extra-dimension scenarios, for the possible existence of brane fluctuations, called branons. Events with a single photon or a single Z-boson and missing energy and momentum collected with the L3 detector in e^+ e^- collisions at centre-of-mass energies sqrt{s}=189-209$ GeV are analysed. No excess over the Standard Model expectations is found and a lower limit at 95% confidence level of 103 GeV is derived for the mass of branons, for a scenario with small brane tensions. Alternatively, under the assumption of a light branon, brane tensions below 180 GeV are excluded

    Ultrarelativistic sources in nonlinear electrodynamics

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    The fields of rapidly moving sources are studied within nonlinear electrodynamics by boosting the fields of sources at rest. As a consequence of the ultrarelativistic limit the delta-like electromagnetic shock waves are found. The character of the field within the shock depends on the theory of nonlinear electrodynamics considered. In particular, we obtain the field of an ultrarelativistic charge in the Born-Infeld theory.Comment: 10 pages, 3 figure

    Measurement of the Cross Section for Open-Beauty Production in Photon-Photon Collisions at LEP

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    The cross section for open-beauty production in photon-photon collisions is measured using the whole high-energy and high-luminosity data sample collected by the L3 detector at LEP. This corresponds to 627/pb of integrated luminosity for electron-positron centre-of-mass energies from 189GeV to 209GeV. Events containing b quarks are identified through their semi-leptonic decay into electrons or muons. The e+e- -> e+e-b b~X cross section is measured within our fiducial volume and then extrapolated to the full phase space. These results are found to be in significant excess with respect to Monte Carlo predictions and next-to-leading order QCD calculations

    Search for Branons at LEP

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    We search, in the context of extra-dimension scenarios, for the possible existence of brane fluctuations, called branons. Events with a single photon or a single Z-boson and missing energy and momentum collected with the L3 detector in e^+ e^- collisions at centre-of-mass energies sqrt{s}=189-209$ GeV are analysed. No excess over the Standard Model expectations is found and a lower limit at 95% confidence level of 103 GeV is derived for the mass of branons, for a scenario with small brane tensions. Alternatively, under the assumption of a light branon, brane tensions below 180 GeV are excluded

    Measurement of Exclusive rho^0 rho^0 Production in Two-Photon Collisions at High Q^2 at LEP

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    Exclusive rho rho production in two-photon collisions involving a single highly virtual photon is studied with data collected at LEP at centre-of-mass energies 89GeV < \sqrt{s} < 209GeV with a total integrated luminosity of 854.7pb^-1 The cross section of the process gamma gamma^* -> rho rho is determined as a function of the photon virtuality, Q^2 and the two-photon centre-of-mass energy, Wgg, in the kinematic region: 1.2GeV^2 < Q^2 < 30GeV^2 and 1.1GeV < Wgg < 3GeV
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