1,457 research outputs found

    Search for a Heavy Particle Decaying into an Electron and a Muon with the ATLAS Detector in √s = 7 TeV pp collisions at the LHC

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    This Letter presents the first search for a heavy particle decaying into an e^±μ^∓ final state in √s=7  TeV pp collisions at the LHC. The data were recorded by the ATLAS detector during 2010 and correspond to a total integrated luminosity of 35  pb^(-1). No excess above the standard model background expectation is observed. Exclusions at 95% confidence level are placed on two representative models. In an R-parity violating supersymmetric model, tau sneutrinos with a mass below 0.75 TeV are excluded, assuming all R-parity violating couplings are zero except λ'_(311)=0.11 and λ_(312)=0.07. In a lepton flavor violating model, a Z′-like vector boson with masses of 0.70–1.00 TeV and corresponding cross sections times branching ratios of 0.175–0.183 pb is excluded. These results extend to higher mass R-parity violating sneutrinos and lepton flavor violating Z’s than previous constraints from the Tevatron

    Measurement of the Υ (1S) production cross-section in pp collisions at √s = 7 TeV in ATLAS

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    A measurement of the cross-section for Υ (1S)→μ^+μ^− production in proton–proton collisions at centre of mass energy of 7 TeV is presented. The cross-section is measured as a function of the Υ (1S) transverse momentum in two bins of rapidity, |y^(Υ(1S))| 4 GeV and pseudorapidity |η^μ| < 2.5 in order to reduce theoretical uncertainties on the acceptance, which depend on the poorly known polarisation. The results are based on an integrated luminosity of 1.13 pb^(−1), collected with the ATLAS detector at the Large Hadron Collider. The cross-section measurement is compared to theoretical predictions: it agrees to within a factor of two with a prediction based on the NRQCD model including colour-singlet and colour-octet matrix elements as implemented in Pythia while it disagrees by up to a factor of ten with the next-to-leading order prediction based on the colour-singlet model

    Search for pair production of first or second generation leptoquarks in proton-proton collisions at √s = 7 TeV using the ATLAS detector at the LHC

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    This paper describes searches for the pair production of first or second generation scalar leptoquarks using 35  pb^(-1) of proton-proton collision data recorded by the ATLAS detector at √s=7  TeV. Leptoquarks are searched in events with two oppositely-charged muons or electrons and at least two jets, and in events with one muon or electron, missing transverse momentum and at least two jets. After event selection, the observed yields are consistent with the predicted backgrounds. Leptoquark production is excluded at the 95% CL for masses M_(LQ)<376 (319) GeV and M_(LQ)<422 (362) GeV for first and second generation scalar leptoquarks, respectively, when assuming the branching fraction of a leptoquark to a charged lepton is equal to 1.0 (0.5)

    Limits on the production of the standard model Higgs boson in pp collisions at √s = 7 TeV with the ATLAS detector

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    A search for the Standard Model Higgs boson at the Large Hadron Collider (LHC) running at a centre-of-mass energy of 7 TeV is reported, based on a total integrated luminosity of up to 40 pb^(−1) collected by the ATLAS detector in 2010. Several Higgs boson decay channels: H→γγ, H→ZZ^((∗))→ℓℓℓℓ, H→ZZ→ℓℓνν, H→ZZ→ℓℓqq, H→WW^((∗))→ℓνℓν and H→WW→ℓνqq (ℓ is e, μ) are combined in a mass range from 110 GeV to 600 GeV. The highest sensitivity is achieved in the mass range between 160 GeV and 170 GeV, where the expected 95% CL exclusion sensitivity is at Higgs boson production cross sections 2.3 times the Standard Model prediction. Upper limits on the cross section for its production are determined. Models with a fourth generation of heavy leptons and quarks with Standard Model-like couplings to the Higgs boson are also investigated and are excluded at 95% CL for a Higgs boson mass in the range from 140 GeV to 185 GeV

    Measurement of the differential cross-sections of inclusive, prompt and non-prompt J/ψ production in proton–proton collisions at √s = 7 TeV

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    The inclusive J/ψ production cross-section and fraction of J/ψ mesons produced in B-hadron decays are measured in proton–proton collisions at √s = 7 TeV with the ATLAS detector at the LHC, as a function of the transverse momentum and rapidity of the J/ψ, using 2.3 pb^(−1) of integrated luminosity. The cross-section is measured from a minimum pT of 1 GeV to a maximum of 70 GeV and for rapidities within |y|<2.4 giving the widest reach of any measurement of J/ψ production to date. The differential production cross-sections of prompt and non-prompt J/ψ are separately determined and are compared to Colour Singlet NNLO^★, filled, Colour Evaporation Model, and FONLL predictions

    Search for Diphoton Events with Large Missing Transverse Energy in 7 TeV Proton-Proton Collisions with the ATLAS Detector

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    A search for diphoton events with large missing transverse energy is presented. The data were collected with the ATLAS detector in proton-proton collisions at √s = 7  TeV at the CERN Large Hadron Collider and correspond to an integrated luminosity of 3.1  pb^(-1). No excess of such events is observed above the standard model background prediction. In the context of a specific model with one universal extra dimension with compactification radius R and gravity-induced decays, values of 1/R < 729  GeV are excluded at 95% C.L., providing the most sensitive limit on this model to date

    A search for new physics in dijet mass and angular distributions in pp collisions at √s = 7 TeV measured with the ATLAS detector

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    A search for new interactions and resonances produced in LHC proton–proton (pp) collisions at a centre-of-mass energy √s = 7 TeV was performed with the ATLAS detector. Using a dataset with an integrated luminosity of 36 pb^(−1), dijet mass and angular distributions were measured up to dijet masses of ~3.5 TeV and were found to be in good agreement with Standard Model predictions. This analysis sets limits at 95% CL on various models for new physics: an excited quark is excluded for mass between 0.60 and 2.64 TeV, an axigluon hypothesis is excluded for axigluon masses between 0.60 and 2.10 TeV and quantum black holes are excluded in models with six extra space–time dimensions for quantum gravity scales between 0.75 and 3.67 TeV. Production cross section limits as a function of dijet mass are set using a simplified Gaussian signal model to facilitate comparisons with other hypotheses. Analysis of the dijet angular distribution using a novel technique simultaneously employing the dijet mass excludes quark contact interactions with a compositeness scale Λ below 9.5 TeV

    Measurement of the W charge asymmetry in the W→μν decay mode in pp collisions at √s = 7 TeV with the ATLAS detector

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    This Letter reports a measurement of the muon charge asymmetry from W bosons produced in proton–proton collisions at a centre-of-mass energy of 7 TeV with the ATLAS experiment at the LHC. The asymmetry is measured in the W→μν decay mode as a function of the muon pseudorapidity using a data sample corresponding to a total integrated luminosity of 31 pb^(−1). The results are compared to predictions based on next-to-leading order calculations with various parton distribution functions. This measurement provides information on the u and d quark momentum fractions in the proton

    Search for Supersymmetry Using Final States with One Lepton, Jets, and Missing Transverse Momentum with the ATLAS Detector in √s = 7 TeV pp Collisions

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    This Letter presents the first search for supersymmetry in final states containing one isolated electron or muon, jets, and missing transverse momentum from √s = 7 TeV proton-proton collisions at the LHC. The data were recorded by the ATLAS experiment during 2010 and correspond to a total integrated luminosity of 35 pb^(-1). No excess above the standard model background expectation is observed. Limits are set on the parameters of the minimal supergravity framework, extending previous limits. Within this framework, for A_0 = GeV, tanβ = 3, and µ > 0 and for equal squark and gluino masses, gluino masses below 700 GeV are excluded at 95% confidence level

    Search for high-mass states with one lepton plus missing transverse momentum in proton–proton collisions at √s = 7 TeV with the ATLAS detector

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    The ATLAS detector is used to search for high-mass states, such as heavy charged gauge bosons (W′,W_*), decaying to a charged lepton (electron or muon) and a neutrino. Results are presented based on the analysis of pp collisions at a center-of-mass energy of 7 TeV corresponding to an integrated luminosity of 36 pb^(−1). No excess beyond standard model expectations is observed. A W′ with sequential standard model couplings is excluded at 95% confidence level for masses below 1.49 TeV, and a Wlow asterisk (charged chiral boson) for masses below 1.35 TeV
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