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At least 1,369 records · Page 76
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DOE OSTI2013

Search for direct third-generation squark pair production in final states with missing transverse momentum and two b -jets in s = 8 TeV p p collisions with the ATLAS detector

CERN-LHC. The results of a search for pair production of supersymmetric partners of the Standard Model third-generation quarks are reported. This search uses 20.1 fb^-1 of pp collisions at sqrt(s) = 8 TeV collected during the period from March to December 2012 by the ATLAS experiment at the Large Hadron Collider. The lightest bottom and top squarks (b1 and t1 respectively) are searched for in a final state with large missing transverse momentum and two jets identified as originating from b-quarks. No excess of events above the expected level of Standard Model background is found. The results are used to set upper limits on the visible cross section for processes beyond the Standard Model. Exclusion limits at the 95% confidence level on the masses of the third-generation squarks are derived in phenomenological supersymmetric R-parity-conserving models in which either the bottom or the top squark is the lightest squark. The b1 is assumed to decay via b1 --> b chi1^0 (where chi1^0 is the lightest neutralino) and the t1 via t1 --> b chi1^{+/-} (where chi1^{+/-} is the lightest chargino), with undetectable products of the subsequent decay of the chi1^{+/-} due to the small mass splitting between the chi1^{+/-} and the chi1^0.

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DOE OSTI2013

Search for Quantum Black Hole Production in High-Invariant-Mass Lepton + Jet Final States Using p p Collisions at s = 8 TeV and the ATLAS Detector

CERN-LHC. Search for quantum black-hole production using 20.3 inverse fb of data collected in 2012 with the ATLAS detector in pp collisions at the LHC at sqrt(s) = 8 TeV. The quantum black holes are assumed to decay into a lepton (electron or muon) and a jet. In either channel, no event with a lepton-jet invariant mass of 3.5 TeV or more is observed, consistent with the expected background. Limits are set on the product of cross sections and branching fractions for the lepton+jet final states of quantum black holes produced in a search region for invariant masses above 1 TeV. The combined 95% confidence level upper limit on this product for quantum black holes with threshold mass above 3.5 TeV is 0.18 fb. This limit constrains the threshold quantum black-hole mass to be above 5.3 TeV in the model considered.

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DOE OSTI2014

Measurement of the production cross section of prompt J / ψ mesons in association with a W ± boson in p p collisions at s = 7 TeV with the ATLAS detector

CERN-LHC. The process P P --> W J/PSI X provides a powerful probe of the production mechanism of charmonium in hadronic collisions, and is also sensitive to multiple parton interactions in the colliding protons. Using the 2011 ATLAS dataset of 4.5 fb^-1 of sqrt(s) = 7 TeV pp collisions at the LHC, the first observation is made of the production of W + prompt J/psi events in hadronic collisions, using W --> MU NU and J/PSI --> MU MU. A yield of 27.4 +7.5,-6.5 W + prompt J/psi events is observed, with a statistical significance of 5.1 sigma. The production rate as a ratio to the inclusive W boson production rate is measured, and the double parton scattering contribution to the cross section is estimated.

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DOE OSTI2012

Measurement of Z Z production in p p collisions at s = 7 TeV and limits on anomalous Z Z Z and Z Z γ couplings with the ATLAS detector

CERN-LHC. Measurements of the cross section for ZZ production using the 4l and 2l2nu decay channels in proton-proton collisions at a centre-of-mass energy of 7 TeV with 4.6 fb^-1 of data collected in 2011. The final states used are 4 electrons, 4 muons, 2 electrons and 2 muons, 2 electrons and missing transverse momentum, and 2 muons and missing transverse momentum (MET).

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DOE OSTI2013

Measurement of jet shapes in top-quark pair events at s = 7 TeV using the ATLAS detector

CERN-LHC. Measurement of the jet shapes in top-quark pair events from proton-proton collisions at a centre-of-mass energy of 7 TeV. The analysis uses a data sample of integrated luminosity 1.8 fb-1. Jets are reconstructed using the anti-kT algorithm with jet radius R=0.4. Jet shapes are presented as differential and integrated defined as: Differential (RHO) - scalar sum of jet PT in an annulus of thickness deltaR around radius r / scalar sum of jet PT in the cone of radius R. Integrated (PSI) - scalar sum of jet PT in cone radius r / scalar sum of jet PT in cone radius R. Data are shown separated into b-jets and light-jet events.

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DOE OSTI2013

Search for charginos nearly mass degenerate with the lightest neutralino based on a disappearing-track signature in pp collisions at s =8 TeV with the ATLAS detector

CERN-LHC. A search is presented for direct chargino production based on a disappearing-track signature using 20.3 fb^-1 of proton-proton collisions at sqrt(s) = 8 TeV collected in 2012 with the ATLAS experiment at the LHC. In anomaly-mediated supersymmetry breaking (AMSB) models, the lightest chargino is nearly mass-degenerate with the lightest neutralino and its lifetime is long enough to be detected in the tracking detectors by identifying decays that result in tracks with no associated hits in the outer region of the tracking system. Some models with supersymmetry also predict charginos with a significant lifetime. This analysis attains sensitivity for charginos with a lifetime between 0.1 ns and 10 ns, and significantly surpasses the reach of the LEP experiments. No significant excess above the background expectation is observed for candidate tracks with large transverse momentum, and constraints on chargino properties are obtained. In the AMSB scenarios, a chargino mass below 270 GeV is excluded at 95% confidence level.

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DOE OSTI2013

Measurement of the distributions of event-by-event flow harmonics in lead-lead collisions at = 2.76 TeV with the ATLAS detector at the LHC

CERN-LHC. The distributions of event-by-event harmonic flow coefficients vn for n = 2-4 are measured in sqrt(sNN) = 2.76 TeV Pb+Pb collisions using the ATLAS detector at the LHC. The measurements are performed using charged particles with transverse momentum pT > 0.5 GeV and in the pseudorapidity range |eta| < 2.5 in a dataset of approximately 7 mub^-1 recorded in 2010. The shapes of the vn distributions suggest that the associated flow vectors are described by a two-dimensional Gaussian function in central collisions for v2 and over most of the measured centrality range for v3 and v4. Significant deviations from this function are observed for v2 in mid-central and peripheral collisions, and a small deviation is observed for v3 in mid-central collisions. In order to be sensitive to these deviations, it is shown that the commonly used multi-particle cumulants, involving four particles or more, need to be measured with a precision better than a few percent. The vn distributions are also measured independently for charged particles with 0.5 < pT < 1 GeV and pT > 1 GeV. When these distributions are rescaled to the same mean values, the adjusted shapes are found to be nearly the same for these two pT ranges. The vn distributions are compared with the eccentricity distributions from two models for the initial collision geometry: a Glauber model and a model that includes corrections to the initial geometry due to gluon saturation effects. Both models fail to describe the experimental data consistently over most of the measured centrality range.

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DOE OSTI2012

Observation of Associated Near-Side and Away-Side Long-Range Correlations in s N N =5.02 TeV Proton-Lead Collisions with the ATLAS Detector

CERN-LHC. Two-particle correlations in relative azimuthal angle, Delta(phi), and pseudorapidity, Delta(eta), are measured in sqrt(sNN) = 5.02 TeV p+Pb collisions recorded in September 2012 using the ATLAS detector at the LHC. The measurements are performed using approximately 1 mub^1 of data as a function of transverse momentum (pT) and the transverse energy summed over 3.1 < eta < 4.9 in the direction of the Pb beam. The correlation function, constructed from charged particles, exhibits a long-range (2 < |Delta(eta)| < 5) near-side (Delta(phi) ~ 0) correlation that grows rapidly with increasing transverse energy. A long-range away-side (Delta(phi) ~ pi) correlation, obtained by subtracting the expected contributions from recoiling dijets and other sources estimated using events with small transverse energy, is found to match the near-side correlation in magnitude, shape (in Delta(eta) and Delta(phi)) and transverse energy dependence. The resultant Delta(phi) correlation is approximately symmetric about pi/2, and is consistent with a dominant cos(2*Delta(phi)) modulation for all transverse energy ranges and particle pT.

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DOE OSTI2014

Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in s = 8TeV p p collisions with the ATLAS detector

CERN-LHC. A search for the direct production of charginos and neutralinos in final states with three leptons and missing transverse momentum is presented. The analysis is based on 20.3 fb^-1 of sqrt(s) = 8 TeV proton--proton collision data delivered by the Large Hadron Collider and recorded in 2012 with the ATLAS detector. Observations are consistent with the Standard Model expectations and limits are set in R-parity-conserving phenomenological Minimal Supersymmetric Standard Models and in simplified supersymmetric models, significantly extending previous results. For simplified supersymmetric models of direct chargino (C1) and next-to-lightest neutralino (N2) production with decays to lightest neutralino (N1) via either all three generations of sleptons, staus only, gauge bosons, or Higgs bosons, C1 and N2 masses are excluded up to 700 GeV, 380 GeV, 345 GeV, or 148 GeV respectively, for a massless N1.

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DOE OSTI2014

Measurement of the production of a W boson in association with a charm quark in p p collisions at s = 7 TeV with the ATLAS detector

CERN-LHC. The production of a W boson in association with a single charm quark is studied using 4.6 fb^-1 of pp collision data at sqrt(s) = 7 TeV collected in 2011 with the ATLAS detector at the Large Hadron Collider. In events in which a W boson decays to an electron or muon, the charm quark is tagged either by its semileptonic decay to a muon or by the presence of a charmed meson. The integrated and differential cross sections as a function of the pseudorapidity of the lepton from the W-boson decay are measured. Results are compared to the predictions of next-to-leading-order QCD calculations obtained from various parton distribution function parameterisations. The ratio of the strange-to-down sea quark distributions is determined to be 0.96 +0.26,-0.30 at Q^2 = 1.9 GeV^2 which supports the hypothesis of an SU(3) symmetric composition of the light quark sea. Additionally, the cross-section ratio sigma(W^+ + cbar)/sigma(W^- + c) is compared to the predictions obtained using parton distribution function parameterisations with different assumptions on the s-sbar asymmetry.

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DOE OSTI2014

Search for direct top squark pair production in events with a Z boson, b-jets and missing transverse momentum in sqrt(s)=8 TeV pp collisions with the ATLAS detector

CERN-LHC. A search is presented for direct top squark pair production using events with at least two leptons including a same-flavour opposite-sign pair with invariant mass consistent with the Z boson mass, jets tagged as originating from b-quarks and missing transverse momentum. The analysis is performed with proton-proton collision data at sqrt(s) = 8 TeV collected with the ATLAS detector at the LHC in 2012 corresponding to an integrated luminosity of 20.3 fb^-1. No excess beyond the Standard Model expectation is observed. Interpretations of the results are provided in models based on the direct pair production of the heavier top squark state (t2) followed by the decay to the lighter top squark state (t1) via t2 --> Z t1, and for t1 pair production in natural gauge-mediated supersymmetry breaking scenarios where the neutralino (chi1^0) is the next-to-lightest supersymmetric particle and decays producing a Z boson and a gravitino (G) via the chi1^0 --> Z G process.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Search for direct production of charginos, neutralinos and sleptons in final states with two leptons and missing transverse momentum in p p collisions at s = 8 TeV with the ATLAS detector

CERN-LHC. Searches for the electroweak production of charginos, neutralinos and sleptons in final states characterized by the presence of two leptons (electrons and muons) and missing transverse momentum are performed using 20.3 fb^-1 of proton-proton collision data at sqrt(s) = 8 TeV recorded in 2012 with the ATLAS experiment at the Large Hadron Collider. No significant excess beyond Standard Model expectations is observed. Limits are set on the masses of the lightest chargino, next-to-lightest neutralino and sleptons for different lightest-neutralino mass hypotheses in simplified models. Results are also interpreted in various scenarios of the phenomenological Minimal Supersymmetric Standard Model.

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DOE OSTI2014

Search for supersymmetry in events with four or more leptons in s = 8 TeV pp collisions with the ATLAS detector

CERN-LHC. Results from a search for supersymmetry in events with four or more leptons including electrons, muons and taus are presented. The analysis uses a data sample corresponding to 20.3 fb^{-1} of proton-proton collisions delivered by the Large Hadron Collider at sqrt(s) = 8 TeV and recorded by the ATLAS detector. Signal regions are designed to target supersymmetric scenarios that can be either enriched in or depleted of events involving the production of a Z boson. No significant deviations are observed in data from Standard Model predictions and results are used to set upper limits on the event yields from processes beyond the Standard Model. Exclusion limits at the 95% confidence level on the masses of relevant supersymmetric particles are obtained. In R-parity-violating simplified models with decays of the lightest supersymmetric particle to electrons and muons, limits of 1350 GeV and 750 GeV are placed on gluino and chargino masses, respectively. In R-parity-conserving simplified models with heavy neutralinos decaying to a massless lightest supersymmetric particle, heavy neutralino masses up to 620 GeV are excluded. Limits are also placed on other supersymmetric scenarios.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Search for supersymmetry at s =8 TeV in final states with jets and two same-sign leptons or three leptons with the ATLAS detector

CERN-LHC. A search for strongly produced supersymmetric particles is conducted using signatures involving multiple energetic jets and either two isolated leptons (e or mu) with the same electric charge, or at least three isolated leptons. The search also utilises jets originating from b-quarks, missing transverse momentum and other observables to extend its sensitivity. The analysis uses a data sample corresponding to a total integrated luminosity of 20.3 fb^-1 of sqrt(s) = 8 TeV proton-proton collisions recorded with the ATLAS detector at the Large Hadron Collider in 2012. No deviation from the Standard Model expectation is observed. New or significantly improved exclusion limits are set on a wide variety of supersymmetric models in which the lightest squark can be of the first, second or third generations, and in which R-parity can be conserved or violated.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2013

Measurement of hard double-parton interactions in W ( &#x2192; l &#x3BD; ) + 2 jet events at s =7 TeV with the ATLAS detector

CERN-LHC. The production of W bosons in association with two jets in proton-proton collisions at a centre-of-mass energy of sqrt(s) = 7 TeV has been analysed for the presence of double-parton interactions using data corresponding to an integrated luminosity of 36 pb^-1, collected in 2010 with the ATLAS detector at the LHC. The fraction of events arising from double-parton interactions, f_{DP}^{(D)}, has been measured through the momentum balance between the two jets and amounts to f_{DP}^{(D)} = 0.08 +- 0.01 (stat.) +- 0.02 (sys) for jets with transverse momentum pT > 20 GeV and rapidity |y| < 2.8. This corresponds to a measurement of the effective area parameter for hard double-parton interactions of sigma_{eff} = 15 +- 3 (stat.) +5,-3 (sys.) mb.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Search for direct top-squark pair production in final states with two leptons in pp collisions at s = 8TeV with the ATLAS detector

CERN-LHC. A search is presented for direct top-squark pair production in final states with two leptons (electrons or muons) of opposite charge using 20.3 fb^-1 of pp collision data at sqrt(s) = 8 TeV, collected by the ATLAS experiment at the Large Hadron Collider in 2012. No excess over the Standard Model expectation is found. The results are interpreted under the separate assumptions (i) that the top squark decays to a b-quark in addition to an on-shell chargino whose decay occurs via a real or virtual W boson, or (ii) that the top squark decays to a t-quark and the lightest neutralino. A top squark with a mass between 150 GeV and 445 GeV decaying to a b-quark and an on-shell chargino is excluded at 95% confidence level for a top squark mass equal to the chargino mass plus 10 GeV, in the case of a 1 GeV lightest neutralino. Top squarks with masses between 215 (90) GeV and 530 (170) GeV decaying to an on-shell (off-shell) t-quark and a neutralino are excluded at 95% confidence level for a 1 GeV neutralino.

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DOE OSTI2014

Evidence for Electroweak Production of W &#xB1; W &#xB1; j j in p p Collisions at s = 8 TeV with the ATLAS Detector

CERN-LHC. We present the first study of W+- W+- JET JET, same-electric-charge diboson production in association with two jets, using 20.3 fb^{-1} of proton-proton collision data at sqrt(s) = 8 TeV recorded by the ATLAS detector at the Large Hadron Collider. Events with two reconstructed same-charge leptons (E+- E+-, E+- MU+-, and MU+- MU+-) and two or more jets are analyzed. Production cross sections are measured in two fiducial regions, with different sensitivities to the electroweak and strong production mechanisms. First evidence for W+- W+- JET JET production and electroweak-only W+- W+- JET JET production is observed with a significance of 4.5 and 3.6 standard deviations respectively.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Measurement of the low-mass Drell-Yan differential cross section at s = 7 TeV using the ATLAS detector

CERN-LHC. The differential cross section for the process Z/gamma*->ll (l=e,mu) as a function of dilepton invariant mass is measured in pp collisions at sqrt(s) = 7 TeV at the LHC using the ATLAS detector. The measurement is performed in the e and mu channels for invariant masses between 26 GeV and 66 GeV using an integrated luminosity of 1.6 fb^-1 collected in 2011 and these measurements are combined. The analysis is extended to invariant masses as low as 12 GeV in the muon channel using 35 pb^-1 of data collected in 2010. The cross sections are determined within fiducial acceptance regions and corrections to extrapolate the measurements to the full kinematic range are provided. Next-to-next-to-leading-order QCD predictions provide a significantly better description of the results than next-to-leading-order QCD calculations, unless the latter are matched to a parton shower calculation.

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