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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.

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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.

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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.

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DOE OSTI2013

Measurement of hard double-parton interactions in W ( → l ν ) + 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.

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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 ± W ± 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.

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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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DOE OSTI2014

Measurement of χ c 1 and χ c 2 production with s = 7 TeV p p collisions at ATLAS

CERN-LHC. The prompt and non-prompt production cross-sections for the chi_c1 and chi_c2 charmonium states are measured in pp collisions at sqrt(s) = 7 TeV with the ATLAS detector at the LHC using 4.5 fb-1 of integrated luminosity. The chi_c states are reconstructed through the radiative decay chi_c --> J/psi + gamma (with J/psi --> mu+ mu-) where photons are reconstructed from gamma --> e+ e- conversions. The differential cross sections for both prompt and non-prompt production of the chi_c1 and chi_c2 states are measured as a function of both J/psi and chi_c pT. The fraction of prompt J/psi produced in feed-down from radiative chi_c decays is also measured in addition to the production rate of chi_c2 relative to chi_c1, for both prompt and non-prompt chi_c production. The measurements have not been corrected for the chi_c --> J/psi + gamma and J/psi --> mu+ mu- branching fractions. All measurements are made within the fiducial region ABS(YRAP(J/PSI)) < 0.75.

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DOE OSTI2014

Measurement of inclusive jet charged-particle fragmentation functions in Pb+Pb collisions at s N N = 2.76 TeV with the ATLAS detector

CERN-LHC. Measurements of charged-particle fragmentation functions of jets produced in ultra-relativistic nuclear collisions can provide insight into the modification of parton showers in the hot, dense medium created in the collisions. ATLAS has measured jets in sqrt(sNN) = 2.76 TeV PB PB collisions at the LHC using a data set recorded in 2011 with an integrated luminosity of 0.14 nb^{-1}. Jets were reconstructed using the anti-kt algorithm with distance parameter values R = 0.2, 0.3, and 0.4.

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DOE OSTI2014

Measurement of the Z / γ ∗ boson transverse momentum distribution in p p collisions at s = 7 TeV with the ATLAS detector

CERN-LHC. This paper describes a measurement of the Z/GAMMA* boson transverse momentum spectrum, using proton-proton collision data at a centre-of-mass energy of sqrt(s) = 7 TeV collected during 2011 by ATLAS at the LHC. The measurement is performed in the Z/GAMMA* --> E+ E- and Z/GAMMA* --> MU+ MU- channels, using data corresponding to an integrated luminosity of 4.7 fb^{-1}. Normalized differential cross sections as a function of the Z/GAMMA* boson transverse momentum, PT(Z), are measured for transverse momenta up to 800 GeV. The measurement is performed inclusively for Z/GAMMA* rapidities, YRAP(Z), up to 2.4, as well as in three rapidity bins.

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DOE OSTI2014

Measurement of the cross section of high transverse momentum Z → b b ¯ production in proton--proton collisions at s = 8 T e V with the ATLAS Detector

CERN-LHC. This Letter reports the observation of a high transverse momentum Z0 --> BOTTOM BOTTOMBAR signal in proton-proton collisions at sqrt(s) = 8 TeV and the measurement of its production cross section. The data analysed were collected in 2012 with the ATLAS detector at the LHC and correspond to an integrated luminosity of 19.5 fb^{-1}. The Z0 --> BOTTOM BOTTOMBAR decay is reconstructed from a pair of b-tagged jets, clustered with the anti-kt jet algorithm with R = 0.4, that have low angular separation and form a dijet with PT > 200 GeV.

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DOE OSTI2014

Search for the direct production of charginos, neutralinos and staus in final states with at least two hadronically decaying taus and missing transverse momentum in p p collisions at s = 8 TeV with the ATLAS detector

CERN-LHC. Results of a search for the electroweak associated production of charginos and next-to-lightest neutralinos, pairs of charginos or pairs of tau sleptons are presented. These processes are characterised by final states with at least two hadronically decaying tau leptons, missing transverse momentum and low jet activity. The analysis is based on an integrated luminosity of 20.3 fb-1 of proton-proton collisions at sqrt(s) = 8 TeV recorded with the ATLAS experiment at the Large Hadron Collider. No significant excess is observed with respect to the predictions from Standard Model processes. Limits are set at 95% confidence level on the masses of the lighter chargino and next-to-lightest neutralino for various hypotheses for the lightest neutralino mass in simplified models. In the scenario of direct production of chargino pairs, with each chargino decaying into the lightest neutralino via an intermediate tau slepton, chargino masses up to 345 GeV are excluded for a massless lightest neutralino. For associated production of mass-degenerate charginos and next-to-lightest neutralinos, both decaying into the lightest neutralino via an intermediate tau slepton, masses up to 410 GeV are excluded for a massless lightest neutralino.

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DOE OSTI2014

Measurements of Four-Lepton Production at the Z Resonance in pp Collisions at s = 7 and 8 TeV with ATLAS

CERN-LHC. Measurements of four-lepton, where lepton refers to electrons and muons, production cross sections at the Z resonance in pp collisions at the LHC with the ATLAS detector are presented. The analysis uses data recorded in 2011 and 2012 and corresponds to an integrated luminosity of 4.5 fb^{-1} at 7 TeV and 20.3 fb^{-1} at 8 TeV.

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DOE OSTI2014

Search for Scalar Diphoton Resonances in the Mass Range 65 − 600 GeV with the ATLAS Detector in p p Collision Data at s = 8 T e V

CERN-LHC-ATLAS. Measurement of the 95% CL limit on the fiducial production cross-section times branching ratio of a Higgs-like resonance decaying into two photons, as a function of the resonance mass mX. The resonance is assumed to be narrow. The narrow width assumption is valid for natural widths smaller than about 0.09 GeV + 0.01*mX. The interference of the resonance with the continuum background is neglected. The cross-section is reported for a fiducial volume defined as follows: photon |eta| < 2.37, photon energies required to be E1 > 22 GeV and E2 > 22 GeV for mX < 110 GeV. For mX > 110 GeV, mass-dependent cuts are applied on top: E1/mgg > 0.4 and E2/mgg > 0.3, where mgg is the diphoton invariant mass. The particle-level isolation within a cone of size DR=0.4 around each photon is required to be less than 14 GeV.

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DOE OSTI2014

Search for direct pair production of the top squark in all-hadronic final states in proton-proton collisions at s = 8 TeV with the ATLAS detector

CERN-LHC. The results of a search for direct pair production of the scalar partner to the top quark using an integrated luminosity of 20.1 fb^{-1} of proton-proton collision data at sqrt(s) = 8 TeV recorded with the ATLAS detector at the LHC are reported. The top squark is assumed to decay via stop --> t chi1^0 or stop --> b chi1^{+/-} --> b W^{(*)} chi1^0, where chi1^0 (chi1^{+/-}) denotes the lightest neutralino (chargino) in supersymmetric models. The search targets a fully-hadronic final state in events with four or more jets and large missing transverse momentum. No significant excess over the Standard Model background prediction is observed, and exclusion limits are reported in terms of the top squark and neutralino masses and as a function of the branching fraction of stop --> t chi1^0. For a branching fraction of 100%, top squark masses in the range 270-645 GeV are excluded for chi1^0 masses below 30 GeV. For a branching fraction of 50% to either stop --> t chi1^0 or stop --> b chi1^{+/-}, and assuming the chi1^{+/-} mass to be twice the chi1^0 mass, top squark masses in the range 250-550 GeV are excluded for chi1^0 masses below 60 GeV.

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