Publications (67)262.68 Total impact
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Article: Measurement of Z Boson Production in Pb-Pb Collisions at sqrt[s_{NN}]=2.76 TeV with the ATLAS Detector.
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ABSTRACT: The ATLAS experiment has observed 1995 Z boson candidates in data corresponding to 0.15 nb^{-1} of integrated luminosity obtained in the 2011 LHC Pb+Pb run at sqrt[s_{NN}]=2.76 TeV. The Z bosons are reconstructed via dielectron and dimuon decay channels, with a background contamination of less than 3%. Results from the two channels are consistent and are combined. Within the statistical and systematic uncertainties, the per-event Z boson yield is proportional to the number of binary collisions estimated by the Glauber model. The elliptic anisotropy of the azimuthal distribution of the Z boson with respect to the event plane is found to be consistent with zero.Physical Review Letters 01/2013; 110(2):022301. · 7.37 Impact Factor -
Article: Search for Dark Matter Candidates and Large Extra Dimensions in Events with a Photon and Missing Transverse Momentum in pp Collision Data at sqrt[s]=7 TeV with the ATLAS Detector.
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ABSTRACT: Results of a search for new phenomena in events with an energetic photon and large missing transverse momentum in proton-proton collisions at sqrt[s]=7 TeV are reported. Data collected by the ATLAS experiment at the LHC corresponding to an integrated luminosity of 4.6 fb^{-1} are used. Good agreement is observed between the data and the standard model predictions. The results are translated into exclusion limits on models with large extra spatial dimensions and on pair production of weakly interacting dark matter candidates.Physical Review Letters 01/2013; 110(1):011802. · 7.37 Impact Factor -
Article: Search for Magnetic Monopoles in sqrt[s]=7 TeV pp Collisions with the ATLAS Detector.
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ABSTRACT: This Letter presents a search for magnetic monopoles with the ATLAS detector at the CERN Large Hadron Collider using an integrated luminosity of 2.0 fb^{-1} of pp collisions recorded at a center-of-mass energy of sqrt[s]=7 TeV. No event is found in the signal region, leading to an upper limit on the production cross section at 95% confidence level of 1.6/ϵ fb for Dirac magnetic monopoles with the minimum unit magnetic charge and with mass between 200 GeV and 1500 GeV, where ϵ is the monopole reconstruction efficiency. The efficiency ϵ is high and uniform in the fiducial region given by pseudorapidity |η|<1.37 and transverse kinetic energy 600-700<E^{kin}sinθ<1400 GeV. The minimum value of 700 GeV is for monopoles of mass 200 GeV, whereas the minimum value of 600 GeV is applicable for higher mass monopoles. Therefore, the upper limit on the production cross section at 95% confidence level is 2 fb in this fiducial region. Assuming the kinematic distributions from Drell-Yan pair production of spin-1/2 Dirac magnetic monopoles, the efficiency is in the range 1%-10%, leading to an upper limit on the cross section at 95% confidence level that varies from 145 fb to 16 fb for monopoles with mass between 200 GeV and 1200 GeV. This limit is weaker than the fiducial limit because most of these monopoles lie outside the fiducial region.Physical Review Letters 12/2012; 109(26):261803. · 7.37 Impact Factor -
Article: Search for Direct Top Squark Pair Production in Final States with One Isolated Lepton, Jets, and Missing Transverse Momentum in sqrt[s]=7 TeV pp Collisions Using 4.7 fb^{-1} of ATLAS Data.
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ABSTRACT: A search is presented for direct top squark pair production in final states with one isolated electron or muon, jets, and missing transverse momentum in proton-proton collisions at sqrt[s]=7 TeV. The measurement is based on 4.7 fb^{-1} of data collected with the ATLAS detector at the LHC. Each top squark is assumed to decay to a top quark and the lightest supersymmetric particle (LSP). The data are found to be consistent with standard model expectations. Top squark masses between 230 GeV and 440 GeV are excluded with 95% confidence for massless LSPs, and top squark masses around 400 GeV are excluded for LSP masses up to 125 GeV.Physical Review Letters 11/2012; 109(21):211803. · 7.37 Impact Factor -
Article: Search for a Supersymmetric Partner to the Top Quark in Final States with Jets and Missing Transverse Momentum at sqrt[s]=7 TeV with the ATLAS Detector.
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ABSTRACT: A search for direct pair production of supersymmetric top squarks (t[over ˜]_{1}) is presented, assuming the t[over ˜]_{1} decays into a top quark and the lightest supersymmetric particle, χ[over ˜]_{1}^{0}, and that both top quarks decay to purely hadronic final states. A total of 16 (4) events are observed compared to a predicted standard model background of 13.5_{-3.6}^{+3.7}(4.4_{-1.3}^{+1.7}) events in two signal regions based on ∫Ldt=4.7 fb^{-1} of pp collision data taken at sqrt[s]=7 TeV with the ATLAS detector at the LHC. An exclusion region in the t[over ˜]_{1} versus χ[over ˜]_{1}^{0} mass plane is evaluated: 370<m_{t[over ˜]_{1}}<465 GeV is excluded for m_{χ[over ˜]_{1}^{0}}∼0 GeV while m_{t[over ˜]_{1}}=445 GeV is excluded for m_{χ[over ˜]_{1}^{0}}≤50 GeV.Physical Review Letters 11/2012; 109(21):211802. · 7.37 Impact Factor -
Article: Search for down-type fourth generation quarks with the ATLAS detector in events with one lepton and hadronically decaying W bosons.
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ABSTRACT: This Letter presents a search for pair production of heavy down-type quarks decaying via b' → Wt in the lepton + jets channel, as b'b' → W- tW+ t → bbW+ W- W+ W- → l± νbbqqqqqq. In addition to requiring exactly one lepton, large missing transverse momentum, and at least six jets, the invariant mass of nearby jet pairs is used to identify high transverse momentum W bosons. In data corresponding to an integrated luminosity of 1.04 fb(-1) from pp collisions at sqrt[s] = 7 TeV recorded with the ATLAS detector, a heavy down-type quark with mass less than 480 GeV can be excluded at the 95% confidence level.Physical Review Letters 07/2012; 109(3):032001. · 7.37 Impact Factor -
Article: Determination of the Strange-Quark Density of the Proton from ATLAS Measurements of the W→ℓν and Z→ℓℓ Cross Sections.
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ABSTRACT: A QCD analysis is reported of ATLAS data on inclusive W^{±} and Z boson production in pp collisions at the LHC, jointly with ep deep-inelastic scattering data from HERA. The ATLAS data exhibit sensitivity to the light quark sea composition and magnitude at Bjorken x∼0.01. Specifically, the data support the hypothesis of a symmetric composition of the light quark sea at low x. The ratio of the strange-to-down sea quark distributions is determined to be 1.00_{-0.28}^{+0.25} at absolute four-momentum transfer squared Q^{2}=1.9 GeV^{2} and x=0.023.Physical Review Letters 07/2012; 109(1):012001. · 7.37 Impact Factor -
Article: Search for massive colored scalars in four-jet final states in proton–proton collisions with the ATLAS detector
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ABSTRACT: A search for pair-produced scalar particles decaying to a four-jet final state is presented. The analysis is performed using an integrated luminosity of 34pb−1 recorded by the ATLAS detector in 2010. No deviation from the Standard Model is observed. For a scalar mass of 100GeV (190GeV) the limit on the scalar gluon pair production cross section at 95% confidence level is 1nb (0.28nb). When these results are interpreted as mass limits, scalar-gluons (hyperpions) with masses of 100 to 185GeV (100 to 155GeV) are excluded at 95% confidence level with the exception of a mass window of width about 5GeV (15GeV) around 140GeV.European Physical Journal C 05/2012; 71(12):1-19. · 3.63 Impact Factor -
Article: Search for scalar bottom quark pair production with the ATLAS detector in pp collisions at sqrt[s]=7 TeV.
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ABSTRACT: The results of a search for pair production of the scalar partners of bottom quarks in 2.05 fb(-1) of pp collisions at sqrt[s]=7 TeV using the ATLAS experiment are reported. Scalar bottom quarks are searched for in events with large missing transverse momentum and two jets in the final state, where both jets are identified as originating from a bottom quark. In an R-parity conserving minimal supersymmetric scenario, assuming that the scalar bottom quark decays exclusively into a bottom quark and a neutralino, 95% confidence-level upper limits are obtained in the b(1) - χ(1)(0) mass plane such that for neutralino masses below 60 GeV scalar bottom masses up to 390 GeV are excluded.Physical Review Letters 05/2012; 108(18):181802. · 7.37 Impact Factor -
Article: Measurement of the jet fragmentation function and transverse profile in proton–proton collisions at a center-of-mass energy of 7 TeV with the ATLAS detector
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ABSTRACT: The jet fragmentation function and transverse profile for jets with 25GeV<p T jet<500GeV and |η jet|<1.2 produced in proton–proton collisions with a center-of-mass energy of 7TeV are presented. The measurement is performed using data with an integrated luminosity of 36 pb−1. Jets are reconstructed and their momentum measured using calorimetric information. The momenta of the charged particle constituents are measured using the tracking system. The distributions corrected for detector effects are compared with various Monte Carlo event generators and generator tunes. Several of these choices show good agreement with the measured fragmentation function. None of these choices reproduce both the transverse profile and fragmentation function over the full kinematic range of the measurement.European Physical Journal C 05/2012; 71(11):1-25. · 3.63 Impact Factor -
Article: Search for an excess of events with an identical flavour lepton pair and significant missing transverse momentum in TeV proton–proton collisions with the ATLAS detector
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ABSTRACT: Results are presented of a search for particles decaying into final states with significant missing transverse momentum and exactly two identical flavour leptons (e, μ) of opposite charge in Ös=7\sqrt{s}=7TeV collisions at the Large Hadron Collider. This channel is particularly sensitive to supersymmetric particle cascade decays producing flavour correlated lepton pairs. Flavour uncorrelated backgrounds are subtracted using a sample of opposite flavour lepton pair events. Observation of an excess beyond Standard Model expectations following this subtraction procedure would offer one of the best routes to measuring the masses of supersymmetric particles. In a data sample corresponding to an integrated luminosity of 35pb−1 no such excess is observed. Model-independent limits are set on the contribution to these final states from supersymmetry and are used to exclude regions of a phenomenological supersymmetric parameter space.European Physical Journal C 04/2012; 71(7):1-18. · 3.63 Impact Factor -
Article: Measurement of the top quark pair production cross-section with ATLAS in the single lepton channel
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ABSTRACT: A measurement of the production cross-section for top quark pairs (tt¯) in pp collisions at s=7 TeV is presented using data recorded with the ATLAS detector at the Large Hadron Collider. Events are selected in the single lepton topology by requiring an electron or muon, large missing transverse momentum and at least three jets. With a data sample of 35 pb-1, two different multivariate methods, one of which uses b-quark jet identification while the other does not, use kinematic variables to obtain cross-section measurements of σ=187±11(stat.)-17+18(syst.)±6(lumi.) pb and σ=173±17(stat.)-16+18(syst.)±6(lumi.) pb respectively. The two measurements are in agreement with each other and with QCD calculations. The first measurement has a better a priori sensitivity and constitutes the main result of this Letter.Physics Letters B 04/2012; 711:244-263. · 3.95 Impact Factor -
Article: Measurement of Wγ and Zγ production in proton-proton collisions at TeV with the ATLAS detector
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ABSTRACT: We present studies of W and Z bosons with associated high energy photons produced in pp collisions at Ös = 7 \sqrt {s} = 7 TeV. The analysis uses 35 pb−1 of data collected by the ATLAS experiment in 2010. The event selection requires W and Z bosons decaying into high p T leptons (electrons or muons) and a photon with E T > 15GeV separated from the lepton(s) by a distance ∆R(l, γ) > 0.7 in η-ϕ space. A total of 95 (97) pp → e ± νγ + X(pp → μ ± νγ + X) and 25 (23) pp → e + e − γ + X(pp → μ + μ − γ + X) event candidates are selected. The kinematic distributions of the leptons and photons and the production cross sections are measured. The data are found to agree with Standard Model predictions that include next-to-leading-order O(αα s ) contributions. KeywordsHadron-Hadron ScatteringJournal of High Energy Physics 04/2012; 2011(9):1-42. · 5.83 Impact Factor -
Article: Search for supersymmetric particles in events with lepton pairs and large missing transverse momentum in proton–proton collisions with the ATLAS experiment
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ABSTRACT: Results are presented of searches for the production of supersymmetric particles decaying into final states with missing transverse momentum and exactly two isolated leptons in Ös=7TeV\sqrt {s}=7~\mbox{TeV} proton–proton collisions at the Large Hadron Collider. Search strategies requiring lepton pairs with identical-sign or opposite-sign electric charges are described. In a data sample corresponding to an integrated luminosity of 35pb−1 collected with the ATLAS detector, no significant excesses are observed. Based on specific benchmark models, limits are placed on the squark mass between 450 and 690GeV for squarks approximately degenerate in mass with gluinos, depending on the supersymmetric mass hierarchy considered.European Physical Journal C 04/2012; 71(7):1-19. · 3.63 Impact Factor -
Article: Studies of the performance of the ATLAS detector using cosmic-ray muons
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ABSTRACT: Muons from cosmic-ray interactions in the atmosphere provide a high-statistics source of particles that can be used to study the performance and calibration of the ATLAS detector. Cosmic-ray muons can penetrate to the cavern and deposit energy in all detector subsystems. Such events have played an important role in the commissioning of the detector since the start of the installation phase in 2005 and were particularly important for understanding the detector performance in the time prior to the arrival of the first LHC beams. Global cosmic-ray runs were undertaken in both 2008 and 2009 and these data have been used through to the early phases of collision data-taking as a tool for calibration, alignment and detector monitoring. These large datasets have also been used for detector performance studies, including investigations that rely on the combined performance of different subsystems. This paper presents the results of performance studies related to combined tracking, lepton identification and the reconstruction of jets and missing transverse energy. Results are compared to expectations based on a cosmic-ray event generator and a full simulation of the detector response.European Physical Journal C 04/2012; 71(3):1-36. · 3.63 Impact Factor -
Article: Measurement of the W → ℓν and Z/γ* → ℓℓ production cross sections in proton-proton collisions at with the ATLAS detector
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ABSTRACT: First measurements of the W → ℓν and Z/γ * → ℓℓ (ℓ = e, μ) production cross sections in proton-proton collisions at Ös = 7\textTeV \sqrt {s} = 7\;{\text{TeV}} are presented using data recorded by the ATLAS experiment at the LHC. The results are based on 2250 W → ℓν and 179 Z/γ * → ℓℓ candidate events selected from a data set corresponding to an integrated luminosity of approximately 320 nb. The measured total W and Z/γ ∗-boson production cross sections times the respective leptonic branching ratios for the combined electron and muon channels are sW\texttot \sigma_W^{\text{tot}} . BR(W → ℓν) = 9.96 ± 0.23(stat) ± 0.50(syst) ± 1.10(lumi) nb and sZ / g \texttot \sigma_{{{Z} \left/ {\gamma } \right.}}^{\text{tot}} BR(Z/γ ∗ → ℓℓ) = 0.82 ± 0.06 (stat) ± 0.05 (syst) ± 0.09(lumi) nb (within the invariant mass window 66 < m ℓℓ < 116GeV). The W/Z cross-section ratio is measured to be 11.7 ± 0.9(stat) ± 0.4(syst). In addition, measurements of the W + and W − production cross sections and of the lepton charge asymmetry are reported. Theoretical predictions based on NNLO QCD calculations are found to agree with the measurements. KeywordsHadron-Hadron ScatteringJournal of High Energy Physics 04/2012; 2010(12):1-65. · 5.83 Impact Factor -
Article: Limits on the production of the standard model Higgs boson in pp collisions at TeV with the ATLAS detector
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ABSTRACT: A search for the Standard Model Higgs boson at the Large Hadron Collider (LHC) running at a centre-of-mass energy of 7TeV is reported, based on a total integrated luminosity of up to 40pb−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 (ℓis e, μ) are combined in a mass range from 110GeV to 600GeV. The highest sensitivity is achieved in the mass range between 160GeV and 170GeV, 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 GeVto 185GeV.European Physical Journal C 04/2012; 71(9):1-30. · 3.63 Impact Factor -
Article: Measurement of the top quark-pair production cross section with ATLAS in pp collisions at TeV
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ABSTRACT: A measurement of the production cross-section for top quark pairs (t[`(t)]t\bar{t}) in pp collisions at Ös=7TeV\sqrt{s}=7\ \ \mbox{TeV} is presented using data recorded with the ATLAS detector at the Large Hadron Collider. Events are selected in two different topologies: single lepton (electron e or muon μ) with large missing transverse energy and at least four jets, and dilepton (ee, μμ or eμ) with large missing transverse energy and at least two jets. In a data sample of 2.9pb−1, 37 candidate events are observed in the single-lepton topology and 9 events in the dilepton topology. The corresponding expected backgrounds from non-t[`(t)]t\bar{t} Standard Model processes are estimated using data-driven methods and determined to be 12.2±3.9 events and 2.5±0.6 events, respectively. The kinematic properties of the selected events are consistent with SM t[`(t)]t\bar{t} production. The inclusive top quark pair production cross-section is measured to be st[`(t)]=145±31(stat.) +42-27(syst.)pb.\sigma_{t\bar{t}}=145\pm31(\mathrm{stat.}) {}^{+42}_{-27}(\mathrm {syst.})\ \mbox{pb}. The measurement agrees with perturbative QCD calculations.European Physical Journal C 04/2012; 71(3):1-36. · 3.63 Impact Factor -
Article: Measurement of inclusive jet and dijet cross sections in proton-proton collisions at 7 TeV centre-of-mass energy with the ATLAS detector
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ABSTRACT: Jet cross sections have been measured for the first time in proton-proton collisions at a centre-of-mass energy of 7TeV using the ATLAS detector. The measurement uses an integrated luminosity of 17nb−1 recorded at the Large Hadron Collider. The anti-k t algorithm is used to identify jets, with two jet resolution parameters, R=0.4 and 0.6. The dominant uncertainty comes from the jet energy scale, which is determined to within 7% for central jets above 60GeV transverse momentum. Inclusive single-jet differential cross sections are presented as functions of jet transverse momentum and rapidity. Dijet cross sections are presented as functions of dijet mass and the angular variableχ. The results are compared to expectations based on next-to-leading-order QCD, which agree with the data, providing a validation of the theory in a new kinematic regime.European Physical Journal C 04/2012; 71(2):1-59. · 3.63 Impact Factor -
Article: Observation of a new χ(b) state in radiative transitions to Υ(1S) and Υ(2S) at ATLAS.
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ABSTRACT: The χ(b)(nP) quarkonium states are produced in proton-proton collisions at the Large Hadron Collider at sqrt[s] = 7 TeV and recorded by the ATLAS detector. Using a data sample corresponding to an integrated luminosity of 4.4 fb(-1), these states are reconstructed through their radiative decays to Υ(1S,2S) with Υ → μ+ μ-. In addition to the mass peaks corresponding to the decay modes χ(b)(1P,2P) → Υ(1S)γ, a new structure centered at a mass of 10.530 ± 0.005(stat) ± 0.009(syst) GeV is also observed, in both the Υ(1S)γ and Υ(2S)γ decay modes. This structure is interpreted as the χ(b)(3P) system.Physical Review Letters 04/2012; 108(15):152001. · 7.37 Impact Factor
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Institutions
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2010–2013
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Universität Freiburg
- Faculty of Mathematics and Physics
Freiburg, Lower Saxony, Germany
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