Publications (96)251.76 Total impact
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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: 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: Performance of the ATLAS detector using first collision data
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ABSTRACT: More than half a million minimum-bias events of LHC collision data were collected by the ATLAS experiment in December 2009 at centre-of-mass energies of 0.9 TeV and 2.36 TeV. This paper reports on studies of the initial performance of the ATLAS detector from these data. Comparisons between data and Monte Carlo predictions are shown for distributions of several track-and calorimeter-based quantities. The good performance of the ATLAS detector in these first data gives confidence for successful running at higher energies. KeywordsHadron-Hadron ScatteringJournal of High Energy Physics 04/2012; 2010(9):1-66. · 5.83 Impact Factor -
Article: Readiness of the ATLAS liquid argon calorimeter for LHC collisions
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ABSTRACT: The ATLAS liquid argon calorimeter has been operating continuously since August 2006. At this time, only part of the calorimeter was readout, but since the beginning of 2008, all calorimeter cells have been connected to the ATLAS readout system in preparation for LHC collisions. This paper gives an overview of the liquid argon calorimeter performance measured in situ with random triggers, calibration data, cosmic muons, and LHC beam splash events. Results on the detector operation, timing performance, electronics noise, and gain stability are presented. High energy deposits from radiative cosmic muons and beam splash events allow to check the intrinsic constant term of the energy resolution. The uniformity of the electromagnetic barrel calorimeter response along η (averaged overφ) is measured at the percent level using minimum ionizing cosmic muons. Finally, studies of electromagnetic showers from radiative muons have been used to cross-check the Monte Carlo simulation. The performance results obtained using the ATLAS readout, data acquisition, and reconstruction software indicate that the liquid argon calorimeter is well-prepared for collisions at the dawn of the LHC era.European Physical Journal C 04/2012; 70(3):723-753. · 3.63 Impact Factor -
Article: Drift Time Measurement in the ATLAS Liquid Argon Electromagnetic Calorimeter using Cosmic Muons
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ABSTRACT: The ionization signals in the liquid argon of the ATLAS electromagnetic calorimeter are studied in detail using cosmic muons. In particular, the drift time of the ionization electrons is measured and used to assess the intrinsic uniformity of the calorimeter gaps and estimate its impact on the constant term of the energy resolution. The drift times of electrons in the cells of the second layer of the calorimeter are uniform at the level of 1.3% in the barrel and 2.8% in the endcaps. This leads to an estimated contribution to the constant term of (0.29+0.05-0.04)(0.29^{+0.05}_{-0.04})% in the barrel and (0.54+0.06-0.04)(0.54^{+0.06}_{-0.04})% in the endcaps. The same data are used to measure the drift velocity of ionization electrons in liquid argon, which is found to be 4.61±0.07mm/μs at 88.5K and 1kV/mm.European Physical Journal C 04/2012; 70(3):755-785. · 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 -
Article: Luminosity determination in pp collisions at TeV using the ATLAS detector at the LHC
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ABSTRACT: Measurements of luminosity obtained using the ATLAS detector during early running of the Large Hadron Collider (LHC) at Ös = 7\sqrt{s} = 7TeV are presented. The luminosity is independently determined using several detectors and multiple algorithms, each having different acceptances, systematic uncertainties and sensitivity to background. The ratios of the luminosities obtained from these methods are monitored as a function of time and of μ, the average number of inelastic interactions per bunch crossing. Residual time- and μ-dependence between the methods is less than 2% for 0<μ<2.5. Absolute luminosity calibrations, performed using beam separation scans, have a common systematic uncertainty of ±11%, dominated by the measurement of the LHC beam currents. After calibration, the luminosities obtained from the different methods differ by at most ±2%. The visible cross sections measured using the beam scans are compared to predictions obtained with the PYTHIA and PHOJET event generators and the ATLAS detector simulation.European Physical Journal C 04/2012; 71(4):1-37. · 3.63 Impact Factor -
Article: Search for the Standard Model Higgs boson in the decay channel H→ZZ→4ℓ with 4.8 fb-1 of pp collision data at s=7 TeV with ATLAS
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ABSTRACT: This Letter presents a search for the Standard Model Higgs boson in the decay channel H→ZZ→ℓ+ℓ-ℓℓ, where ℓ,ℓ′=e or μ, using proton-proton collisions at s=7 TeV recorded with the ATLAS detector and corresponding to an integrated luminosity of 4.8 fb-1. The four-lepton invariant mass distribution is compared with Standard Model background expectations to derive upper limits on the cross section of a Standard Model Higgs boson with a mass between 110 GeV and 600 GeV. The mass ranges 134-156 GeV, 182-233 GeV, 256-265 GeV and 268-415 GeV are excluded at the 95% confidence level. The largest upward deviations from the background-only hypothesis are observed for Higgs boson masses of 125 GeV, 244 GeV and 500 GeV with local significances of 2.1, 2.2 and 2.1 standard deviations, respectively. Once the look-elsewhere effect is considered, none of these excesses are significant.Physics Letters B 03/2012; 710:383-402. · 3.95 Impact Factor -
Article: Measurement of the centrality dependence of the charged particle pseudorapidity distribution in lead-lead collisions at sNN=2.76 TeV with the ATLAS detector
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ABSTRACT: The ATLAS experiment at the LHC has measured the centrality dependence of charged particle pseudorapidity distributions over |η|<2 in lead-lead collisions at a nucleon-nucleon centre-of-mass energy of sNN=2.76 TeV. In order to include particles with transverse momentum as low as 30 MeV, the data were recorded with the central solenoid magnet off. Charged particles were reconstructed with two algorithms (2-point "tracklets" and full tracks) using information from the pixel detector only. The lead-lead collision centrality was characterized by the total transverse energy in the forward calorimeter in the range 3.2<|η|<4.9. Measurements are presented of the per-event charged particle pseudorapidity distribution, dNch/dη, and the average charged particle multiplicity in the pseudorapidity interval |η|<0.5 in several intervals of collision centrality. The results are compared to previous mid-rapidity measurements at the LHC and RHIC. The variation of the mid-rapidity charged particle yield per colliding nucleon pair with the number of participants is consistent with lower sNN results. The shape of the dNch/dη distribution is found to be independent of centrality within the systematic uncertainties of the measurement.Physics Letters B 03/2012; 710:363-382. · 3.95 Impact Factor
Top co-authors
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Institutions
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2010–2012
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Universität Freiburg
- Faculty of Mathematics and Physics
Freiburg, Lower Saxony, Germany
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2000–2008
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Bergische Universität Wuppertal
Wuppertal, North Rhine-Westphalia, Germany
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