• on behalf of the ATLAS Collaboration

      Articles written in Pramana – Journal of Physics

    • Standard Model Higgs boson searches with the ATLAS detector at the Large Hadron Collider

      Aleandro Nisati on behalf of the ATLAS Collaboration

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      The investigation of the mechanism responsible for electroweak symmetry breaking is one of the most important tasks of the scientific program of the Large Hadron Collider. The experimental results on the search of the Standard Model Higgs boson with 1 to 2 fb-1 of proton–proton collision data at $\sqrt{s} = 7$ TeV recorded by the ATLAS detector are presented and discussed. No significant excess of events is found with respect to the expectations from Standard Model processes, and the production of a Higgs boson is excluded at 95% Confidence Level for the mass regions 144–232, 256–282 and 296–466 GeV.

    • Measurements of the electron and muon inclusive cross-sections in proton–proton collisions at $\sqrt{s} = 7$ TeV with the ATLAS detector

      Moritz Backes on behalf of the ATLAS Collaboration

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      We present the measurements of the differential cross-sections for inclusive electron and muon production in proton–proton collisions at a centre-of-mass energy of $\sqrt{s} = 7$ TeV, using ∼ 1.4 pb-1 of data collected by the ATLAS detector at the Large Hadron Collider. The muon cross-section is measured as a function of muon transverse momentum $p_{\text{T}}$ in the range $4 < p_{\text{T}} < 100$ GeV and within pseudorapidity $|\eta| < 2.5$. In addition, the electron and muon cross-sections are measured in the range $7 < p_{\text{T}} < 26$ GeV and within $|\eta| < 2.0$, excluding $1.37 < |\eta| < 1.52$. After subtraction of the$ W/Z/\gamma^{*}$ contribution, the differential cross-sections are found to be in good agreement with theoretical predictions for heavy-flavour production obtained from fixed order NLO calculations with NLL high-$p_{\text{T}}$ resummation, and to be sensitive to the effects of NLL resummation.

    • Top physics with 0.70–1.08 fb-1 of $pp$ collisions with the ATLAS detector at the LHC

      Markus Cristinziani on behalf of the ATLAS Collaboration

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      With data collected during the first half of the 2011 $pp$ run of the Large Hadron Collider at $\sqrt{s} = 7$ TeV, a substantial data sample of high $p_{T}$ triggers, corresponding to an integrated luminosity of 1.08 fb-1, has been collected by the ATLAS detector. Measurements of the productions of top-quark pairs and single top quarks in different channels, the top-quark mass, the top-quark pair charge asymmetry and spin correlations, and the 𝑊 helicity fractions in top-quark decays are presented, as well as two searches for new physics ecffects involving top-quark pairs.

    • Search for new physics in dijet mass and angular distributions in $pp$ collisions at $\sqrt{s} = 7$ TeV measured with the ATLAS detector

      Thorsten Dietzsch on behalf of the ATLAS Collaboration

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      We present a search for physics beyond the Standard Model in proton–proton collisions at a centre-of-mass energy of $\sqrt{s} = 7$ TeV, performed with the ATLAS detector at the Large Hadron Collider (LHC). No evidence for new physics is found in dijet mass and angular distributions and stringent limits are set on a variety of models of new physics, including excited quarks, quark contact interactions, axigluons, and quantum black holes.

    • $W +$ jets in $pp$ collisions at 7 TeV with ATLAS

      Giovanni Zevi Della Porta on behalf of the ATLAS Collaboration

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      Measurements of differential cross-sections of 𝑊 boson production in association with jets, and jets containing 𝑏 hadrons are presented. The measurements are based on 35 pb-1 of $pp$ collisions at $\sqrt{s} = 7$ TeV collected with the ATLAS detector at the LHC, using the electron and muon decay channels of the 𝑊 boson. The data are found to be compatible with next-to-leading order predictions. In addition, a study of the invariant mass distribution of jet pairs produced in association with a 𝑊 boson is presented, based on an integrated luminosity of 1.02 fb-1. The measured dijet mass shows no significant excess over the Standard Model expectation.

    • Search with the ATLAS detector for new physics with significant missing transverse energy and two isolated leptons

      João Firmino Da Costa on behalf of the ATLAS Collaboration

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      Results of searches for supersymmetry in events with significant missing transverse energy and two isolated leptons with the ATLAS experiment at the LHC are presented. Three analyses are presented here, the first two are analyses with leptons of opposite charge and same charge, respectively. The third one is an analysis that searches for an excess of same-flavour oppositecharge lepton pairs over those of different-flavour. Data corresponding to an integrated luminosity of 1 fb-1 are analysed.

    • $W^{\pm} Z$ production in $pp$ collisions at 7 TeV with ATLAS

      Michael Kagan on behalf of the ATLAS Collaboration

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      The measurement of the $W^{\pm} Z$ production cross-section in proton–proton interactions at $\sqrt{s} = 7$ TeV is reported from the ATLAS experiment at the Large Hadron Collider using 1.02 fb-1 of data. The measurement is derived from events with leptonically decaying bosons with electrons, muons, and missing energy in the final state. Limits on anomalous triple gauge boson couplings are derived.

    • ATLAS fast physics monitoring

      Karsten Köneke on behalf of the ATLAS Collaboration

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      The ATLAS experiment at the Large Hadron Collider is recording data from proton–proton collisions at a centre-of-mass energy of 7 TeV since the spring of 2010. The integrated luminosity has grown nearly exponentially since then and continues to rise fast. The ATLAS Collaboration has set up a framework to automatically process the rapidly growing dataset and produce performance and physics plots for the most interesting analyses. The system is designed to give fast feedback. The histograms are produced within hours of data reconstruction (2–3 days after data taking). Hints of potentially interesting physics signals obtained this way are followed up by physics groups.

    • Triggering on hadronic tau decays: ATLAS meets the challenge

      Mark Scarcella on behalf of the ATLAS Collaboration

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      Hadronic tau decays play a crucial role in taking Standard Model (SM) measurements as well as in the search for physics beyond the SM. However, hadronic tau decays are difficult to identify and trigger on due to their resemblance to QCD jets. Given the large production crosssection of QCD processes, designing and operating a trigger system to efficiently select hadronic tau decays, while maintaining the rate within the bandwidth limits, is a difficult challenge. This contribution will summarize the status and performance of the ATLAS tau trigger system during the 2010–2011 data taking period. Different methods that have been explored to obtain the trigger efficiency curves from data will be shown. Finally, the status of the measurements, which include hadronic tau decays in the final state, will be summarized. In light of the vast statistics collected in 2011, future prospects for triggering on hadronic tau decays in this exciting new period of increased instantaneous luminosity will be presented.

    • Tau reconstruction, energy calibration and identification at ATLAS

      Michel Trottier-McDonald on behalf of the ATLAS Collaboration

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      Tau leptons play a central role in the LHC physics programme, in particular as an important signature in many Higgs boson and supersymmetry searches. They are further used in Standard Model electroweak measurements, as well as detector-related studies like the determination of the missing transverse energy scale. Copious backgrounds from QCD processes call for both efficient identification of hadronically decaying tau leptons, as well as large suppression of fake candidates. A solid understanding of the combined performance of the calorimeter and tracking detectors is also required. We present the current status of the tau reconstruction, energy calibration and identification with the ATLAS detector at the LHC. Identification efficiencies are measured in $W \rightarrow \tau \nu$ events in data and compared with predictions from Monte Carlo simulations, whereas the misidentification probabilities of QCD jets and electrons are determined from various jet-enriched data samples and from $Z \rightarrow ee$ events, respectively. The tau energy scale calibration is described and systematic uncertainties on both energy scale and identification efficiencies discussed.

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