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Anger, P.

Publications and source records attributed to Anger, P..

At least 271 records · Page 15
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DOE OSTI2014

Search for high-mass dilepton resonances in p p collisions at s = 8 TeV with the ATLAS detector

Here, the ATLAS detector at the Large Hadron Collider is used to search for high-mass resonances decaying to dielectron or dimuon final states. Results are presented from an analysis of proton-proton (pp) collisions at a center-of-mass energy of 8 TeV corresponding to an integrated luminosity of 20.3 fb –1 in the dimuon channel. A narrow resonance with Standard Model Z couplings to fermions is excluded at 95% confidence level for masses less than 2.79 TeV in the dielectron channel, 2.53 TeV in the dimuon channel, and 2.90 TeV in the two channels combined. Limits on other model interpretations are also presented, including a grand-unification model based on the E 6 gauge group, Z* bosons, minimal Z' models, a spin-2 graviton excitation from Randall-Sundrum models, quantum black holes, and a minimal walking technicolor model with a composite Higgs boson.

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DOE OSTI2014

Search for supersymmetry in events with large missing transverse momentum, jets, and at least one tau lepton in 20 fb -1 of $ \sqrt{s} $ = 8 TeV proton-proton collision data with the ATLAS detector

A search for supersymmetry (SUSY) in events with large missing transverse momentum, jets, at least one hadronically decaying tau lepton and zero or one additional light leptons (electron/muon), has been performed using 20.3fb -1 of proton-proton collision data at $ \sqrt{s} $ = 8 TeV recorded with the ATLAS detector at the Large Hadron Collider. No excess above the Standard Model background expectation is observed in the various signal regions and 95% confidence level upper limits on the visible cross section for new phenomena are set. The results of the analysis are interpreted in several SUSY scenarios, significantly extending previous limits obtained in the same final states. In the framework of minimal gauge-mediated SUSY breaking models, values of the SUSY breaking scale Λ below 63 TeV are excluded, independently of tan β. Exclusion limits are also derived for an mSUGRA/CMSSM model, in both the R-parity-conserving and R-parity-violating case. A further interpretation is presented in a framework of natural gauge mediation, in which the gluino is assumed to be the only light coloured sparticle and gluino masses below 1090 GeV are excluded.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Muon reconstruction efficiency and momentum resolution of the ATLAS experiment in proton–proton collisions at √s = 7 TeV in 2010

This paper presents a study of the performance of the muon reconstruction in the analysis of proton–proton collisions at √s = 7 TeV at the LHC, recorded by the ATLAS detector in 2010. This performance is described in terms of reconstruction and isolation efficiencies and momentum resolutions for different classes of reconstructed muons. The results are obtained from an analysis of J/ψ meson and Z boson decays to dimuons, reconstructed from a data sample corresponding to an integrated luminosity of 40 pb -1 . The measured performance is compared to Monte Carlo predictions and deviations from the predicted performance are discussed.

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DOE OSTI2014

Search for new particles in events with one lepton and missing transverse momentum in pp collisions at $ \sqrt{s} $ = 8 TeV with the ATLAS detector

This paper presents a search for new particles in events with one lepton (electron or muon) and missing transverse momentum using 20.3 fb -1 of proton-proton collision data at $ \sqrt{s} $ = 8 TeV recorded by the ATLAS experiment at the Large Hadron Collider. No significant excess beyond Standard Model expectations is observed. A W' with Sequential Standard Model couplings is excluded at the 95% confidence level for masses up to 3.24 TeV. Excited chiral bosons (W*) with equivalent coupling strengths are excluded for masses up to 3.21 TeV. In the framework of an effective field theory limits are also set on the dark matter-nucleon scattering cross-section as well as the mass scale M* of the unknown mediating interaction for dark matter pair production in association with a leptonically decaying W.[Figure not available: see fulltext.]

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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 $ \sqrt{s} $ = 8 TeV with the ATLAS detector

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 $ \bar{t} $ → t $ ^{-0} _ { χ_1 } $ or t → b $ ^{-±} _ { χ_1 } $ → bW(*)$ ^{-0} _ { χ_1 } $, where $ ^{-0} _ { χ_1 } $ ($ ^{-±} _ { χ_1 } $) 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 $ \bar{t} $ → t $ ^{-0} _ { χ_1 } $. For a branching fraction of 100%, top squark masses in the range 270-645 GeV are excluded for $ ^{-0} _ { χ_1 } $ masses below 30GeV. For a branching fraction of 50% to either $ \bar{t} $ → t $ ^{-0} _ { χ_1 } $ or $ \bar{t} $ → b $ ^{-±} _ { χ_1 } $, and assuming the $ ^{-±} _ { χ_1 } $ mass to be twice the $ ^{-0} _ { χ_1 } $ mass, top squark masses in the range 250-550 GeV are excluded for $ ^{-0} _ { χ_1 } $ masses below 60 GeV.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Light-quark and gluon jet discrimination in pp collisions at √s = 7 TeV with the ATLAS detector

A likelihood-based discriminant for the identification of quark- and gluon-initiated jets is built and validated using 4.7 fb -1 of proton–proton collision data at √s = 7 TeV collected with the ATLAS detector at the LHC. Data samples with enriched quark or gluon content are used in the construction and validation of templates of jet properties that are the input to the likelihood-based discriminant. The discriminating power of the jet tagger is established in both data and Monte Carlo samples within a systematic uncertainty of ≈ 10–20 %. In data, light-quark jets can be tagged with an efficiency of ≈ 50% while achieving a gluon-jet mis-tag rate of ≈ 25% in a p T range between 40 GeV and 360 GeV for jets in the acceptance of the tracker. The rejection of gluon-jets found in the data is significantly below what is attainable using a Pythia 6 Monte Carlo simulation, where gluon-jet mis-tag rates of 10 % can be reached for a 50 % selection efficiency of light-quark jets using the same jet properties.

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DOE OSTI2014

Search for microscopic black holes and string balls in final states with leptons and jets with the ATLAS detector at s $ \sqrt{s} $ = 8 TeV

A search for an excess of events with multiple high transverse momentum objects including charged leptons and jets is presented, using 20.3 fb -1 of proton-proton collision data recorded by the ATLAS detector at the Large Hadron Collider in 2012 at a centre-of-mass energy of $ \sqrt{s} $ = 8 TeV. No excess of events beyond Standard Model expectations is observed. Using extra-dimensional models for black hole and string ball production and decay, exclusion contours are determined as a function of the mass threshold for production and the fundamental gravity scale for two, four and six extra dimensions. For six extra dimensions, mass thresholds of 4.8-6.2 TeV are excluded at 95 % confidence level, depending on the fundamental gravity scale and model assumptions. Upper limits on the fiducial cross-sections for non-Standard Model production of these final states are set.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Measurement of the centrality and pseudorapidity dependence of the integrated elliptic flow in lead–lead collisions at $\sqrt{s_{\mathrm {NN}}}=2.76$ TeV with the ATLAS detector

The integrated elliptic flow of charged particles produced in Pb+Pb collisions at $\sqrt{s_{\mathrm {NN}}} TeV has been measured with the ATLAS detector using data collected at the Large Hadron Collider. The anisotropy parameter, v 2 , was measured in the pseudorapidity range |η| ≤ 2.5 with the event-plane method. In order to include tracks with very low transverse momentum p T , thus reducing the uncertainty in v 2 integrated over p T , a 1 μ b -1 data sample recorded without a magnetic field in the tracking detectors is used. The centrality dependence of the integrated v 2 is compared to other measurements obtained with higher p T thresholds. The integrated elliptic flow is weakly decreasing with |η|. The integrated v 2 transformed to the rest frame of one of the colliding nuclei is compared to the lower-energy RHIC data.

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DOE OSTI2014

Measurement of event-plane correlations in s NN = 2.76 TeV lead-lead collisions with the ATLAS detector

A measurement of event-plane correlations involving two or three event planes of different order is presented as a function of centrality for 7 μb -1 Pb+Pb collision data at s NN = 2.76 TeV, recorded by the ATLAS experiment at the Large Hadron Collider. Fourteen correlators are measured using a standard event-plane method and a scalar-product method, and the latter method is found to give a systematically larger correlation signal. Several different trends in the centrality dependence of these correlators are observed. These trends are not reproduced by predictions based on the Glauber model, which includes only the correlations from the collision geometry in the initial state. Calculations that include the final-state collective dynamics are able to describe qualitatively, and in some cases also quantitatively, the centrality dependence of the measured correlators. These observations suggest that both the fluctuations in the initial geometry and the nonlinear mixing between different harmonics in the final state are important for creating these correlations in momentum space.

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DOE OSTI2014

Measurement of the underlying event in jet events from 7 TeV proton–proton collisions with the ATLAS detector

Distributions sensitive to the underlying event in QCD jet events have been measured with the ATLAS detector at the LHC, based on 37 pb -1 of proton–proton collision data collected at a centre-of-mass energy of 7 TeV. Charged-particle mean p T and densities of all-particle E T and charged-particle multiplicity and p T have been measured in regions azimuthally transverse to the hardest jet in each event. These are presented both as one-dimensional distributions and with their mean values as functions of the leading-jet transverse momentum from 20 to 800 GeV. The correlation of charged-particle mean p T with charged-particle multiplicity is also studied, and the E T densities include the forward rapidity region; these features provide extra data constraints for Monte Carlo modelling of colour reconnection and beam-remnant effects respectively. For the first time, underlying event observables have been computed separately for inclusive jet and exclusive dijet event selections, allowing more detailed study of the interplay of multiple partonic scattering and QCD radiation contributions to the underlying event. Comparisons to the predictions of different Monte Carlo models show a need for further model tuning, but the standard approach is found to generally reproduce the features of the underlying event in both types of event selection.

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DOE OSTI2014

Electron reconstruction and identification efficiency measurements with the ATLAS detector using the 2011 LHC proton–proton collision data

Many of the interesting physics processes to be measured at the LHC have a signature involving one or more isolated electrons. The electron reconstruction and identification efficiencies of the ATLAS detector at the LHC have been evaluated using proton–proton collision data collected in 2011 at √s = 7 TeV and corresponding to an integrated luminosity of 4.7 fb -1 . Tag-and-probe methods using events with leptonic decays of W and Z bosons and J/ψ mesons are employed to benchmark these performance parameters. The combination of all measurements results in identification efficiencies determined with an accuracy at the few per mil level for electron transverse energy greater than 30 GeV.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Search for dark matter in events with a Z boson and missing transverse momentum in p p collisions at s = 8 TeV with the ATLAS detector

A search is presented for production of dark-matter particles recoiling against a leptonically decaying Z boson in 20.3 fb -1 of pp collisions at $ \sqrt{s} $=8 TeV with the ATLAS detector at the Large Hadron Collider. Events with large missing transverse momentum and two oppositely charged electrons or muons consistent with the decay of a Z boson are analyzed. No excess above the Standard Model prediction is observed. Limits are set on the mass scale of the contact interaction as a function of the dark-matter particle mass using an effective field theory description of the interaction of dark matter with quarks or with Z bosons. Limits are also set on the coupling and mediator mass of a model in which the interaction is mediated by a scalar particle.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

The differential production cross section of the $\phi $ (1020) meson in $\sqrt{s}$ TeV $pp$ collisions measured with the ATLAS detector

Ameasurement is presented of the $\phi $×BR($\phi $ →K < sup > + < /sup > K < sup > - < /sup > ) production cross section at √s = 7 TeV using pp collision data corresponding to an integrated luminosity of 383 μb -1 , collected with the ATLAS experiment at the HC. Selection of $\phi $(1020) mesons is based on the identification of charged kaons by their energy loss in the pixel detector. The differential cross section ismeasured as a function of the transverse momentum, pT,$\phi $ , and rapidity, y$\phi $, of the $\phi $(1020) meson in the fiducial region 500 < pT,$\phi $ < 1200MeV, |y$\phi $ | < 0.8, kaon p T,K > 230 MeV and kaon momentum p K < 800 MeV. The integrated $\phi $(1020)-meson production cross section in this fiducial range is measured to be sφ×BR($\phi $ →K < sup > + < /sup > K < sup > - < /sup > ) = 570 ± 8 (stat) ± 66 (syst) ± 20 (lumi) μb.

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 $ \sqrt{s} $ = 8 TeV with the ATLAS detector

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. [Figure not available: see fulltext.]

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

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

Measurements of four-lepton (4,$\ell$,$\ell$=$e,μ$) production cross sections at the Z resonance in pp collisions at the LHC with the ATLAS detector are presented. For dilepton and four-lepton invariant mass regions m &ell;+&ell;- >5 GeV and 80 < m 4&ell; < 100 Gev, the measured cross sections are 76 ± 18(stat) ±4 (syst) ±1.4(lumi) fb and 107 ±9 (stat) ±4 (syst) ±3.0(lumi) fb at $ \sqrt{s} $ = 7 and 8 Tev, respectively. By subtracting the nonresonant 4&ell; production contributions and normalizing with Z→μ + μ - events, the branching fraction for the Z boson decay to 4&ell; is determined to be (3.20±0.25(stat)±0.13(syst))×10 -6 , consistent with the standard model prediction.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

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

A search for strongly produced supersymmetric particles is conducted using signatures involving multiple energetic jets and either two isolated leptons (e or μ) 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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