Engineering Papers⌕ Search

Engineering topics

Alexandre, G.

Publications and source records attributed to Alexandre, G..

At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
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.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Search for top quark decays t → qH with H → γγ using the ATLAS detector

A search is performed for flavour-changing neutral currents in the decay of a top quark to an up-type ( c, u ) quark and a Higgs boson, where the Higgs boson decays to two photons. The proton-proton collision data set used corresponds to 4.7 fb -1 at $ \sqrt{s}$ = 7 TeV and 20.3 fb -1 at $ \sqrt{s}$ = 8 TeV collected by the ATLAS experiment at the LHC. Top quark pair events are searched for in which one top quark decays to qH and the other decays to bW . Both the hadronic and the leptonic decay modes of the W boson are used. No significant signal is observed and an upper limit is set on the t → qH branching ratio of 0.79 at the 95% confidence level. The corresponding limit on the t qH coupling combination $ \sqrt{λ ^2_{tcH} + λ^2_{tuH}}$ is 0.17.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Measurement of the parity-violating asymmetry parameter α b and the helicity amplitudes for the decay Λ b 0 → J / ψ Λ 0 with the ATLAS detector

A measurement of the parity-violating decay asymmetry parameter, α b , and the helicity amplitudes for the decay Λ$ ^0_b $→J/ψ(μ + μ - )Λ 0 (pπ - ) is reported. The analysis is based on 1400 Λ$ ^0_b $ and $ \bar{Λ} ^0_b $ baryons selected in 4.6 fb -1 of proton-proton collision data with a center-of-mass energy of 7 TeV recorded by the ATLAS experiment at the LHC. By combining the Λ$ ^0_b $ and $ \bar{Λ} ^0_b $ samples under the assumption of CP conservation, the value of α b is measured to be 0.30 ± 0.16(stat) ± 0.06(syst). This measurement provides a test of theoretical models based on perturbative QCD or heavy-quark effective theory.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Search for Invisible Decays of a Higgs Boson Produced in Association with a Z Boson in ATLAS

A search for evidence of invisible-particle decay modes of a Higgs boson produced in association with a Z boson at the Large Hadron Collider is presented. No deviation from the standard model expectation is observed in 4.5 fb -1 (20.3 fb -1 ) of 7 (8) TeV pp collision data collected by the ATLAS experiment. Assuming the standard model rate for ZH production, an upper limit of 75%, at the 95% confidence level is set on the branching ratio to invisible-particle decay modes of the Higgs boson at a mass of 125.5 GeV. The limit on the branching ratio is also interpreted in terms of an upper limit on the allowed dark matter-nucleon scattering cross section within a Higgs-portal dark matter scenario. Within the constraints of such a scenario, the results presented in this Letter provide the strongest available limits for low-mass dark matter candidates. Limits are also set on an additional neutral Higgs boson, in the mass range 110 < m H < 400 GeV, produced in association with a Z boson and decaying to invisible particles.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Measurement of the production of a W boson in association with a charm quark in pp collisions at $ \sqrt{s} $ = 7 TeV with the ATLAS detector

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 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 σ(W + + $ \bar{c} $)/σ(W - +c) is compared to the predictions obtained using parton distribution function parameterisations with different assumptions about the s–$ \bar{s} $ quark asymmetry.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Search for direct production of charginos, neutralinos and sleptons in final states with two leptons and missing transverse momentum in $pp$ collisions at $ \sqrt{s} $ = 8TeV with the ATLAS detector

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} $ = 8TeV recorded 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.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Measurement of dijet cross-sections in pp collisions at 7 TeV centre-of-mass energy using the ATLAS detector

Double-differential dijet cross-sections measured in pp collisions at the LHC with a 7 TeV centre-of-mass energy are presented as functions of dijet mass and half the rapidity separation of the two highest-p T jets. These measurements are obtained using data corresponding to an integrated luminosity of 4.5 fb -1 , recorded by the ATLAS detector in 2011. The data are corrected for detector effects so that cross-sections are presented at the particle level. Cross-sections are measured up to 5TeV dijet mass using jets reconstructed with the anti- k t algorithm for values of the jet radius parameter of 0.4 and 0.6. The cross-sections are compared with next-to-leading-order perturbative QCD calculations by NLOJet++ corrected to account for non-perturbative effects. Comparisons with POWHEG predictions, using a next-to-leading-order matrix element calculation interfaced to a partonshower Monte Carlo simulation, are also shown. Electroweak effects are accounted for in both cases. The quantitative comparison of data and theoretical predictions obtained using various parameterizations of the parton distribution functions is performed using a frequentist method. In general, good agreement with data is observed for the NLOJet++ theoretical predictions when using the CT10, NNPDF2.1 and MSTW 2008 PDF sets. Disagreement is observed when using the ABM11 and HERAPDF1.5 PDF sets for some ranges of dijet mass and half the rapidity separation. An example setting a lower limit on the compositeness scale for a model of contact interactions is presented, showing that the unfolded results can be used to constrain contributions to dijet production beyond that predicted by the Standard Model.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in $ \sqrt{s} $ = 8 TeV pp collisions with the ATLAS detector

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} $ = 8TeV proton-proton collision data delivered by the Large Hadron Collider and recorded 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 (&sim;$ \tilde{X} ^± _1 $) and next-to-lightest neutralino ($ \tilde{X} ^0 _2 $) production with decays to lightest neutralino (&sim;$ \tilde{X} ^0 _1 $) via either all three generations of sleptons, staus only, gauge bosons, or Higgs bosons, $ \tilde{X} ^± _1 $ and $ \tilde{X} ^0 _2 $ masses are excluded up to 700GeV, 380GeV, 345GeV, or 148GeV respectively, for a massless $ \tilde{X} ^0 _1 $.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Measurement of the production cross section of prompt J/ψ mesons in association with a W ± boson in pp collisions at $ \sqrt{s} $ = 7 TeV with the ATLAS detector

The process pp → W ± J/ψ provides a powerful probe of the production mechanism of charmonium in hadronic collisions, and is also sensitive to multiple parton interactions in the colliding protons. Using the 2011 ATLAS dataset of 4.5 fb -1 of $ \sqrt{s} $ = 7 TeV pp collisions at the LHC, the first observation is made of the production of W ± + prompt J/ψ events in hadronic collisions, using W ± → μν μ and J/ψ → μ + μ - . A yield of 27.4$ ^{+7.5} _{-6.5} $ W ± + prompt J/ψ events is observed, with a statistical significance of 5.1σ. The production rate as a ratio to the inclusive W ± boson production rate is measured, and the double parton scattering contribution to the cross section is estimated.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Study of heavy-flavor quarks produced in association with top-quark pairs at s = 7 TeV using the ATLAS detector

Using a sample of dilepton top-quark pair (t$\bar{t}$) candidate events, a study is performed of the production of top-quark pairs together with heavy-flavor (HF) quarks, the sum of t$\bar{t}$+b+X and t$\bar{t}$+c+X, collectively referred to as t$\bar{t}$+HF. The data set used corresponds to an integrated luminosity of 4.7 fb -1 of proton-proton collisions at a center-of-mass energy of 7 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. The presence of additional HF (b or c) quarks in the t$\bar{t}$ sample is inferred by looking for events with at least three b-tagged jets, where two are attributed to the b quarks from the t$\bar{t}$decays and the third to additional HF production. The dominant background to t$\bar{t}$+HF in this sample is t$\bar{t}$+jet events in which a light-flavor jet is misidentified as a heavy-flavor jet. To determine the heavy- and light-flavor content of the additional b -tagged jets, a fit to the vertex mass distribution of b -tagged jets in the sample is performed. The result of the fit shows that 79±14(stat)±22(syst) of the 105 selected extra b -tagged jets originate from HF quarks, 3 standard deviations away from the hypothesis of zero t$\bar{t}$+HF production. The result for extra HF production is quoted as a ratio (R HF ) of the cross section for t$\bar{t}$+HF production to the cross section for t$\bar{t}$ production with at least one additional jet. Both cross sections are measured in a fiducial kinematic region within the ATLAS acceptance. R HF is measured to be [6.2±1.1(stat)±1.8(syst)]% for jets with p T >25GeV and |η|<2.5, in agreement with the expectations from Monte Carlo generators.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Measurement of the electroweak production of dijets in association with a Z-boson and distributions sensitive to vector boson fusion in proton-proton collisions at $ \sqrt{s} $ = 8 TeV using the ATLAS detector

Measurements of fiducial cross sections for the electroweak production of two jets in association with a Z-boson are presented. The measurements are performed using 20.3 fb -1 of proton-proton collision data collected at a centre-of-mass energy of √s = 8 TeV by the ATLAS experiment at the Large Hadron Collider. The electroweak component is extracted by a fit to the dijet invariant mass distribution in a fiducial region chosen to enhance the electroweak contribution over the dominant background in which the jets are produced via the strong interaction. The electroweak cross sections measured in two fiducial regions are in good agreement with the Standard Model expectations and the background-only hypothesis is rejected with significance above the 5σ level. The electroweak process includes the vector boson fusion production of a Z-boson and the data are used to place limits on anomalous triple gauge boson couplings. In addition, measurements of cross sections and differential distributions for inclusive Z-boson-plus-dijet production are performed in five fiducial regions, each with different sensitivity to the electroweak contribution. The results are corrected for detector effects and compared to predictions from the Sherpa and Powheg event generators.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Measurement of the inclusive isolated prompt photons cross section in p p collisions at s = 7 TeV with the ATLAS detector using 4.6 fb - 1

A measurement of the cross section for the production of isolated prompt photons in pp collisions at a center-of-mass energy $ \sqrt{s}$ =7 TeV is presented. The results are based on an integrated luminosity of 4.6 fb -1 collected with the ATLAS detector at the LHC. The cross section is measured as a function of photon pseudorapidity ηγ and transverse energy E γ T in the kinematic range 100 ≤ E γ T < 1000 GeV and in the regions |η γ | < 1.37 and 1.52 ≤ |η γ | < 2.37. The results are compared to leading-order parton-shower Monte Carlo models and next-to-leading-order perturbative QCD calculations. Next-to-leading-order perturbative QCD calculations agree well with the measured cross sections as a function of E γ T and η γ .

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Search for Quantum Black Hole Production in High-Invariant-Mass Lepton + Jet Final States Using p p Collisions at s = 8 TeV and the ATLAS Detector

This Letter presents a search for quantum black-hole production using 20.3 fb -1 of data collected with the ATLAS detector in $pp$ collisions at the LHC at $ \sqrt{s}$ = 8 TeV. The quantum black holes are assumed to decay into a final state characterized by a lepton (electron or muon) and a jet. In either channel, no event with a lepton-jet invariant mass of 3.5 TeV or more is observed, consistent with the expected background. Limits are set on the product of cross sections and branching fractions for the lepton + jet final states of quantum black holes produced in a search region for invariant masses above 1 TeV. The combined 95% confidence level upper limit on this product for quantum black holes with threshold mass above 3.5 TeV is 0.18 fb. This limit constrains the threshold quantum black-hole mass to be above 5.3 TeV in the model considered.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗