Lou, Xinchou
Permanent URI for this collectionhttps://hdl.handle.net/10735.1/2305
Dr. Lou’s research interests focus on elementary particle physics experiments at electron-position colliders, high performance cluster computing, and application of data mining techniques to other fields of research. He is currently searching for new elementary particles, measuring charmonium production with the BABAR detector located at the Stanford Linear Accelerator Center.
Browse
Browsing Lou, Xinchou by Author "Abajyan, T."
Now showing 1 - 20 of 26
- Results Per Page
- Sort Options
Item Jet Energy Measurement and its Systematic Uncertainty in Proton-Proton Collisions at √s̅ = 7 TeV with the ATLAS Detector(Springer, 2015-01-15) Aad, G.; Abajyan, T.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdinov, O.; Aben, R.; Haleem, M.; Izen, Joseph M.; Lou, Xinchou; Namasivayam, Harisankar; Reeves, Kendall; Wong, Wei-Cheng; ATLAS Collaboration; 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Namasivayam, Harisankar; Reeves, Kendall; Lou, XinchouThe jet energy scale (JES) and its systematic uncertainty are determined for jets measured with the ATLAS detector using proton-proton collision data with a centre-of-mass energy of √s̅ = 7 TeV corresponding to an integrated luminosity of 4.7 fb⁻¹. Jets are reconstructed from energy deposits forming topological clusters of alorimeter cells using the anti-_k algorithm with distance parameters R = 0.4 or R = 0.6, and are calibrated using MC simulations. A residual JES correction is applied to account for differences between data and MC simulations. This correction and its systematic uncertainty are estimated using a combination of in situ techniques exploiting the transverse momentum balance between a jet and a reference object such as a photon or a Z boson, for 20 ≤ p_T ^{jet} < 1000 GeV and pseudorapidities |η| < 4.5. The effect of multiple proton-proton interactions is corrected for, and an uncertainty is evaluated using in situ techniques. The smallest JES uncertainty of less than 1% is found in the central calorimeter region (|η| < 1.2) for jets with 55 ≤ p{T}^{jet} < 500 GeV. For central jets at lower p{T}, the uncertainty is about 3%. A consistent JES estimate is found using measurements of the calorimeter response of single hadrons in proton?proton collisions and test-beam data, which also provide the estimate for p{T}^{jet} > 1 TeV. The calibration of forward jets is derived from dijet p{T} balance measurements. The resulting uncertainty reaches its largest value of 6% for low-p{T} jets at |η| = 4.5. Additional JES uncertainties due to specific event topologies, such as close-by jets or selections of event samples with an enhanced content of jets originating from light quarks or gluons, are also discussed. The magnitude of these uncertainties depends on the event sample used in a given physics analysis, but typically amounts to 0.5-3%.Item Measurement of Charged-Particle Event Shape Variables in Inclusive √(s̅) = 7 TeV Proton-Proton Interactions with the ATLAS Detector(American Physical Society, 2013-08-06) Aad, G.; Abajyan, T.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdelalim, A. A.; Abdinov, O.; Ahsan, Mahsana; Izen, Joseph M .; Lou, Xinchou; Reeves, Kendall; Wong, Wei-Cheng; ATLAS Collaboration; 0000-0003-0867-2189 (Lou, X); 250674117 (Izen, JM); 250674117 (Izen, JM); |Ahsan, Mahsana; Izen, Joseph M.; Lou, Xinchou; Reeves, Kendall; Wong, Wei-ChengThe measurement of charged-particle event shape variables is presented in inclusive inelastic pp collisions at a center-of-mass energy of 7 TeV using the ATLAS detector at the LHC. The observables studied are the transverse thrust, thrust minor, and transverse sphericity, each defined using the final-state charged particles' momentum components perpendicular to the beam direction. Events with at least six charged particles are selected by a minimum-bias trigger. In addition to the differential distributions, the evolution of each event shape variable as a function of the leading charged-particle transverse momentum, charged-particle multiplicity, and summed transverse momentum is presented. Predictions from several Monte Carlo models show significant deviations from data.Item Measurement of Dijet Cross-Sections in pp Collisions at 7 TeV Centre-of-Mass Energy Using the ATLAS Detector(Springer, 2014-05-014) Aad, G.; Abajyan, T.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdinov, O.; Aben, R.; Ashan, Mahsana; Haleem, M.; Izen, Joseph M.; Lou, Xinchou; Namasivayam, Harisankar; Reeves, Kendall; Wong, Wei-Cheng; ATLAS Collaboration; 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Namasivayam, Harisankar; Reeves, Kendall; Lou, Xinchou; Wong, Wei-ChengDouble-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 5 TeV 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 parton-shower 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.Item 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 √s=8 TeV using the ATLAS Detector(Springer, 2014-04-07) Aad, G.; Abajyan, T.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdinov, O.; Aben, R.; Izen, Joseph M.; Leyton, Michael; Lou, Xinchou; Namasivayam, Harisankar; Reeves, Kendall; ATLAS Collaboration; 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Namasivayam, Harisankar; Reeves, Kendall; Lou, XinchouMeasurements 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.Item Measurement of the Inclusive Isolated Prompt Photons Cross Section in pp Collisions at √s=7 TeV with the ATLAS Detector Using 4.6 fb⁻¹(American Physical Society, 2014-03-24) Aad, G.; Abajyan, T.; Abbott, B.; Abdallah, J.; Khalek, S. Abdel; Abdelalim, A. A.; Abdinov, O.; Wong, Wei-Cheng; Reeves, Kendall; Lou, Xinchou; Izen, Joseph M .; Namasivayam, Harisankar; ATLAS Collaboration; 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Namasivayam, Harisankar; Reeves, Kendall; Lou, Xinchou; Wong, Wei-ChengA measurement of the cross section for the production of isolated prompt photons in pp collisions at a center-of-mass energy √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 η^γ and transverse energy E_T^γ in the kinematic range 100 ≤ E_T^γ < 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_T^γ and η^γ.Item Measurement of the Low-Mass Drell-Yan Differential Cross Section at √s=7 TeV Using the ATLAS Detector(Springer, 2014-06-18) Aad, G.; Abajyan, T.; Abbott, B.; Abdallah, J.; Khalek, S. Abdel; Abdinov, O.; Aben, R.; Izen, Joseph M.; Leyton, Michael; Lou, Xinchou; Namasivayam, Harisankar; Reeves, Kendall; ATLAS Collaboration; 0000-0003-0867-2189 (Lou, X); 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Namasivayam, Harisankar; Reeves, Kendall; Lou, XinchouThe differential cross section for the process Z/γ^∗ → ℓℓ (ℓ = e, μ) as a function of dilepton invariant mass is measured in pp collisions at s√ = 7 TeV at the LHC using the ATLAS detector. The measurement is performed in the e and μ channels for invariant masses between 26 GeV and 66 GeV using an integrated luminosity of 1.6 fb⁻¹ collected in 2011 and these measurements are combined. The analysis is extended to invariant masses as low as 12 GeV in the muon channel using 35 pb⁻¹ of data collected in 2010. The cross sections are determined within fiducial acceptance regions and corrections to extrapolate the measurements to the full kinematic range are provided. Next-to-next-to-leading-order QCD predictions provide a significantly better description of the results than next-to-leading-order QCD calculations, unless the latter are matched to a parton shower calculation.Item Measurement of the Parity-Violating Asymmetry Parameter α_b and the Helicity Amplitudes for the Decay Λ_b^0 → J/ψΛ⁰ with the ATLAS Detector(American Physical Society, 2014-05-27) Aad, G.; Abajyan, T.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdinov, O.; Aben, R.; Izen, Joseph M.; Leyton, Michael; Lou, Xinchou; Namasivayam, Harisankar; Reeves, Kendall; ATLAS Collaboration; 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Namasivayam, Harisankar; Reeves, Kendall; Lou, XinchouA measurement of the parity-violating decay asymmetry parameter, α_b, and the helicity amplitudes for the decay Λ_b^0 → J/ψ(μ⁺μ⁻)Λ⁰(pπ⁻) is reported. The analysis is based on 1400 Λ_b^0 and Λ̅_b^0 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_b^0 and Λ̅_b^0 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.Item Measurement of the Production Cross Section of Jets in Association with a Z Boson in pp Collisions at √s=7 TeV with the ATLAS Detector(Springer, 2013-07-05) Aad, G.; Abajyan, T.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdelalim, A. A.; Abdinov, O.; Ahsan, Mahsana; Izen, Joseph M.; Lou, Xinchou; Namasivayam, Harisankar; Reeves, Kendall; Wong, Wei-Cheng; ATLAS Collaboration; 250674117 (Izen, JM); 250674117 (Izen, JM); |Ahsan, Mahsana; Izen, Joseph M.; Lou, Xinchou; Reeves, Kendall; Wong, Wei-Cheng; Namasivayam, HarisankarMeasurements of the production of jets of particles in association with a Z boson in pp collisions at √s=7 TeV are presented, using data corresponding to an integrated luminosity of 4.6 fb⁻¹ collected by the ATLAS experiment at the Large Hadron Collider. Inclusive and differential jet cross sections in Z events, with Z decaying into electron or muon pairs, are measured for jets with transverse momentum p_T > 30 GeV and rapidity |y| < 4.4. The results are compared to next-to-leading-order perturbative QCD calculations, and to predictions from different Monte Carlo generators based on leading-order and next-to-leading-order matrix elements supplemented by parton showers.Item Measurement of the Production Cross Section of Prompt J/ψ Mesons in Association with a W^± Boson in pp Collisions √s=7 TeV with the ATLAS Detector(Springer, 2014-04-28) Aad, G.; Abajyan, T.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdinov, O.; Aben, R.; Haleem, M.; Izen, Joseph M.; Leyton, Michael; Lou, Xinchou; Namasivayam, Harisankar; Reeves, Kendall; Wong, Wei-Cheng; 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Namasivayam, Harisankar; Reeves, Kendall; Lou, Xinchou; Wong, Wei-ChengThe 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 √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_(-6.5)^(+7.5) 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.Item Measurement of the Production of a W Boson in Association with a Charm Quark in pp Collisions at √s=7 TeV with the ATLAS Detector(Springer, 2014-05-16) Aad, G.; Abajyan, T.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdinov, O.; Aben, R.; Izen, Joseph M.; Leyton, Michael; Lou, Xinchou; Namasivayam, Harisankar; Reeves, Kendall; ATLAS Collaboration; 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Namasivayam, Harisankar; Reeves, Kendall; Lou, XinchouThe production of a W boson in association with a single charm quark is studied using 4.6 fb⁻¹ of pp collision data at√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₋{-0.30}^{+0.26} at Q² = 1.9 GeV², which supports the hypothesis of an SU(3)-symmetric composition of the light-quark sea. Additionally, the cross-section ratio σ(W⁺ +c̅) / σ(W⁻ +c) is compared to the predictions obtained using parton distribution function parameterisations with different assumptions about the s-s̅ quark asymmetry.Item Measurement of the t-Channel Single Top-Quark Production Cross Section in pp Collisions at √s = 7 TeV with the ATLAS Detector(Elsevier B. V., 2012-09-17) Aad, G.; Abajyan, T.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdelalim, A. A.; Abdinov, O.; Ahsan, Mahsana; Izen, Joseph M.; Lou, Xinchou; Reeves, Kendall; Wong, Wei-Cheng; ATLAS Collaboration; 250674117 (Izen, JM); 250674117 (Izen, JM); |Ahsan, Mahsana; Izen, Joseph M.; Lou, Xinchou; Reeves, Kendall; Wong, Wei-ChengWe report a measurement of the cross section of single top-quark production in the t-channel using 1.04 fb⁻¹ of pp collision data at √s̅ = 7 TeV recorded with the ATLAS detector at the LHC. Selected events feature one electron or muon, missing transverse momentum, and two or three jets, exactly one of them identified as originating from a b quark. The cross section is measured by fitting the distribution of a multivariate discriminant constructed with a neural network, yielding σ_t = 83 ±4 (stat.) _{-19} ^{+20}(syst.) pb, which is in good agreement with the prediction of the Standard Model. Using the ratio of the measured to the theoretically predicted cross section and assuming that the top-quark-related CKM matrix elements obey the relation |V_{tb}| » |V_{ts}|, |V_{td}|, the coupling strength at the W-t-b vertex is determined to be |V_{tb}| = 1.13_{-0.13}^{+0.14}. If it is assumed that |V_{tb}| ≤ 1 a lower limit of |V_{tb}| > 0.75 is obtained at the 95% confidence level.Item Measurement of the Top Quark Pair Production Charge Asymmetry in Proton-Proton Collisions at √s̅ = 7 TeV Using the ATLAS Detector(Springer, 2014-02-25) Aad, G.; Abajyan, T.; Abbott, B.; Abdallah, J.; Khalek, S. Abdel; Abdinov, O.; Aben, R.; Namasivayam, Harisankar; Izen, Joseph M.; Lou, Xinchou; Reeves, Kendall; Wong, Wei-Cheng; Haleem, M.; ATLAS Collaboration; 0000-0003-0867-2189 (Lou, X); 250674117 (Izen, JM); 250674117 (Izen, JM); |Ahsan, Mahsana; Izen, Joseph M.; Lou, Xinchou; Reeves, Kendall; Wong, Wei-Cheng; Namasivayam, HarisankarThis paper presents a measurement of the top quark pair (tt̅) production charge asymmetry A_C using 4.7 fb−1 of proton-proton collisions at a centre-of-mass energy √s̅ = 7 TeV collected by the ATLAS detector at the LHC. A tt̅ -enriched sample of events with a single lepton (electron or muon), missing transverse momentum and at least four high transverse momentum jets, of which at least one is tagged as coming from a b-quark, is selected. A likelihood fit is used to reconstruct the tt̅ event kinematics. A Bayesian unfolding procedure is employed to estimate A_C at the parton-level. The measured value of the tt̅ production charge asymmetry is A_C = 0.006 ± 0.010, where the uncertainty includes both the statistical and the systematic components. Differential A_C measurements as a function of the invariant mass, the rapidity and the transverse momentum of the tt̅ system are also presented. In addition, A_C is measured for a subset of events with large tt̅ velocity, where physics beyond the Standard Model could contribute. All measurements are consistent with the Standard Model predictions.Item Measurement of the Underlying Event in Jet Events from 7 TeV Proton−Proton Collisions with the ATLAS Detector(Springer New York LLC, 2014-08-12) Aad, G.; Abajyan, T.; Abbott, B.; Abdallah, J.; Khalek, S. A.; Abdinov, O.; Aben, R.; Izen, Joseph M.; Leyton, Michael; Lou, Xinchou; Namasivayam, Harisankar; Reeves, Kendall; ATLAS Collaboration; 0000-0003-0867-2189 (Lou, X); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Namasivayam, Harisankar; Reeves, Kendall; Lou, XinchouDistributions 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.Item Measurement of W⁺W⁻ Production in pp Collisions at √s=7 TeV with the ATLAS Detector and Limits on Anomalous WWZ and WWγ Couplings(American Physical Society, 2013-06-03) Aad, G.; Abajyan, T.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdelalim, A. A.; Abdinov, O.; Ahsan, Mahsana; Izen, Joseph M.; Lou, Xinchou; Reeves, Kendall; Wong, Wei-Cheng; ATLAS Collaboration; 250674117 (Izen, JM); 250674117 (Izen, JM); |Ahsan, Mahsana; Izen, Joseph M.; Lou, Xinchou; Reeves, Kendall; Wong, Wei-ChengThis paper presents a measurement of the W⁺W⁻ production cross section in pp collisions at √s=7 TeV. The leptonic decay channels are analyzed using data corresponding to an integrated luminosity of 4.6 fb⁻¹ collected with the ATLAS detector at the Large Hadron Collider. The W⁺W⁻ production cross section σ(pp → W⁺W⁻ + X) is measured to be 51.9 ± 2.0(stat) ± 3.9(syst) ± 2.0(lumi) pb, compatible with the Standard Model prediction of 44.7_{-1.9}^{+2.1} pb. A measurement of the normalized fiducial cross section as a function of the leading lepton transverse momentum is also presented. The reconstructed transverse momentum distribution of the leading lepton is used to extract limits on anomalous WWZ and WWγ couplings.Item Measurements of Normalized Differential Cross Sections for tt̅ Production in pp Collisions at √s = 7 TeV using the ATLAS Detector(American Physical Society, 2014-10-13) Aad, G.; Abajyan, T.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdinov, O.; Aben, R.; Izen, Joseph M.; Leyton, Michael; Lou, Xinchou; Namasivayam, Harisankar; Reeves, Kendall; ATLAS Collaboration; 0000-0003-0867-2189 (Lou, X); 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Namasivayam, Harisankar; Reeves, Kendall; Lou, XinchouMeasurements of normalized differential cross sections for top-quark pair production are presented as a function of the top-quark transverse momentum, and of the mass, transverse momentum, and rapidity of the tt̅ system, in proton proton collisions at a center-of-mass energy of √s = 7 TeV. The data set corresponds to an integrated luminosity of 4.6 fb⁻¹, recorded in 2011 with the ATLAS detector at the CERN Large Hadron Collider. Events are selected in the lepton + jets channel, requiring exactly one lepton and at least four jets with at least one of the jets tagged as originating from a b-quark. The measured spectra are corrected for detector efficiency and resolution effects and are compared to several Monte Carlo simulations and theory calculations. The results are in fair agreement with the predictions in a wide kinematic range. Nevertheless, data distributions are softer than predicted for higher values of the mass of the tt̅ system and of the top-quark transverse momentum. The measurements can also discriminate among different sets of parton distribution functions.Item Measurements of Wγ and Zγ Production in pp Collisions at √s = 7 TeV with the ATLAS Detector at the LHC(American Physical Society, 2013-06-04) Aad, G.; Abajyan, T.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdelalim, A. A.; Abdinov, O.; Ahsan, Mahsana; Izen, Joseph M.; Lou, Xinchou; Namasivayam, Harisankar; Reeves, Kendall; Wong, Wei-Cheng; ATLAS Collaboration; 250674117 (Izen, JM); 250674117 (Izen, JM); |Ahsan, Mahsana; Izen, Joseph M.; Lou, Xinchou; Reeves, Kendall; Wong, Wei-Cheng; Namasivayam, HarisankarThe integrated and differential fiducial cross sections for the production of a W or Z boson in association with a high-energy photon are measured using pp collisions at √s = 7 TeV. The analyses use a data sample with an integrated luminosity of 4.6 fb⁻¹ collected by the ATLAS detector during the 2011 LHC data-taking period. Events are selected using leptonic decays of the W and Z bosons [W(eν, μν) and Z(e⁺e⁻,μ⁺μ⁻, νν¯ )] with the requirement of an associated isolated photon. The data are used to test the electroweak sector of the Standard Model and search for evidence for new phenomena. The measurements are used to probe the anomalous WWγ, ZZγ, and Zγγ triple-gauge-boson couplings and to search for the production of vector resonances decaying to Zγ and Wγ. No deviations from Standard Model predictions are observed and limits are placed on anomalous triple-gauge-boson couplings and on the production of new vector meson resonances.Item Muon Reconstruction Efficiency and Momentum Resolution of the ATLAS Experiment in Proton−Proton Collisions at √s =7 TeV in 2010(Springer New York LLC, 2014-09-16) Aad, G.; Abajyan, T.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdelalim, A. A.; Abdinov, O.; Ahsan, Mahsana; Izen, Joseph M.; Leyton, Michael; Lou, Xinchou; Namasivayam, Harisankar; Reeves, Kendall; Wong, Wei-Cheng; ATLAS Collaboration; 0000-0003-0867-2189 (Lou, X); 250674117 (Izen, JM); 250674117 (Izen, JM); Ahsan, Mahsana; Izen, Joseph M.; Leyton, Michael; Lou, Xinchou; Namasivayam, Harisankar; Reeves, Kendall; Wong, Wei-ChengThis 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⁻¹d. The measured performance is compared to Monte Carlo predictions and deviations from the predicted performance are discussed.Item Search for Dark Matter Candidates and Large Extra Dimensions in Events with a Photon and Missing Transverse Momentum in pp Collision Data at √s̅ = 7 TeV with the ATLAS Detector(American Physical Society, 2013-01-03) Aad, G.; Abajyan, T.; Abbott, B.; Abdallah, J.; Khalek, S. A.; Abdelalim, A. A.; Abdinov, O.; Ahsan, Mahsana; Izen, Joseph M.; Lou, Xinchou; Reeves, Kendall; Wong, Wei-Cheng; ATLAS Collaboration; 0000-0003-0867-2189 (Lou, X); 250674117 (Izen, JM); 250674117 (Izen, JM); |Ahsan, Mahsana; Izen, Joseph M.; Lou, Xinchou; Reeves, Kendall; Wong, Wei-ChengResults of a search for new phenomena in events with an energetic photon and large missing transverse momentum in proton-proton collisions at √s̅ = 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.Item Search for Dark Matter in Events with a Hadronically Decaying W or Z Boson and Missing Transverse Momentum in pp Collisions at √s̅ = 8 TeV with the ATLAS Detector(American Physical Society, 2014-01-29) Aad, G.; Abajyan, T.; Abbott, B.; Abdallah, J.; Khalek, S. Abdel; Abdinov, O.; Aben, R.; Ahsan, Mahsana; Izen, Joseph M.; Lou, Xinchou; Reeves, Kendall; Wong, Wei-Cheng; Namasivayam, Harisankar; ATLAS Collaboration; 0000-0003-0867-2189 (Lou, X); 250674117 (Izen, JM); 250674117 (Izen, JM); |Ahsan, Mahsana; Izen, Joseph M.; Lou, Xinchou; Reeves, Kendall; Wong, Wei-ChengA search is presented for dark matter pair production in association with a W or Z boson in pp collisions representing 20.3 fb(-1) of integrated luminosity at √s = 8 TeV using data recorded with the ATLAS detector at the Large Hadron Collider. Events with a hadronic jet with the jet mass consistent with a W or Z boson, and with large missing transverse momentum are analyzed. The data are consistent with the standard model expectations. Limits are set on the mass scale in effective field theories that describe the interaction of dark matter and standard model particles, and on the cross section of Higgs production and decay to invisible particles. In addition, cross section limits on the anomalous production of W or Z bosons with large missing transverse momentum are set in two fiducial regions.Item Search for Dark Matter in Events with a Z Boson and Missing Transverse Momentum in pp Collisions at √s=8 TeV with the ATLAS Detector(American Physical Society, 2014-07-10) Aad, G.; Abajyan, T.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdinov, O.; Aben, R.; Izen, Joseph M.; Lou, Xinchou; Reeves, Kendall; Leyton, Michael; Namasivayam, Harisankar; ATLAS Collaboration; 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Namasivayam, Harisankar; Reeves, Kendall; Lou, XinchouA 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 √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.