Browsing by Author "Abdelalim, A. A."
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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 the Azimuthal Anisotropy for Charged Particle Production in √{S_{NN}} = 2.76 TeV Lead-Lead Collisions with the ATLAS Detector(American Physical Society, 2012-07-24) Aad, G.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdelalim, A. A.; Abdesselam, A.; Abdinov, O.; Ahsan, Mahsana; Izen, Joseph M.; Lou, Xinchou; Reeves, Kendall; 0000-0003-0867-2189 (Lou, X); 250674117 (Izen, JM); 250674117 (Izen, JM); Lou, Xinchou; Izen, Joseph M.Differential measurements of charged particle azimuthal anisotropy are presented for lead-lead collisions at √{S_{NN}} = 2.76 TeV with the ATLAS detector at the LHC, based on an integrated luminosity of approximately 8 μb⁻¹. This anisotropy is characterized via a Fourier expansion of the distribution of charged particles in azimuthal angle relative to the reaction plane, with the coefficients vn denoting the magnitude of the anisotropy. Significant v₂–v₆ values are obtained as a function of transverse momentum (0.5 < p_T < 20 GeV), pseudorapidity (|η| < 2.5), and centrality using an event plane method. The ν_n values for n ≥ 3 are found to vary weakly with both η and centrality, and their p_T dependencies are found to follow an approximate scaling relation, v_{1/n / n} (p_T) ∝ v{1/2 / 2} (p_T), except in the top 5% most central collisions. A Fourier analysis of the charged particle pair distribution in relative azimuthal angle (Δφ = φ_a − φ_b) is performed to extract the coefficients v_{n,n} = (cos nΔφ). For pairs of charged particles with a large pseudorapidity gap (|Δη = η_a−η_b| > 2) and one particle with pT < 3 GeV, the v₂,₂ – v₆,₆ values are found to factorize as v_{n,n}(p_T^a, pp_T^b) ≈ v_n(p_T^a) ν_n (p_T^b) in central and midcentral events. Such factorization suggests that these values of v₂,₂ – v₆,₆ are primarily attributable to the response of the created matter to the fluctuations in the geometry of the initial state. A detailed study shows that the v₁,₁ (p_T^a,p_T^b) data are consistent with the combined contributions from a rapidity-even v₁ and global momentum conservation. A two-component fit is used to extract the v₁ contribution. The extracted v₁ is observed to cross zero at p_T ≈ 1.0 GeV, reaches a maximum at 4–5 GeV with a value comparable to that for v₃, and decreases at higher p_T.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 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 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 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 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 Magnetic Monopoles in √s=7 TeV pp Collisions with the ATLAS Detector(American Physical Society, 2012-12-27) 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 Letter presents a search for magnetic monopoles with the atlas detector at the CERN Large Hadron Collider using an integrated luminosity of 2.0 fb⁻¹ of pp collisions recorded at a center-of-mass energy of √s=7 TeV. No event is found in the signal region, leading to an upper limit on the production cross section at 95% confidence level of 1.6∈ fb for Dirac magnetic monopoles with the minimum unit magnetic charge and with mass between 200 GeV and 1500 GeV, where ∈ is the monopole reconstruction efficiency. The efficiency ∈ is high and uniform in the fiducial region given by pseudorapidity |η| <1.37 and transverse kinetic energy 600–700 < E^{kin} sinθ < 1400 GeV. The minimum value of 700 GeV is for monopoles of mass 200 GeV, whereas the minimum value of 600 GeV is applicable for higher mass monopoles. Therefore, the upper limit on the production cross section at 95% confidence level is 2 fb in this fiducial region. Assuming the kinematic distributions from Drell-Yan pair production of spin-1/2 Dirac magnetic monopoles, the efficiency is in the range 1%–10%, leading to an upper limit on the cross section at 95% confidence level that varies from 145 fb to 16 fb for monopoles with mass between 200 GeV and 1200 GeV. This limit is weaker than the fiducial limit because most of these monopoles lie outside the fiducial region.Item Search for Resonant Diboson Production in the WW/WZ → ℓνjj Decay Channels with the ATLAS Detector at √s=7 TeV(American Physical Society, 2013-06-17) 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-ChengA search for resonant diboson production using a data sample corresponding to 4.7 fb⁻¹ of integrated luminosity collected by the ATLAS experiment at the Large Hadron Collider in pp collisions at √s=7 TeV is presented. The search for a narrow resonance in the WW or WZ mass distribution is conducted in a final state with an electron or a muon, missing transverse momentum, and at least two jets. No significant excess is observed and limits are set using three benchmark models: WW resonance masses below 940 and 710 GeV are excluded at 95% confidence level for spin-2 Randall-Sundrum and bulk Randall-Sundrum gravitons, respectively; WZ resonance masses below 950 GeV are excluded at 95% confidence level for a spin-1 extended gauge model W ′ boson.Item Search for Squarks and Gluinos with the ATLAS Detector in Final States with Jets and Missing Transverse Momentum Using 4.7 fb⁻¹ of √s̅ = 7 TeV Proton-Proton Collision Data(American Physical Society, 2013-01-22) Aad, G.; Abajyan, T.; Abbott, B.; Abdallah, J.; Khalek, S. Abdel; 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-ChengA search for squarks and gluinos in final states containing jets, missing transverse momentum and no high-p_T electrons or muons is presented. The data represent the complete sample recorded in 2011 by the ATLAS experiment in 7 TeV proton-proton collisions at the Large Hadron Collider, with a total integrated luminosity of 4.7 fb⁻¹. No excess above the Standard Model background expectation is observed. Gluino masses below 860 GeV and squark masses below 1320 GeV are excluded at the 95% confidence level in simplified models containing only squarks of the first two generations, a gluino octet and a massless neutralino, for squark or gluino masses below 2 TeV, respectively. Squarks and gluinos with equal masses below 1410 GeV are excluded. In minimal supergravity/constrained minimal supersymmetric Standard Model models with tan β = 10, A₀ = ⋅0 and μ > 0, squarks and gluinos of equal mass are excluded for masses below 1360 GeV. Constraints are also placed on the parameter space of supersymmetric models with compressed spectra. These limits considerably extend the region of supersymmetric parameter space excluded by previous measurements with the ATLAS detector.Item Study of Heavy-Flavor Quarks Produced in Association with Top-Quark Pairs at √s̅=7 TeV using the ATLAS Detector(American Physical Society, 2014-04-21) Aad, G.; Abajyan, T.; Abbott, B.; Abdallah, J.; Khalek, S. Abdel; Abdelalim, A. A.; Abdinov, O.; Ashan, Mahsana; Namasivayam, Harisankar; Reeves, Kendall; Wong, Wei-Cheng; Izen, Joseph M.; Lou, Xinchou; 250674117 (Izen, JM); 250674117 (Izen, JM); Ashan, Mahsana; Namasivayam, Harisankar; Reeves, Kendall; Wong, Wei-Cheng; Izen, Joseph M.; Lou, XinchouUsing a sample of dilepton top-quark pair (tt̅) candidate events, a study is performed of the production of top-quark pairs together with heavy-flavor (HF) quarks, the sum of tt̅ + b + X and tt̅ + c + X, collectively referred to as tt̅ + 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 tt̅ sample is inferred by looking for events with at least three b-tagged jets, where two are attributed to the b quarks from the tt̅ decays and the third to additional HF production. The dominant background to tt̅ + HF in this sample is tt̅ + 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 + HF production. The result for extra HF production is quoted as a ratio (R-HF) of the cross section for tt̅ + HF production to the cross section for tt̅ 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 > 25 GeV and |η| < 2.5, in agreement with the expectations from Monte Carlo generators.