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.
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Browsing Lou, Xinchou by Author "Abbott, B."
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Item Centrality and Rapidity Dependence of Inclusive Jet Production in √{S_{NN}} = 5.02 TeV Proton-Lead Collisions with the ATLAS Detector(Elsevier B.V., 2015-07-17) Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdinov, O.; Aben, R.; Abi, B.; Abolins, M.; 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 the centrality and rapidity dependence of inclusive jet production in the √s̅̅ {̅N̅N̅}=5.02 TeV proton–lead (p+Pb) collisions and the jet cross-section in √s̅=2.76 TeV proton–proton collisions are presented. These quantities are measured in datasets corresponding to an integrated luminosity of 27.8 nb⁻¹ and 4.0 pb⁻¹, respectively, recorded with the ATLAS detector at the Large Hadron Collider in 2013. The p+Pb collision centrality was characterised using the total transverse energy measured in the pseudorapidity interval −4.9 < η < −3.2 in the direction of the lead beam. Results are presented for the double-differential per-collision yields as a function of jet rapidity and transverse momentum (p_T) for minimum-bias and centrality-selected p+Pb collisions, and are compared to the jet rate from the geometric expectation. The total jet yield in minimum-bias events is slightly enhanced above the expectation in a p_T-dependent manner but is consistent with the expectation within uncertainties. The ratios of jet spectra from different centrality selections show a strong modification of jet production at all p_T at forward rapidities and for large p_t at mid-rapidity, which manifests as a suppression of the jet yield in central events and an enhancement in peripheral events. These effects imply that the factorisation between hard and soft processes is violated at an unexpected level in proton–nucleus collisions. Furthermore, the modifications at forward rapidities are found to be a function of the total jet energy only, implying that the violations may have a simple dependence on the hard parton–parton kinematics.Item Comprehensive Measurements of T-Channel Single Top-Quark Production Cross Sections at √s = 7 TeV with the ATLAS Detector(American Physical Society, 2014-12-11) Aad, G.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdinov, O.; Aben, R.; Abi, B.; 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, XinchouThis article presents measurements of the t-channel single top-quark (t) and top-antiquark (t̅) total production cross sections σ(tq) and σ(t̅q), their ratio R_t= σ(tq) / σ(tq), and a measurement of the inclusive production cross section σ(tq+t̅q) in proton-proton collisions at √s = 7 TeV at the LHC. Differential cross sections for the tq and t̅q processes are measured as a function of the transverse momentum and the absolute value of the rapidity of t and t̅, respectively. The analyzed data set was recorded with the ATLAS detector and corresponds to an integrated luminosity of 4.59 fb⁻¹. Selected events contain one charged lepton, large missing transverse momentum, and two or three jets. The cross sections are measured by performing a binned maximum-likelihood fit to the output distributions of neural networks. The resulting measurements are σ(tq) = 46 ± 1(stat) ± 6(syst) pb, σ(t̅q )= 23 ± 1(stat) ± 3(syst) pb, R_t = 2.04 ± 0.13(stat) ± 0.12(syst), and σ(tq+t̅q) = 68 ± 2(stat) ± 8(syst) pb, consistent with the Standard Model expectation. The uncertainty on the measured cross sections is dominated by systematic uncertainties, while the uncertainty on R_t is mainly statistical. Using the ratio of σ(tq+t̅q) to its theoretical prediction, and assuming that the top-quark-related CKM matrix elements obey the relation |V_{tb} ≫ |V_{ts}|,|V_td|, we determine |V_{tb}| = 1.02 ± 0.07.Item Electron and Photon Energy Calibration with the ATLAS Detector using LHC Run 1 Data(Springer New York LLC, 2014-10-01) Aad, G.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdinov, O.; Aben, R.; Abi, B.; Izen, Joseph M.; Leyton, Michael; Lou, Xinchou; Namasivayam, Harisankar; Reeves, Kendall; ATLAS Collaboration; 56366505900 (Lou, X); 250674117 (Izen, JM); 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Namasivayam, Harisankar; Reeves, Kendall; Lou, XinchouThis paper presents the electron and photon energy calibration achieved with the ATLAS detector using about 25 fb⁻¹ of LHC proton−proton collision data taken at centre-of-mass energies of √s = 7 and 8 TeV. The reconstruction of electron and photon energies is optimised using multivariate algorithms. The response of the calorimeter layers is equalised in data and simulation, and the longitudinal profile of the electromagnetic showers is exploited to estimate the passive material in front of the calorimeter and reoptimise the detector simulation. After all corrections, the Z resonance is used to set the absolute energy scale. For electrons from Z decays, the achieved calibration is typically accurate to 0.05% in most of the detector acceptance, rising to 0.2% in regions with large amounts of passive material. The remaining inaccuracy is less than 0.2−1% for electrons with a transverse energy of 10 GeV, and is on average 0.3% for photons. The detector resolution is determined with a relative inaccuracy of less than 10% for electrons and photons up to 60 GeV transverse energy, rising to 40% for transverse energies above 500 GeV.Item Evidence for Electroweak Production of W^±W^±jj in pp Collisions at √s̅ = 8 TeV with the ATLAS Detector(2014-10-03) Aad, G.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdinov, O.; Aben, R.; Abi, B.; Reeves, Kendall; Namasivayam, Harisankar; Lou, Xinchou; Leyton, Michael; Izen, Joseph M.; ATLAS Collaboration; 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Namasivayam, Harisankar; Reeves, Kendall; Lou, XinchouThis Letter presents the first study of W^±W^±jj, same-electric-charge diboson production in association with two jets, using 20.3 fb⁻¹ of proton-proton collision data at √s̅ = 8 TeV recorded by the ATLAS detector at the Large Hadron Collider. Events with two reconstructed same-charge leptons (e^±e^±, e^±μ^±, and μ^±μ^±) and two or more jets are analyzed. Production cross sections are measured in two fiducial regions, with different sensitivities to the electroweak and strong production mechanisms. First evidence for W^±W^±jj production and electroweak-only W^±W^±jj production is observed with a significance of 4.5 and 3.6 standard deviations, respectively. The measured production cross sections are in agreement with standard model predictions. Limits at 95% confidence level are set on anomalous quartic gauge couplings.Item Fiducial and Differential Cross Sections of Higgs Boson Production Measured in the Four-Lepton Decay Channel in pp Collisions at √s̅ = 8 TeV with the ATLAS Detector(Elsevier B. V., 2014-09-28) Aad, G.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdinov, O.; Aben, R.; Abi, B.; 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); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Namasivayam, Harisankar; Reeves, Kendall; Lou, XinchouMeasurements of fiducial and differential cross sections of Higgs boson production in the H → ZZ* → 4ℓ decay channel are presented. The cross sections are determined within a fiducial phase space and corrected for detection efficiency and resolution effects. They are based on 20.3 fb⁻¹ of pp collision data, produced at √s̅ = 8 TeV centre-of-mass energy at the LHC and recorded by the ATLAS detector. The differential measurements are performed in bins of transverse momentum and rapidity of the four-lepton system, the invariant mass of the subleading lepton pair and the decay angle of the leading lepton pair with respect to the beam line in the four-lepton rest frame, as well as the number of jets and the transverse momentum of the leading jet. The measured cross sections are compared to selected theoretical calculations of the Standard Model expectations. No significant deviation from any of the tested predictions is found..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 Light-Quark and Gluon Jet Discrimination in pp Collisions at √s̅ = 7 TeV with the ATLAS Detector(Springer, 2014-08-21) Aad, G.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdinov, O.; Aben, R.; Abi, B.; Reeves, Kendall; Namasivayam, Harisankar; Lou, Xinchou; Leyton, Michael; Izen, Joseph M.; 0000-0003-0867-2189 (Lou, X); 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Namasivayam, Harisankar; Reeves, Kendall; Lou, XinchouA 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 √s̅ = 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.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 Distributions Sensitive to the Underlying Event in Inclusive Z-Boson Production in pp Collisions at √s = 7 TeV with the ATLAS Detector(Springer New York LLC, 2014-12-10) Aad, G.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdinov, O.; Aben, R.; Abi, B.; 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, XinchouA measurement of charged-particle distributions sensitive to the properties of the underlying event is presented for an inclusive sample of events containing a Z-boson, decaying to an electron or muon pair. The measurement is based on data collected using the ATLAS detector at the LHC in proton-proton collisions at a centre-of-mass energy of 7 TeV with an integrated luminosity of 4.6 fb⁻¹. Distributions of the charged particle multiplicity and of the charged particle transverse momentum are measured in regions of azimuthal angle defined with respect to the Z-boson direction. The measured distributions are compared to similar distributions measured in jet events, and to the predictions of various Monte Carlo generators implementing different underlying event models.Item Measurement of Flow Harmonics with Multi-Particle Cumulants in Pb + Pb Collisions at √{S_{NN}} = 2.76 TeV with the ATLAS Detector(Springer, 2014-11-26) Aad, G.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdinov, O.; Aben, R.; Abi, B.; 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, XinchouATLAS measurements of the azimuthal anisotropy in lead-lead collisions at √sNN = 2.76 TeV are shown using a dataset of approximately 7 μb⁻¹ collected at the LHC in 2010. The measurements are performed for charged particles with transverse momenta 0.5 < p_T < 20 GeV and in the pseudorapidity range |η| < 2.5. The anisotropy is characterized by the Fourier coefficients, v_n, of the charged-particle azimuthal angle distribution for n = 2-4. The Fourier coefficients are evaluated using multi-particle cumulants calculated with the generating function method. Results on the transverse momentum, pseudorapidity and centrality dependence of the v_n coefficients are presented. The elliptic flow, v₂, is obtained from the two-, four-, six- and eight-particle cumulants while higher-order coefficients, v₃ and v₄, are determined with two-and four-particle cumulants. Flow harmonics v(n) measured with four-particle cumulants are significantly reduced compared to the measurement involving two-particle cumulants. A comparison to v_n measurements obtained using different analysis methods and previously reported by the LHC experiments is also shown. Results of measurements of flow fluctuations evaluated with multiparticle cumulants are shown as a function of transverse momentum and the collision centrality. Models of the initial spatial geometry and its fluctuations fail to describe the flow fluctuations measurements.Item Measurement of Higgs Boson Production in the Diphoton Decay Channel in pp Collisions at Center-Of-Mass Energies of 7 and 8 TeV with the ATLAS Detector(American Physical Society, 2014-12-24) Aad, G.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdinov, O.; Aben, R.; Abi, B.; 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, XinchouA measurement of the production processes of the recently discovered Higgs boson is performed in the two-photon final state using 4.5 fb⁻¹ of proton-proton collisions data at √s = 7 TeV and 20.3 fb⁻¹ at √s = 8 TeV collected by the ATLAS detector at the Large Hadron Collider. The number of observed Higgs boson decays to diphotons divided by the corresponding Standard Model prediction, called the signal strength, is found to be μ=1.17 ± 0.27 at the value of the Higgs boson mass measured by ATLAS, m_H = 125.4 GeV. The analysis is optimized to measure the signal strengths for individual Higgs boson production processes at this value of m_H. They are found to be μ_{ggF} = 1.32 ± 0.38, μ_{VBF} = 0.8 ± 0.7, μ_{WH} = 1.0 ± 1.6, μ_{ZH} = 0.1{−0.1} ^{+3.7}, and μ_{tt̅H} = 1.6 {−1.8} ^{+ 2.7} , for Higgs boson production through gluon fusion, vector-boson fusion, and in association with a W or Z boson or a top-quark pair, respectively. Compared with the previously published ATLAS analysis, the results reported here also benefit from a new energy calibration procedure for photons and the subsequent reduction of the systematic uncertainty on the diphoton mass resolution. No significant deviations from the predictions of the Standard Model are found.Item Measurement of Inclusive Jet Charged-Particle Fragmentation Functions in pb + pb Collisions at √{S_{NN}} = 2.76 TeV with the ATLAS Detector(Elsevier, 2014-11-04) Aad, G.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdinov, O.; Aben, R.; Abi, B.; 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 charged-particle fragmentation functions of jets produced in ultra-relativistic nuclear collisions can provide insight into the modification of parton showers in the hot, dense medium created in the collisions. ATLAS has measured jets in √{S_{NN}} = 2.76 TeV Pb+Pb collisions at the LHC using a data set recorded in 2011 with an integrated luminosity of 0.14 nb⁻¹. Jets were reconstructed using the anti-k_t algorithm with distance parameter values R = 0.2, 0.3, and 0.4. Distributions of charged-particle transverse momentum and longitudinal momentum fraction are reported for seven bins in collision centrality for R = 0.4 jets with p_T^{jet} > 100 GeV. Commensurate minimum p_T values are used for the other radii. Ratios of fragment distributions in each centrality bin to those measured in the most peripheral bin are presented. These ratios show a reduction of fragment yield in central collisions relative to peripheral collisions at intermediate z values, 0.04 ≲ z ≲ 0.2, and an enhancement in fragment yield for z less than or similar to 0.04. A smaller, less significant enhancement is observed at large z and large p_T in central collisions.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 Centrality and Pseudorapidity Dependence of the Integrated Elliptic Flow in Lead−Lead Collisions at \sqrt{^sNN} = 2.76 TeV with the ATLAS Detector(Springer New York LLC, 2014-08-13) Aad, G.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdinov, O.; Aben, R.; Abi, B.; 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 integrated elliptic flow of charged particles produced in Pb+Pb collisions at √^SNN 2.76 TeV has been measured with the ATLAS detector using data collected at the Large Hadron Collider. The anisotropy parameter, v₂, was measured in the pseudorapidity range |η| ≤ 2.5 with the event-plane method. In order to include tracks with very low transverse momentum pT, thus reducing the uncertainty in v₂ integrated over p_T, a 1 μb⁻¹ data sample recorded without a magnetic field in the tracking detectors is used. The centrality dependence of the integrated v₂ is compared to other measurements obtained with higher p_T thresholds. The integrated elliptic flow is weakly decreasing with |η|. The integrated v₂ transformed to the rest frame of one of the colliding nuclei is compared to the lower-energy RHIC data.Item Measurement of the Cross Section of High Transverse Momentum Z → bb̅ Production in Proton-Proton Collisions at √s = 8 TeV with the ATLAS Detector(Elsevier, 2014-09-16) Aad, G.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdinov, O.; Aben, R.; Abi, B.; Izen, Joseph M.; Leyton, Michael; Lou, Xinchou; Namasivayam, Harisankar; Reeves, Kendall; 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Namasivayam, Harisankar; Reeves, Kendall; Lou, XinchouThis Letter reports the observation of a high transverse momentum Z → bb̅ signal in proton-proton collisions at √s = 8 TeV and the measurement of its production cross section. The data analysed were collected in 2012 with the ATLAS detector at the LHC and correspond to an integrated luminosity of 19.5 fb⁻¹. The Z → bb̅ decay is reconstructed from a pair of b-tagged jets, clustered with the anti- k_t jet algorithm with R = 0.4, that have low angular separation and form a dijet with p_T > 200 GeV. The signal yield is extracted from a fit to the dijet invariant mass distribution, with the dominant, multi-jet background mass shape estimated by employing a fully data-driven technique that reduces the dependence of the analysis on simulation. The fiducial cross section is determined to be σ_{Z → bb̅} ^{fid} = 2.02 ± 0.20 (stat.) ± 0.25 (syst.) ± 0.06 (lumi.) pb = 2.02 ± 0.33 pb, in good agreement with next-to-leading-order theoretical predictions.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 Higgs Boson Mass from the H → γγ and H → ZZ* → 4ℓ Channels in pp Collisions at Center-Of-Mass Energies of 7 and 8 TeV with the ATLAS Detector(American Physical Society, 2014-09-09) Aad, G.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdinov, O.; Aben, R.; Abi, B.; 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, XinchouAn improved measurement of the mass of the Higgs boson is derived from a combined fit to the reconstructed invariant mass spectra of the decay channels H → γγ and H → ZZ∗ → 4ℓ. The analysis uses the pp collision data sample recorded by the ATLAS experiment at the CERN Large Hadron Collider at center-of-mass energies of 7 TeV and 8 TeV, corresponding to an integrated luminosity of 25 fb⁻¹. The measured value of the Higgs boson mass is m_H = 125.36 ± 0.37(stat) ± 0.18(syst) GeV. This result is based on improved energy-scale calibrations for photons, electrons, and muons as well as other analysis improvements, and supersedes the previous result from ATLAS. Upper limits on the total width of the Higgs boson are derived from fits to the invariant mass spectra of the H → γγ and H → ZZ∗ → 4ℓ decay channels.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 Inclusive Jet Cross-Section in Proton-Proton Collisions at √s̅ = 7 TeV using 4.5 fb⁻¹ of Data with the ATLAS Detector(2015-02-24) Aad, G.; Abbott, B.; Abdallah, J.; Khalek, S. Abdel; Abdinov, O.; Aben, R.; Abi, B.; Abolins, M.; Izen, J. M.; Leyton, Michael; Lou, Xinchou; Meirose, Bernhard; Namasivayam, Harisankar; ATLAS Collaboration; 0000-0003-0867-2189 (Lou, X.); 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, KendallThe inclusive jet cross-section is measured in proton-proton collisions at a centre-of-mass energy of 7 TeV using a data set corresponding to an integrated luminosity of 4.5 fb⁻¹ collected with the ATLAS detector at the Large Hadron Collider in 2011. Jets are identified using the anti-kT algorithm with radius parameter values of 0.4 and 0.6. The double-differential cross-sections are presented as a function of the jet transverse momentum and the jet rapidity, covering jet transverse momenta from 100 GeV to 2 TeV. Next-to-leading-order QCD calculations corrected for non-perturbative effects and electroweak effects, as well as Monte Carlo simulations with next-to-leading-order matrix elements interfaced to parton showering, are compared to the measured cross-sections. A quantitative comparison of the measured cross-sections to the QCD calcula tions using several sets of parton distribution functions is performed.