Browsing by Author "Abi, 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 Determination of the Top-Quark Pole Mass Using tt̅+1-Jet Events Collected with the ATLAS Experiment in 7 TeV pp Collisions(Springer, 2015-10-19) Aad, G.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdinov, O.; Aben, R.; Abi, B.; Izen, Joseph M.; Leyton, Michael; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, Kendall; ATLAS Collaboration; 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, KendallThe normalized differential cross section for top-quark pair production in association with at least one jet is studied as a function of the inverse of the invariant mass of the tt̅ + 1-jet system. This distribution can be used for a precise determination of the top-quark mass since gluon radiation depends on the mass of the quarks. The experimental analysis is based on proton-proton collision data collected by the ATLAS detector at the LHC with a centre-of-mass energy of 7 TeV corresponding to an integrated luminosity of 4.6 fb⁻¹. The selected events were identified using the lepton+jets top-quark-pair decay channel, where lepton refers to either an electron or a muon. The observed distribution is compared to a theoretical prediction at next-to-leading-order accuracy in quantum chromodynamics using the pole-mass scheme. With this method, the measured value of the top-quark pole mass, m(^(pole)(t)), is: m({t}^{pole}) = 173.7 ± 1.5 (stat.) ± 1.4 (syst.){-0.5}^{+1.0} (theory) GeV. This result represents the most precise measurement of the top-quark pole mass to date.Item Differential Top-Antitop Cross-Section Measurements as a Function of Observables Constructed from Final-State Particles Using pp Collisions at √s̅ = 7 TeV in the ATLAS Detector(Springer, 2015-06-16) Aad, G.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdinov, O.; Aben, R.; Abi, B.; Izen, Joseph M.; Leyton, Michael; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, Kendall; ATLAS Collaboration; 250674117 (Izen, JM); 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, KendallVarious differential cross-sections are measured in top-quark pair tt̅ events produced in proton-proton collisions at a centre-of-mass energy of √s̅ = 7 TeV at the LHC with the ATLAS detector. These differential cross-sections are presented in a data set corresponding to an integrated luminosity of 4.6 fb⁻¹. The differential cross-sections are presented in terms of kinematic variables, such as momentum, rapidity and invariant mass, of a top-quark proxy referred to as the pseudo-top-quark as well as the pseudo-top-quark pair system. The dependence of the measurement on theoretical models is minimal. The measurements are performed on tt̅ events in the lepton+jets channel, requiring exactly one charged lepton and at least four jets with at least two of them tagged as originating from a b-quark. The hadronic and leptonic pseudo-top-quarks are defined via the leptonic or hadronic decay mode of the W boson produced by the top-quark decay in events with a single charged lepton. Differential cross-section measurements of the pseudo-top-quark variables are compared with several Monte Carlo models that implement next-to-leading order or leading-order multi-leg matrix-element calculations.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 Evidence for the Higgs-Boson Yukawa Coupling to Tau Leptons with the ATLAS Detector(Springer, 2015-04-21) Aad, G.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdinov, O.; Aben, R.; Abi, B.; Izen, Joseph M.; Leyton, Michael l; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, Kendall; ATLAS Collaboration; 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, KendallResults of a search for H → ττ decays are presented, based on the full set of proton-proton collision data recorded by the ATLAS experiment at the LHC during 2011 and 2012. The data correspond to integrated luminosities of 4.5 fb⁻¹ and 20.3 fb⁻¹ at centre-of-mass energies of √s̅ = 7 TeV and √s̅ = 8 TeV respectively. All combinations of leptonic (τ → ℓνν̅ with ℓ = e, μ) and hadronic (τ → hadrons ν) tau decays are considered. An excess of events over the expected background from other Standard Model processes is found with an observed (expected) significance of 4.5 (3.4) standard deviations. This excess provides evidence for the direct coupling of the recently discovered Higgs boson to fermions. The measured signal strength, normalised to the Standard Model expectation, of μ = 1.43 {-0.37)^(+0.43) is consistent with the predicted Yukawa coupling strength in the Standard Model.Item Evidence of Wγγ Production in pp Collisions at √s̅ = 8 TeV and Limits on Anomalous Quartic Gauge Couplings with the ATLAS Detector(American Physical Society, 2015-07-16) Aad, G.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdinov, O.; Aben, R.; Abi, B.; Izen, Joseph M.; Leyton, Michael; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, Kendall; ATLAS Collaboration; 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, KendallThis Letter reports evidence of triple gauge boson production pp → W(ℓν)γγ + X, which is accessible for the first time with the 8 TeV LHC data set. The fiducial cross section for this process is measured in a data sample corresponding to an integrated luminosity of 20.3 fb⁻¹, collected by the ATLAS detector in 2012. Events are selected using the W boson decay to eν or μν as well as requiring two isolated photons. The measured cross section is used to set limits on anomalous quartic gauge couplings in the high diphoton mass region.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 Identification and Energy Calibration of Hadronically Decaying Tau Leptons with the ATLAS Experiment in pp Collisions at √s̅ = 8 TeV(Springer, 2015-05) Aad, G.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdinov, O.; Aben, R.; Abi, B.; Izen, Joseph M.; Leyton, Michael |Lou, Xinchou; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, Kendall; ATLAS Collaboration; 0000-0003-0867-2189 (Lou, X); 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, Kendall; Lou, XinchouThis paper describes the trigger and offline reconstruction, identification and energy calibration algorithms for hadronic decays of tau leptons employed for the data collected from pp collisions in 2012 with the ATLAS detector at the LHC center-of-mass energy √s̅ = 8 TeV. The performance of these algorithms is measured in most cases with Z decays to tau leptons using the full 2012 dataset, corresponding to an integrated luminosity of 20.3 fb⁻¹ An uncertainty on the offline reconstructed tau energy scale of 2-4%, depending on transverse energy and pseudorapidity, is achieved using two independent methods. The offline tau identification efficiency is measured with a precision of 2.5% for hadronically decaying tau leptons with one associated track, and of 4% for the case of three associated tracks, inclusive in pseudorapidity and for a visible transverse energy greater than 20 GeV. For hadronic tau lepton decays selected by offline algorithms, the tau trigger identification efficiency is measured with a precision of 2-8%, depending on the transverse energy. The performance of the tau algorithms, both offline and at the trigger level, is found to be stable with respect to the number of concurrent proton-proton interactions and has supported a variety of physics results using hadronically decaying tau leptons at ATLAS.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 Spectra in Pb + Pb Collisions at √s̅{n̅n̅} = 2.76 TeV with the ATLAS Detector at the LHC(Springer, 2015-09-09) Aad, G.; Abbott, B.; Abdallah, J.; Khalek, S. Abdel; Abdinov, O.; Aben, R.; Abi, B.; Izen, Joseph M.; Leyton, Michael; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, Kendall; ATLAS Collaboration; 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, KendallCharged-particle spectra obtained in Pb+Pb interactions at √s̅{̅N̅N̅}̅ = 2.76 TeV and pp interactions at √s̅{̅N̅N̅}̅ = 2.76 TeV with the ATLAS detector at the LHC are presented, using data with integrated luminosities of 0.15 nb⁻¹ and 4.2 pb⁻¹, respectively, in a wide transverse momentum (0.5 < p{T} < 150 GeV) and pseudorapidity (|η| < 2) range. For Pb+Pb collisions, the spectra are presented as a function of collision centrality, which is determined by the response of the forward calorimeters located on both sides of the interaction point. The nuclear modification factors R{AA} and R{CP} are presented in detail as a function of centrality, p{T} and η. They show a distinct p{T}-dependence with a pronounced minimum at about 7 GeV. Above 60 GeV, R{AA} is consistent with a plateau at a centrality-dependent value, within the uncertainties. The value is 0.55 ± 0.01(stat.) ± 0.04(syst.) in the most central collisions. The R{AA} distribution is consistent with flat |η| dependence over the whole transverse momentum range in all centrality classes.Item Measurement of Differential J/ψ Production Cross Sections and Forward-Backward Ratios in P + Pb Collisions with the ATLAS Detector(American Physical Society, 2015-09-14) Aad, G.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdinov, O.; Aben, R.; Abi, B.; Izen, Joseph M.; Leyton, Michael; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, Kendall; ATLAS Collaboration; 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, KendallMeasurements of differential cross sections for J/ψ production in p+Pb collisions at √{sNN}=5.02 TeV at the CERN Large Hadron Collider with the ATLAS detector are presented. The data set used corresponds to an integrated luminosity of 28.1 nb⁻¹. The J/ψ mesons are reconstructed in the dimuon decay channel over the transverse momentum range 8 < pT < 30 GeV and over the center-of-mass rapidity range -2.87 < y∗ < 1.94. Prompt J/ψ are separated from J/ψ resulting from b-hadron decays through an analysis of the distance between the J/ψ decay vertex and the event primary vertex. The differential cross section for production of nonprompt J/ψ is compared to a FONLL calculation that does not include nuclear effects. Forward-backward production ratios are presented and compared to theoretical predictions. These results complement previously published results by covering a region of higher transverse momentum and more central rapidity. They thus constrain the kinematic dependence of nuclear modifications of charmonium and b-quark production in p+Pb collisions.Item Measurement of Differential Production Cross-Sections for a Z Boson in Association with B-Jets in 7 TeV Proton-Proton Collisions with the ATLAS Detector(Springer, 2014-10-24) 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; 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Namasivayam, Harisankar; Reeves, Kendall; Lou, XinchouMeasurements of differential production cross-sections of a Z boson in association with b-jets in pp collisions at √s = 7 TeV are reported. The data analysed correspond to an integrated luminosity of 4.6 fb⁻¹ recorded with the ATLAS detector at the Large Hadron Collider. Particle-level cross-sections are determined for events with a Z boson decaying into an electron or muon pair, and containing b-jets. For events with at least one b-jet, the cross-section is presented as a function of the Z boson transverse momentum and rapidity, together with the inclusive b-jet cross-section as a function of b-jet transverse momentum, rapidity and angular separations between the b-jet and the Z boson. For events with at least two b-jets, the cross-section is determined as a function of the invariant mass and angular separation of the two highest transverse momentum b-jets, and as a function of the Z boson transverse momentum and rapidity. Results are compared to leading-order and next-to-leading-order perturbative QCD calculations.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 Long-Range Pseudorapidity Correlations and Azimuthal Harmonics in √sNN = 5.02 TeV Proton-Lead Collisions with the ATLAS Detector(American Physical Society, 2014-10-01) Aad, G.; Abbott, B.; Abdallah, J.; Khalek, S. Abdel; 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, XinchouMeasurements of two-particle correlation functions and the first five azimuthal harmonics, v₁ to v₅, are presented, using 28 nb⁻¹ of p + Pb collisions at a nucleon-nucleon center-of-mass energy of √sNN = 5.02 TeV measured with the ATLAS detector at the LHC. Significant long-range "ridgelike" correlations are observed for pairs with small relative azimuthal angle (|Δϕ| < π/3) back-to-back pairs (|Δϕ| > )π/3) over the transverse momentum range 0.4 < pT < 12 GeV and in different intervals of event activity. The event activity is defined by either the number of reconstructed tracks or the total transverse energy on the Pb-fragmentation side. The azimuthal structure of such long-range correlations is Fourier decomposed to obtain the harmonics vn as a function of pT and event activity. The extracted vn values for n=2 to 5 decrease with n. The v₂ and v₃ values are found to be positive in the measured pT range. The v₁ is also measured as a function of pT and is observed to change sign around pT ≈ 1.5–2.0 GeV and then increase to about 0.1 for pT > 4 GeV. The v₂(pT), v₃(pT), and v₄(pT) are compared to the vn coefficients in Pb+Pb collisions at √sNN = 2.76 TeV with similar event multiplicities. Reasonable agreement is observed after accounting for the difference in the average pT of particles produced in the two collision systems.Item Measurement of Spin Correlation in Top-Antitop Quark Events and Search for Top Squark Pair Production in pp Collisions at √s = 8 TeV using the ATLAS Detector(American Physical Society, 2015-04-08) Aad, G.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdinov, O.; Aben, R.; Abi, B.; Izen, Joseph M.; Leyton, Michael; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, Kendall; ATLAS Collaboration; 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, KendallA measurement of spin correlation in tt̅ production is presented using data collected with the ATLAS detector at the Large Hadron Collider in proton-proton collisions at a center-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 20.3 fb⁻¹. The correlation between the top and antitop quark spins is extracted from dilepton tt̅ events by using the difference in the azimuthal angle between the two charged leptons in the laboratory frame. In the helicity basis the measured degree of correlation corresponds to A{helicity} = 0.38 ± 0.04, in agreement with the standard model prediction. A search is performed for pair production of top squarks with masses close to the top quark mass decaying to predominantly right-handed top quarks and a light neutralino, the lightest supersymmetric particle. Top squarks with masses between the top quark mass and 191 GeV are excluded at the 95% confidence level.;