Browsing by Author "Khalek, S. Abdel"
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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 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 the Charge Asymmetry in Dileptonic Decays of Top Quark Pairs in pp Collisions at √s=7 TeV Using the ATLAS Detector(Springer, 2015-05-01) 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; 56366505900 (Lou, X); 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Namasivayam, Harisankar; Reeves, Kendall; Lou, XinchouA measurement of the top-antitop (tt̅) charge asymmetry is presented using data corresponding to an integrated luminosity of 4.6 fb⁻¹ of LHC pp collisions at a centre-of-mass energy of 7 TeV collected by the ATLAS detector. Events with two charged leptons, at least two jets and large missing transverse momentum are selected. Two observables are studied: A^(ℓℓ)(C) based on the identified charged leptons, and A^(tt̅)(C), based on the reconstructed tt̅ final state. The asymmetries are measured to be A^(ℓℓ)(C) = 0.024 ± 0.015 (stat.) ± 0.009 (syst.), A^(tt̅)(C) = 0.021 ± 0.025 (stat.) ± 0.017 (syst.). The measured values are in agreement with the Standard Model predictions.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.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 Production Cross-Section of ψ(2s) → J/ψ(→ μ⁺μ⁻) π⁺π⁻ in pp Collisions at √s = 7 TeV at ATLAS(Springer, 2014-09-12) 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, XinchouThe prompt and non-prompt production cross-sections for ψ(2S) mesons are measured using 2.1 fb⁻¹ of pp collision data at a centre-of-mass energy of 7 TeV recorded by the ATLAS experiment at the LHC. The measurement exploits the ψ(2s) → J/ψ(→ μ⁺μ⁻ )π⁺π⁻ decay mode, and probes ψ(2S) mesons with transverse momenta in the range 10 ≤ p_T < 100 GeV and rapidity |y| < 2.0. The results are compared to other measurements of ψ(2S) production at the LHC and to various theoretical models for prompt and non-prompt quarkonium production.Item Measurement of the Production of Neighbouring Jets in Lead-Lead Collisions at √s̅_{NN}} = 2.76 TeV with the ATLAS Detector(Elsevier Science B.V., 2015-12-17) 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; Lou, Xinchou; 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 Letter presents measurements of correlated production of nearby jets in Pb+Pb collisions at √s̅_(NN) = 2.76 TeV using the ATLAS detector at the Large Hadron Collider. The measurement was performed using 0.14 nb⁻¹of data recorded in 2011. The production of correlated jet pairs was quantified using the rate, R_{ΔR}, of "neighbouring" jets that accompany "test" jets within a given range of angular distance, ΔR, in the pseudorapidity-azimuthal angle plane. The jets were measured in the ATLAS calorimeter and were reconstructed using the anti-k_t algorithm with radius parameters d = 0.2, 0.3, and 0.4. R_{ΔR} was measured in different Pb+Pb collision centrality bins, characterized by the total transverse energy measured in the forward calorimeters. A centrality dependence of R_{ΔR} is observed for all three jet radii with R_{ΔR} found to be lower in central collisions than in peripheral collisions. The ratios formed by the R_{ΔR} values in different centrality bins and the values in the 40-80% centrality bin are presented.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 tt̅ Production Cross-Section as a Function of Jet Multiplicity and Jet Transverse Momentum in 7 TeV Proton-Proton Collisions with the ATLAS Detector(Springer, 2015-01-08) 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; 0000-0003-0867-2189 (Lou, X); 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Namasivayam, Harisankar; Reeves, Kendall; Lou, XinchouThe tt̅ production cross-section dependence on jet multiplicity and jet transverse momentum is reported for proton-proton collisions at a centre-of-mass energy of 7 TeV in the single-lepton channel. The data were collected with the ATLAS detector at the CERN Large Hadron Collider and comprise the full 2011 data sample corresponding to an integrated luminosity of 4.6 fb⁻¹. Differential cross-sections are presented as a function of the jet multiplicity for up to eight jets using jet transverse momentum thresholds of 25, 40, 60, and 80 GeV, and as a function of jet transverse momentum up to the fifth jet. The results are shown after background subtraction and corrections for all known detector effects, within a kinematic range closely matched to the experimental acceptance. Several QCD-based Monte Carlo models are compared with the results. Sensitivity to the parton shower modelling is found at the higher jet multiplicities, at high transverse momentum of the leading jet and in the transverse momentum spectrum of the fifth leading jet. The MC@NLO+HERWIG MC is found to predict too few events at higher jet multiplicities.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 H → γγ Produced in Association with Top Quarks and Constraints on the Yukawa Coupling between the Top Quark and the Higgs Boson using Data Taken at 7 TeV and 8 TeV with the ATLAS Detector(Elsevier, 2015-01-06) 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; 0000-0003-0867-2189 (Lou, X); 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Namasivayam, Harisankar; Reeves, Kendall; Lou, XinchouA search is performed for Higgs bosons produced in association with top quarks using the diphoton decay mode of the Higgs boson. Selection requirements are optimized separately for leptonic and fully hadronic final states from the top quark decays. The dataset used corresponds to an integrated luminosity of 4.5 fb⁻¹ of proton-proton collisions at a center-of-mass energy of 7 TeV and 20.3 fb⁻¹ at 8 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. No significant excess over the background prediction is observed and upper limits are set on the tt̅H production cross section. The observed exclusion upper limit at 95% confidence level is 6.7 times the predicted Standard Model cross section value. In addition, limits are set on the strength of the Yukawa coupling between the top quark and the Higgs boson, taking into account the dependence of the tt̅H and tH cross sections as well as the H → γγ branching fraction on the Yukawa coupling. Lower and upper limits at 95% confidence level are set at -1.3 and +8.0 times the Yukawa coupling strength in the Standard Model.Item Search for High-Mass Dilepton Resonances in pp Collisions at √s = 8 TeV with the ATLAS Detector(American Physical Society, 2014-09-19) 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, XinchouThe ATLAS detector at the Large Hadron Collider is used to search for high-mass resonances decaying to dielectron or dimuon final states. Results are presented from an analysis of proton-proton (pp) collisions at a center-of-mass energy of 8 TeV corresponding to an integrated luminosity of 20.3 fb⁻¹ in the dimuon channel. A narrow resonance with Standard Model Z couplings to fermions is excluded at 95% confidence level for masses less than 2.79 TeV in the dielectron channel, 2.53 TeV in the dimuon channel, and 2.90 TeV in the two channels combined. Limits on other model interpretations are also presented, including a grand-unification model based on the E₆ gauge group, Z* bosons, minimal Z' models, a spin-2 graviton excitation from Randall-Sundrum models, quantum black holes, and a minimal walking technicolor model with a composite Higgs boson.Item A Search for High-Mass Resonances Decaying to τ⁺τ⁻ in pp Collisions at √s̅ = 8 TeV with the ATLAS Detector(Springer, 2015-07) 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, KendallA search for high-mass resonances decaying into τ⁺τ⁻ final states using proton-proton collisions at √s̅ = 8 TeV produced by the Large Hadron Collider is presented. The data were recorded with the ATLAS detector and correspond to an integrated luminosity of 19.5-20.3 fb⁻¹. No statistically significant excess above the Standard Model expectation is observed; 95% credibility upper limits are set on the cross section times branching fraction of Zʹ resonances decaying into τ⁺τ⁻ pairs as a function of the resonance mass. As a result, Zʹ bosons of the Sequential Standard Model with masses less than 2.02 TeV are excluded at 95% credibility. The impact of the fermionic couplings on the Z' acceptance is investigated and limits are also placed on a Zʹ model that exhibits enhanced couplings to third-generation fermions.Item Search for New Resonances in Wγ and Zγ Final States in PP Collisions at √s = 8 TeV with the ATLAS Detector(Elsevier Science B.V., 2014-10-06) 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; 0000-0003-0867-2189 (Lou, X); 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Namasivayam, Harisankar; Reeves, Kendall; Lou, XinchouThis Letter presents a search for new resonances decaying to final states with a vector boson produced in association with a high transverse momentum photon, Vγ, with V = W(→ ℓν) or Z(ℓ⁺ℓ⁻), where ℓ = e or μ. The measurements use 20.3 fb⁻¹ of proton-proton collision data at a center-of-mass energy of √s = 8 TeV recorded with the ATLAS detector. No deviations from the Standard Model expectations are found, and production cross section limits are set at 95% confidence level. Masses of the hypothetical a(T) and ω(T) states of a benchmark Low Scale Technicolor model are excluded in the ranges [275, 960] GeV and [200, 700]u[750, 890] GeV, respectively. Limits at 95% confidence level on the production cross section of a singlet scalar resonance decaying to Zγ final states have also been obtained for masses below 1180GeV.Item Search for Pair-Produced Third-Generation Squarks Decaying via Charm Quarks or in Compressed Supersymmetric Scenarios in Pp Collisions at √S = 8 TeV with the ATLAS Detector(American Physical Society, 2014-09-24) 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, XinchouResults of a search for supersymmetry via direct production of third-generation squarks are reported, using 20.3 fb⁻¹ of proton-proton collision data at √s = 8 TeV recorded by the ATLAS experiment at the LHC in 2012. Two different analysis strategies based on monojetlike and c-tagged event selections are carried out to optimize the sensitivity for direct top squark-pair production in the decay channel to a charm quark and the lightest neutralino (t ̃ → c + χ ̃₁⁰) across the top squark-neutralino mass parameter space. No excess above the Standard Model background expectation is observed. The results are interpreted in the context of direct pair production of top squarks and presented in terms of exclusion limits in the (m-_{t₁}, m_{χ ̃_1 ^0} parameter space. A top squark of mass up to about 240 GeV is excluded at 95% confidence level for arbitrary neutralino masses, within the kinematic boundaries. Top squark masses up to 270 GeV are excluded for a neutralino mass of 200 GeV. In a scenario where the top squark and the lightest neutralino are nearly degenerate in mass, top squark masses up to 260 GeV are excluded. The results from the monojetlike analysis are also interpreted in terms of compressed scenarios for top squark-pair production in the decay channel t ̃¹ → b + ʄʄ′ + χ ̃₁⁰ and sbottom pair production with b ̃₁ → b + χ ̃₁⁰, leading to a similar exclusion for nearly mass-degenerate third-generation squarks and the lightest neutralino. The results in this paper significantly extend previous results at colliders.Item Search for S-Channel Single Top-Quark Production in Proton-Proton Collisions at √s = 8 TeV with the ATLAS Detector(Elsevier, 2014-11-22) 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; 0000-0003-0867-2189 (Lou, X); 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Namasivayam, Harisankar; Reeves, Kendall; Lou, XinchouThis Letter presents a search at the LHC for s-channel single top-quark production in proton-proton collisions at a centre-of-mass energy of 8 TeV. The analyzed data set was recorded by the ATLAS detector and corresponds to an integrated luminosity of 20.3 fb⁻¹. Selected events contain one charged lepton, large missing transverse momentum and exactly two b-tagged jets. A multivariate event classifier based on boosted decision trees is developed to discriminate s-channel single top-quark events from the main background contributions. The signal extraction is based on a binned maximum-likelihood fit of the output classifier distribution. The analysis leads to an upper limit on the s-channel single top-quark production cross-section of 14.6 pb at the 95% confidence level. The fit gives a cross-section of σ(s) = 5.0 ± 4.3 pb, consistent with the Standard Model expectation.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 Search for Supersymmetry in Events with Large Missing Transverse Momentum, Jets, and at Least One Tau Lepton In Proton-Proton Collision Data with the ATLAS Detector(Springer, 2014-09-18) 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, XinchouA search for supersymmetry (SUSY) in events with large missing transverse momentum, jets, at least one hadronically decaying tau lepton and zero or one additional light leptons (electron/muon), has been performed using 20.3 fb⁻¹ of proton-proton collision data at √s = 8 TeV recorded with the ATLAS detector at the Large Hadron Collider. No excess above the Standard Model background expectation is observed in the various signal regions and 95% confidence level upper limits on the visible cross section for new phenomena are set. The results of the analysis are interpreted in several SUSY scenarios, significantly extending previous limits obtained in the same final states. In the framework of minimal gauge-mediated SUSY breaking models, values of the SUSY breaking scale Λ below 63 TeV are excluded, independently of tan β. Exclusion limits are also derived for an mSUGRA/CMSSM model, in both the R-parity-conserving and R-parity-violating case. A further interpretation is presented in a framework of natural gauge mediation, in which the gluino is assumed to be the only light coloured sparticle and gluino masses below 1090 GeV are excluded.Item Search for W ' → bb → qqbb Decays in pp Collisions at √s̅ = 8 TeV with the ATLAS Detector(Springer, 2015-04-24) 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; Lou, Xinchou; ATLAS Collaboration; 0000-0003-0867-2189 (Lou, X); 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, KendallA search for a massive W' gauge boson decaying to a top quark and a bottom quark is performed with the ATLAS detector in pp collisions at the LHC. The datasetwas taken at a centre-of-mass energy of √s̅ = 8 TeV and corresponds to 20.3 fb⁻¹ of integrated luminosity. This analysis is done in the hadronic decay mode of the top quark, where novel jet substructure techniques are used to identify jets from high-momentum top quarks. This allows for a search for high-mass W' bosons in the range 1.5-3.0 TeV. b-tagging is used to identify jets originating from b-quarks. The data are consistent with Standard Model background-only expectations, and upper limits at 95% confidence level are set on the W' → tb cross section times branching ratio ranging from 0.16 pb to 0.33 pb for left-handed W' bosons, and ranging from 0.10 pb to 0.21 pb for W' bosons with purely righthanded couplings. Upper limits at 95% confidence level are set on the W'-boson coupling to tb as a function of the W' mass using an effective field theory approach, which is independent of details of particular models predicting a W' boson.