Browsing by Author "AbouZeid, O. S."
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Item Analysis of Events with B-Jets and a Pair of Leptons of the Same Charge in pp Collisions at √s̅ = 8 TeV with the ATLAS Detector(Springer, 2015-10-22) Aad, G.; Abbott, B.; Abdallah, J.; Abdinov, O.; Aben, R.; Abolins, M.; AbouZeid, O. S.; Izen, Joseph M.; Leyton, Michael; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, Kendall; ATLAS Collaboration; 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, KendallAn analysis is presented of events containing jets including at least one b tagged jet, sizeable missing transverse momentum, and at least two leptons including a pair of the same electric charge, with the scalar sum of the jet and lepton transverse momenta being large. A data sample with an integrated luminosity of 20.3 fb(-1) of p p collisions at p root s = 8TeV recorded by the ATLAS detector at the Large Hadron Collider is used. Standard Model processes rarely produce these final states, but there are several models of physics beyond the Standard Model that predict an enhanced rate of production of such events; the ones considered here are production of vector-like quarks, enhanced fourtop- quark production, pair production of chiral b'-quarks, and production of two positively charged top quarks. Eleven signal regions are defined; subsets of these regions are combined when searching for each class of models. In the three signal regions primarily sensitive to positively charged top quark pair production, the data yield is consistent with the background expectation. There are more data events than expected from background in the set of eight signal regions defined for searching for vector-like quarks and chiral b'-quarks, but the significance of the discrepancy is less than two standard deviations. The discrepancy reaches 2.5 standard deviations in the set of five signal regions defined for searching for four-top-quark production. The results are used to set 95% CL limits on various models.Item ATLAS Run 1 Searches for Direct Pair Production of Third-Generation Squarks at the Large Hadron Collider(Springer New York Llc, 2015-10-29) Aad, G.; Abbott, B.; Abdallah, J.; Abdinov, O.; Aben, R.; Abolins, M.; AbouZeid, O. S.; Izen, Joseph M.; Leyton, Michael; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, Kendall; ATLAS Collaboration; 250674117 (Izen, JM)This paper reviews and extends searches for the direct pair production of the scalar supersymmetric partners of the top and bottom quarks in proton−proton collisions collected by the ATLAS collaboration during the LHC Run 1. Most of the analyses use 20 fb⁻¹ of collisions at a centre-of-mass energy of √s = 8 TeV, although in some case an additional 4.7 fb⁻¹ of collision data at √s = 7 TeV are used. New analyses are introduced to improve the sensitivity to specific regions of the model parameter space. Since no evidence of third-generation squarks is found, exclusion limits are derived by combining several analyses and are presented in both a simplified model framework, assuming simple decay chains, as well as within the context of more elaborate phenomenological supersymmetric models.Item Combination of Inclusive and Differential tt̅ Charge Asymmetry Measurements Using ATLAS and CMS Data at √s =7 and 8 TeV(Springer) Aaboud, M.; Aad, G.; Abbott, B.; Abdinov, O.; Abeloos, B.; Abidi, S. H.; AbouZeid, O. S.; Izen, Joseph M.; Meirose, Bernhard; Reeves, Kendall; ATLAS Collaboration; 250674117 (Izen, JM); Izen, Joseph M.; Meirose, Bernhard; Reeves, KendallThis paper presents combinations of inclusive and differential measurements of the charge asymmetry (AC) in top quark pair (tt̅) events with a lepton + jets signature by the ATLAS and CMS Collaborations, using data from LHC proton-proton collisions at centre-of-mass energies of 7 and 8 TeV. The data correspond to integrated luminosities of about 5 and 20 fb⁻¹ for each experiment, respectively. The resulting combined LHC measurements of the inclusive charge asymmetry are A _C ^{CHC7} = 0.005 ± 0.007 (stat) ± 0.006(syst) at 7 TeV and A _C ^{CHC8) = 0.0055 ± 0.0023 (stat) ± 0.0025 (syst) at 8 TeV. These values, as well as the combination of AC measurements as a function of the invariant mass of the tt̅ system at 8 TeV, are consistent with the respective standard model predictions.Item Combination of Searches for WW, WZ, and ZZ Resonances in pp Collisions at √s̅ = 8 TeV with the ATLAS Detector(Elsevier B.V., 2016-02-11) Aad, G.; Abbott, B.; Abdallah, J.; Abdinov, O.; Aben, R.; Abolins, M.; AbouZeid, O. S.; 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, KendallThe ATLAS experiment at the CERN Large Hadron Collider has performed searches for new, heavy bosons decaying to WW, WZ and ZZ final states in multiple decay channels using 20.3 fb⁻¹ of pp collision data at √s̅ = 8 TeV. In the current study, the results of these searches are combined to provide a more stringent test of models predicting heavy resonances with couplings to vector bosons. Direct searches for a charged diboson resonance decaying to WZ in the ℓνℓ'ℓ' (ℓ = μ, e), ℓℓqq̅, ℓνqq̅ and fully hadronic final states are combined and upper limits on the rate of production times branching ratio to the WZ bosons are compared with predictions of an extended gauge model with a heavy W' boson. In addition, direct searches for a neutral diboson resonance decaying to WW and ZZ in the ℓℓqq̅, ℓνqq̅, and fully hadronic final states are combined and upper limits on the rate of production times branching ratio to the WW and ZZ bosons are compared with predictions for a heavy, spin-2 graviton in an extended Randall-Sundrum model where the Standard Model fields are allowed to propagate in the bulk of the extra dimension.Item Combined Measurement of the Higgs Boson Mass in pp Collisions at √s̅ = 7 and 8 TeV with the ATLAS and CMS Experiments(Amer Physical Soc, 2015-05-14) Aad, G.; Abbott, B.; Abdallah, J.; Abdinov, O.; Aben, R.; Abolins, M.; AbouZeid, O. S.; Izen, Joseph M.; Leyton, Michael; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, Kendall; CMS Collaboration; ATLAS Collaboration; 250674117 (Izen, JM); 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, KendallA measurement of the Higgs boson mass is presented based on the combined data samples of the ATLAS and CMS experiments at the CERN LHC in the H → γγ and H → ZZ → 4ℓ decay channels. The results are obtained from a simultaneous fit to the reconstructed invariant mass peaks in the two channels and for the two experiments. The measured masses from the individual channels and the two experiments are found to be consistent among themselves. The combined measured mass of the Higgs boson is m(H) = 125.09 ± 0.21 (stat) ± 0.11 (syst) GeV.Item Constraints on New Phenomena via Higgs Boson Couplings and Invisible Decays with the ATLAS Detector(Springer, 2015-11-30) Aad, G.; Abbott, B.; Abdallah, J.; Abdinov, O.; Aben, R.; Abolins, M.; AbouZeid, O. S.; 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 ATLAS experiment at the LHC has measured the Higgs boson couplings and mass, and searched for invisible Higgs boson decays, using multiple production and decay channels with up to 4.7 fb⁻¹ of pp collision data at √s̅ = 7 TeV and 20.3 fb⁻¹ at √s̅ = 8 TeV. In the current study, the measured production and decay rates of the observed Higgs boson in the γγ, ZZ, WW, Zγ, bb, ττ, and μμ decay channels, along with results from the associated production of a Higgs boson with a top-quark pair, are used to probe the scaling of the couplings with mass. Limits are set on parameters in extensions of the Standard Model including a composite Higgs boson, an additional electroweak singlet, and two-Higgs-doublet models. Together with the measured mass of the scalar Higgs boson in the γγ and ZZ decay modes, a lower limit is set on the pseudoscalar Higgs boson mass of m{A} > 370 GeV in the "hMSSM" simplified Minimal Supersymmetric Standard Model. Results from direct searches for heavy Higgs bosons are also interpreted in the hMSSM. Direct searches for invisible Higgs boson decays in the vector-boson fusion and associated production of a Higgs boson with W/Z (Z→ ℓℓ, W/Z → jj) modes are statistically combined to set an upper limit on the Higgs boson invisible branching ratio of 0.25. The use of the measured visible decay rates in a more general coupling fit improves the upper limit to 0.23, constraining a Higgs portal model of dark matter.Item Constraints on Non-Standard Model Higgs Boson Interactions in an Effective Lagrangian Using Differential Cross Sections Measured in the H → γγ Decay Channel at √s̅ = 8 TeV with the ATLAS Detector(Elsevier Science B.V., 2015-12-02) Aad, G.; Abbott, B.; Abdallah, J.; Abdinov, O.; Aben, R.; Abolins, M.; AbouZeid, O. S.; 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 strength and tensor structure of the Higgs boson's interactions are investigated using an effective Lagrangian, which introduces additional CP-even and CP-odd interactions that lead to changes in the kinematic properties of the Higgs boson and associated jet spectra with respect to the Standard Model. The parameters of the effective Lagrangian are probed using a fit to five differential cross sections previously measured by the ATLAS experiment in the H → γγ decay channel with an integrated luminosity of 20.3 fb⁻¹ at √s̅ = 8 TeV. In order to perform a simultaneous fit to the five distributions, the statistical correlations between them are determined by re-analysing the H → γγ candidate events in the proton-proton collision data. No significant deviations from the Standard Model predictions are observed and limits on the effective Lagrangian parameters are derived. The statistical correlations are made publicly available to allow for future analysis of theories with non-Standard Model interactions.Item Constraints on the Off-Shell Higgs Boson Signal Strength in the High-Mass ZZ and WW Final States with the ATLAS Detector(Springer New York LLC, 2015-07-17) Aad, G.; Abbott, B.; Abdallah, J.; Abdinov, O.; Aben, R.; Abolins, M.; AbouZeid, O. S.; 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 the ZZ and WW final states in the mass range above the 2m(Z) and 2m(W) thresholds provide a unique opportunity to measure the off-shell coupling strength of the Higgs boson. This paper presents constraints on the off-shell Higgs boson event yields normalised to the Standard Model prediction (signal strength) in the ZZ → 4ℓ, ZZ → 2ℓ2ν and WW → eνμν final states. The result is based on pp collision data collected by the ATLAS experiment at the LHC, corresponding to an integrated luminosity of 20.3 fb⁻¹ at a collision energy of √s=8 TeV. Using the CL(s) method, the observed 95% confidence level (CL) upper limit on the off-shell signal strength is in the range 5.1–8.6, with an expected range of 6.7–11.0. In each case the range is determined by varying the unknown gg → ZZ and gg → WW background K-factor from higher-order quantum chromodynamics corrections between half and twice the value of the known signal K-factor. Assuming the relevant Higgs boson couplings are independent of the energy scale of the Higgs boson production, a combination with the on-shell measurements yields an observed (expected) 95% CL upper limit on (formula presented) in the range 4.5–7.5 (6.5–11.2) using the same variations of the background K-factor. Assuming that the unknown gg → VV background K-factor is equal to the signal K-factor, this translates into an observed (expected) 95% CL upper limit on the Higgs boson total width of 22.7 (33.0) MeV.Item Determination of Spin and Parity of the Higgs Boson in the WW* → eνμν Decay Channel with the ATLAS Detector(Springer, 2015-05-27) Aad, G.; Abbott, B.; Abdallah, J.; Abdinov, O.; Aben, R.; Abolins, M.; AbouZeid, O. S.; 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, KendallStudies of the spin and parity quantum numbers of the Higgs boson in the WW* → eνμν final state are presented, based on proton-proton collision data collected by the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of 20.3 fb⁻¹ at a centre-of-mass energy of √s̅ = 8 TeV. The Standard Model spin-parity J^{CP} = 0⁺⁺ hypothesis is compared with alternative hypotheses for both spin and CP. The case where the observed resonance is a mixture of the Standard-Model-like Higgs boson and CP-even (J^{CP} = 0⁺⁺) or CP-odd (J^{CP} = 0⁺⁻) Higgs boson in scenarios beyond the Standard Model is also studied. The data are found to be consistent with the Standard Model prediction and limits are placed on alternative spin and CP hypotheses, including CP mixing in different scenarios.Item Dijet Production in √s̅ = 7 TeV pp Collisions with Large Rapidity Gaps at the ATLAS Experiment(Elsevier B.V., 2016-01-18) Aad, G.; Abbott, B.; Abdallah, J.; Abdinov, O.; Aben, R.; Abolins, M.; AbouZeid, O. S.; 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, KendallA 6.8 nb⁻¹ sample of pp collision data collected under low-luminosity conditions at √s̅ = 7 TeV by the ATLAS detector at the Large Hadron Collider is used to study diffractive dijet production. Events containing at least two jets with p{T} > 20 GeV are selected and analysed in terms of variables which discriminate between diffractive and non-diffractive processes. Cross sections are measured differentially in Δη^F, the size of the observable forward region of pseudorapidity which is devoid of hadronic activity, and in an estimator, ξ̃, of the fractional momentum loss of the proton assuming single diffractive dissociation (pp → pX). Model comparisons indicate a dominant non-diffractive contribution up to moderately large Δη^F and small ξ̃, with a diffractive contribution which is significant at the highest Δη^F and the lowest ξ̃. The rapidity-gap survival probability is estimated from comparisons of the data in this latter region with predictions based on diffractive parton distribution functions.Item Direct Top-Quark Decay Width Measurement in the tt̅ Lepton+Jets Channel at √s =8 TeV with the ATLAS Experiment(Springer) Aaboud, M.; Aad, G.; Abbott, B.; Abdinov, O.; Abeloos, B.; Abidi, S. H.; AbouZeid, O. S.; Izen, Joseph M.; Meirose, Bernhard; Reeves, Kendall; ATLAS Collaboration; 250674117 (Izen, JM); Izen, Joseph M.; Meirose, Bernhard; Reeves, KendallThis paper presents a direct measurement of the decay width of the top quark using tt̅ events in the lepton+jets final state. The data sample was collected by the ATLAS detector at the LHC in proton-proton collisions at a centre-of-mass energy of 8 TeV and corresponds to an integrated luminosity of 20.2 fb⁻¹. The decay width of the top quark is measured using a template fit to distributions of kinematic observables associated with the hadronically and semileptonically decaying top quarks. The result, Γ_t = 1.76 ± 0.33 (stat.) _{-0.68} ^{+0.70} (syst.) GeV for a top-quark mass of 172.5 GeV, is consistent with the prediction of the Standard Model.Item Evidence for Single Top-Quark Production in the S-Channel in Proton-Proton Collisions at √s̅ = 8 TeV with the ATLAS Detector Using the Matrix Element Method(Elsevier Science Bv, 2016-03-08) Aad, G.; Abbott, B.; Abdallah, J.; Abdinov, O.; Aben, R.; Abolins, M.; AbouZeid, O. S.; Izen, Joseph M.; Leyton, Michael; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, Kendall; ATLAS Collaboration; 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, KendallThis Letter presents evidence for single top-quark production in the s-channel using proton-proton collisions at a centre-of-mass energy of 8 TeV with the ATLAS detector at the CERN Large Hadron Collider. The analysis is performed on events containing one isolated electron or muon, large missing transverse momentum and exactly two b-tagged jets in the final state. The analysed data set corresponds to an integrated luminosity of 20.3 fb⁻¹. The signal is extracted using a maximum-likelihood fit of a discriminant which is based on the matrix element method and optimized in order to separate single-top-quark s-channel events from the main background contributions, which are top-quark pair production and W boson production in association with heavy-flavour jets. The measurement leads to an observed signal significance of 3.2 standard deviations and a measured cross-section of σ_s = 4.8 ± 0.8(stat.)_{(-1.3)^(+1.6)}(syst.) pb, which is consistent with the Standard Model expectation. The expected significance for the analysis is 3.9 standard deviations.Item Evidence for the Associated Production of the Higgs Boson and a Top Quark Pair with the ATLAS Detector(Amer Physical Soc) Aaboud, M.; Aad, G.; Abbott, B.; Abdinov, O.; Abeloos, B.; Abidi, S. H.; AbouZeid, O. S.; Izen, Joseph M.; Meirose, Bernhard; Reeves, Kendall; ATLAS Collaboration; 250674117 (Izen, JM); Izen, Joseph M.; Meirose, Bernhard; Reeves, KendallA search for the associated production of the Higgs boson with a top quark pair (tt̅ H) is reported. The search is performed in multilepton final states using a data set corresponding to an integrated luminosity of 36.1 fb⁻¹ of proton-proton collision data recorded by the ATLAS experiment at a center-of-mass energy √s = 13 TeV at the Large Hadron Collider. Higgs boson decays to WW*, ττ, and ZZ* are targeted. Seven final states, categorized by the number and flavor of charged-lepton candidates, are examined for the presence of the Standard Model Higgs boson with a mass of 125 GeV and a pair of top quarks. An excess of events over the expected background from Standard Model processes is found with an observed significance of 4.1 standard deviations, compared to an expectation of 2.8 standard deviations. The best fit for the tt̅ H production cross section is σ(tt̅ H) = 790_{-210}^{+230} fb in agreement with the Standard Model prediction of 507_{-50}^{+35} fb. The combination of this result with other tt̅ H searches of the ATLAS experiment using the Higgs boson decay modes to bb̅, γγ and ZZ* → 4ℓ, has an observed significance of 4.2 standard deviations, compared to an expectation of 3.8 standard deviations. This provides evidence for the tt̅ H production mode.Item Evidence for the H → bb̅ Decay with the ATLAS Detector(Springer, 2018-09-27) Aaboud, M.; Aad, G.; Abbott, B.; Abdinov, O.; Abeloos, B.; Abidi, S. H.; AbouZeid, O. S.; Abraham, N. L.; Izen, Joseph M.; Meirose, Bernhard; Reeves, Kendall; ATLAS Collaboration; 250674117 (Izen, JM); Izen, Joseph M.; Meirose, Bernhard; Reeves, KendallA search for the decay of the Standard Model Higgs boson into a bb̅ pair when produced in association with a W or Z boson is performed with the ATLAS detector. The analysed data, corresponding to an integrated luminosity of 36.1 fb⁻¹, were collected in proton-proton collisions in Run 2 of the Large Hadron Collider at a centre-of-mass energy of 13 TeV. Final states containing zero, one and two charged leptons (electrons or muons) are considered, targeting the decays Z → νν, W → ℓν and Z → ℓℓ. For a Higgs boson mass of 125 GeV, an excess of events over the expected background from other Standard Model processes is found with an observed significance of 3.5 standard deviations, compared to an expectation of 3.0 standard deviations. This excess provides evidence for the Higgs boson decay into b-quarks and for its production in association with a vector boson. The combination of this result with that of the Run 1 analysis yields a ratio of the measured signal events to the Standard Model expectation equal to 0.90 ± 0.18(stat.)(-0.19)(+0.21)(syst.). Assuming the Standard Model production cross-section, the results are consistent with the value of the Yukawa coupling to b-quarks in the Standard Model.Item Fiducial, Total and Differential Cross-Section Measurements of T-Channel Single Top-Quark Production in pp Collisions at 8 TeV Using Data Collected by the ATLAS Detector(Springer New York LLC, 2018-05-15) Aaboud, M.; Aad, G.; Abbott, B.; Abdallah, J.; Abdinov, O.; Abeloos, B.; AbouZeid, O. S.; Izen, Joseph M.; Meirose, Bernhard; Reeves, Kendall; Kurth, Matthew G.; Leyton, Michael; ATLAS Collaboration; 250674117 (Izen, JM); Izen, Joseph M.; Kurth, Matthew G.; Leyton, Michael; Meirose, Bernhard; Reeves, KendallThis paper describes the algorithms for the reconstruction and identification of electrons in the central region of the ATLAS detector at the Large Hadron Collider (LHC). These algorithms were used for all ATLAS results with electrons in the final state that are based on the 2012 pp collision data produced by the LHC at √s̅ = 8 TeV. The efficiency of these algorithms, together with the charge misidentification rate, is measured in data and evaluated in simulated samples using electrons from Z→ ee, Z→ eeγ and J/ ψ→ ee decays. For these efficiency measurements, the full recorded data set, corresponding to an integrated luminosity of 20.3 fb⁻¹, is used. Based on a new reconstruction algorithm used in 2012, the electron reconstruction efficiency is 97% for electrons with E_T= 15 GeV and 99% at E_T= 50 GeV. Combining this with the efficiency of additional selection criteria to reject electrons from background processes or misidentified hadrons, the efficiency to reconstruct and identify electrons at the ATLAS experiment varies from 65 to 95%, depending on the transverse momentum of the electron and background rejection.Item High-E_T Isolated-Photon Plus Jets Production in pp Collisions at √s̅ = 8 TeV with the ATLAS Detector(Springer New York LLC, 2018-05-15) Aaboud, M.; Aad, G.; Abbott, B.; Abdallah, J.; Abdinov, O.; Abeloos, B.; AbouZeid, O. S.; Izen, Joseph M.; Meirose, Bernhard; Reeves, Kendall; Kurth, Matthew G.; Leyton, Michael; ATLAS Collaboration; 250674117 (Izen, JM); Izen, Joseph M.; Kurth, Matthew G.; Leyton, Michael; Meirose, Bernhard; Reeves, KendallDetailed measurements of t-channel single top-quark production are presented. They use 20.2 fb⁻¹ of data collected by the ATLAS experiment in proton–proton collisions at a centre-of-mass energy of 8 TeV at the LHC. Total, fiducial and differential cross-sections are measured for both top-quark and top-antiquark production. The fiducial cross-section is measured with a precision of 5.8% (top quark) and 7.8% (top antiquark), respectively. The total cross-sections are measured to be σ_{tot}(tq) = 56.7_{-3.8}^{+4.3} pb for top-quark production and σ_{tot}(t̅q) = 32.9_{-2.7}^{+3.0} pb for top-antiquark production, in agreement with the Standard Model prediction. In addition, the ratio of top-quark to top-antiquark production cross-sections is determined to be R_t = 1.72 ± 0.09. The differential cross-sections as a function of the transverse momentum and rapidity of both the top quark and the top antiquark are measured at both the parton and particle levels. The transverse momentum and rapidity differential cross-sections of the accompanying jet from the t-channel scattering are measured at particle level. All measurements are compared to various Monte Carlo predictions as well as to fixed-order QCD calculations where available.Item Identification of Boosted, Hadronically Decaying W Bosons and Comparisons with ATLAS Data Taken at √s̅ = 8 TeV(Springer New York LLC, 2016-03-17) Aad, G.; Abbott, B.; Abdallah, J.; Abdinov, O.; Aben, R.; Abolins, M.; AbouZeid, O. S.; Izen, Joseph M.; Leyton, Michael; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, Kendall; ATLAS Collaboration; 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, KendallThis paper reports a detailed study of techniques for identifying boosted, hadronically decaying W bosons using 20.3 fb⁻¹ of proton–proton collision data collected by the ATLAS detector at the LHC at a centre-of-mass energy √s̅ = 8 TeV. A range of techniques for optimising the signal jet mass resolution are combined with various jet substructure variables. The results of these studies in Monte Carlo simulations show that a simple pairwise combination of groomed jet mass and one substructure variable can provide a 50 % efficiency for identifying W bosons with transverse momenta larger than 200 GeV while maintaining multijet background efficiencies of 2–4 % for jets with the same transverse momentum. These signal and background efficiencies are confirmed in data for a selection of tagging techniques.Item Measurement of Colour Flow with the Jet Pull Angle in tt̅ Events Using the ATLAS Detector a √s̅ = 8 TeV(Elsevier Science B.V., 2015-09-26) Aad, G.; Abbott, B.; Abdallah, J.; Abdinov, O.; Aben, R.; Abolins, M.; AbouZeid, O. S.; 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 distribution and orientation of energy inside jets is predicted to be an experimental handle on colour connections between the hard-scatter quarks and gluons initiating the jets. This Letter presents a measurement of the distribution of one such variable, the jet pull angle. The pull angle is measured for jets produced in tt̅ events with one W boson decaying leptonically and the other decaying to jets using 20.3 fb⁻¹ of data recorded with the ATLAS detector at a centre-of-mass energy of √s̅ = 8 TeV at the LHC. The jet pull angle distribution is corrected for detector resolution and acceptance effects and is compared to various models.Item Measurement of D^{*±}, D^± and {D_s ^±} Meson Production Cross Sections in pp Collisions at √s̅ = 7 TeV with the ATLAS Detector(Elsevier, 2016-04-25) Aad, G.; Abbott, B.; Abdallah, J.; Abdinov, O.; Aben, R.; Abolins, M.; AbouZeid, O. S.; Izen, Joseph M.; Leyton, Michael; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, Kendall; ATLAS Collaboration; 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, KendallThe production of D*^±, D^± and {D_s ^±} charmed mesons has been measured with the ATLAS detector in pp collisions at √s̅ = 7 TeV at the LHC, using data corresponding to an integrated luminosity of 280 nb⁻¹. The charmed mesons have been reconstructed in the range of transverse momentum 3.5 < p_T(D) <100 GeV and pseudorapidity |η(D)| < 2.1. The differential cross sections as a function of transverse momentum and pseudorapidity were measured for D*^± and D^± production. The next-to-leading-order QCD predictions are consistent with the data in the visible kinematic region within the large theoretical uncertainties. Using the visible D cross sections and an extrapolation to the full kinematic phase space, the strangeness-suppression factor in charm fragmentation, the fraction of charged non-strange D mesons produced in a vector state, and the total cross section of charm production at √s̅ = 7 TeV were derived.Item Measurement of Differential Cross Sections and W⁺/W⁻ Cross-Section Ratios for W Boson Production in Association with Jets at √s=8 TeV with the ATLAS Detector(Springer Verlag, 2018-11-05) Aaboud, M.; Aad, G.; Abbott, B.; Abdinov, O.; Abeloos, B.; Abidi, S. H.; AbouZeid, O. S.; Izen, Joseph M.; Meirose, Bernhard; Reeves, Kendall; ATLAS collaboration; 250674117 (Izen, JM); Izen, Joseph M.; Meirose, Bernhard; Reeves, KendallThis paper presents a measurement of the W boson production cross section and the W⁺/W⁻ cross-section ratio, both in association with jets, in proton-proton collisions at √s=8 TeV with the ATLAS experiment at the Large Hadron Collider. The measurement is performed in final states containing one electron and missing transverse momentum using data corresponding to an integrated luminosity of 20.2 fb⁻¹. Differential cross sections for events with at least one or two jets are presented for a range of observables, including jet transverse momenta and rapidities, the scalar sum of transverse momenta of the visible particles and the missing transverse momentum in the event, and the transverse momentum of the W boson. For a subset of the observables, the differential cross sections of positively and negatively charged W bosons are measured separately. In the cross-section ratio of W⁺/W⁻ the dominant systematic uncertainties cancel out, improving the measurement precision by up to a factor of nine. The observables and ratios selected for this paper provide valuable input for the up quark, down quark, and gluon parton distribution functions of the proton.