Browsing by Author "Abeloos, B."
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Item Analysis of the Wtb Vertex from the Measurement of Triple-Differential Angular Decay Rates of Single Top Quarks Produced in the T-Channel at √s=8 TeV with the ATLAS Detector(Springer, 2018-09-24) Izen, Joseph M.; Meirose, Bernhard; Reeves, Kendall; Aaboud, M.; Aad, G.; Abbott, B.; Abdallah, J.; Abdinov, O.; Abeloos, B.; Abidi, S. H.; ATLAS Collaboration; 250674117 (Izen, JM); Izen, Joseph M.; Meirose, Bernhard; Reeves, KendallThe electroweak production and subsequent decay of single top quarks in the t-channel is determined by the properties of the Wtb vertex, which can be described by the complex parameters of an effective Lagrangian. An analysis of a triple-differential decay rate in t-channel production is used to simultaneously determine five generalised helicity fractions and phases, as well as the polarisation of the produced top quark. The complex parameters are then constrained. This analysis is based on 20.2 fb⁻¹ of proton-proton collision data at a centre-of-mass energy of 8 TeV collected with the ATLAS detector at the LHC. The fraction of decays containing transversely polarised W bosons is measured to be f(1) = 0.30 ± 0.05. The phase between amplitudes for transversely and longitudinally polarised W bosons recoiling against left-handed b-quarks is measured to be δ = 0.002 π_{+0.017π}^{+0.016π), giving no indication of CP violation. The fractions of longitudinal or transverse W bosons accompanied by right-handed b-quarks are also constrained. Based on these measurements, limits are placed at 95% CL on the ratio of the complex coupling parameters Re [g_R/V_L ∈ [-0.12, 0.17] and Im [g_R/V_L ∈ [-0.07, 0.06]. Constraints are also placed on the ratios |V_R/V_L| and |g_L/V_L|. In addition, the polarisation of single top quarks in the t-channel is constrained to be P > 0.72 (95% CL). None of the above measurements make assumptions about the value of any of the other parameters or couplings and all of them are in agreement with the Standard Model.Item Charged-Particle Distributions in √s̅ = 13 TeV pp Interactions Measured with the ATLAS Detector at the LHC(Elsevier B.V., 2016-07-10) Aad, G.; Abbott, B.; Abdallah, J.; Abdinov, O.; Abeloos, B.; Aben, R.; Abolins, M.; 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 distributions are measured in proton-proton collisions at a centre-of-mass energy of 13 TeV, using a data sample of nearly 9 million events, corresponding to an integrated luminosity of 170 μb⁻¹, recorded by the ATLAS detector during a special Large Hadron Collider fill. The charged-particle multiplicity, its dependence on transverse momentum and pseudorapidity and the dependence of the mean transverse momentum on the charged-particle multiplicity are presented. The measurements are performed with charged particles with transverse momentum greater than 500 MeV and absolute pseudorapidity less than 2.5, in events with at least one charged particle satisfying these kinematic requirements. Additional measurements in a reduced phase space with absolute pseudorapidity less than 0.8 are also presented, in order to compare with other experiments. The results are corrected for detector effects, presented as particle-level distributions and are compared to the predictions of various Monte Carlo event generators.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 Cyclopamine Tartrate, An Inhibitor of Hedgehog Signaling, Strongly Interferes with Mitochondrial Function and Suppresses Aerobic Respiration in Lung Cancer Cells(Springer New York LLC, 2018-05-15) Aad, G.; Abbott, B.; Abdallah, J.; Abdinov, O.; Abeloos, B.; Aben, R.; Abolins, M.; 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 reconstruction and calibration algorithms used to calculate missing transverse momentum (E_T^{miss}) with the ATLAS detector exploit energy deposits in the calorimeter and tracks reconstructed in the inner detector as well as the muon spectrometer. Various strategies are used to suppress effects arising from additional proton–proton interactions, called pileup, concurrent with the hard-scatter processes. Tracking information is used to distinguish contributions from the pileup interactions using their vertex separation along the beam axis. The performance of the E_T^{miss} reconstruction algorithms, especially with respect to the amount of pileup, is evaluated using data collected in proton–proton collisions at a centre-of-mass energy of 8 TeV during 2012, and results are shown for a data sample corresponding to an integrated luminosity of 20.3 fb⁻¹. The simulation and modelling of E_T^{miss} in events containing a Z boson decaying to two charged leptons (electrons or muons) or a W boson decaying to a charged lepton and a neutrino are compared to data. The acceptance for different event topologies, with and without high transverse momentum neutrinos, is shown for a range of threshold criteria for E_T^{miss}, and estimates of the systematic uncertainties in the E_T^{miss} measurements are presented.Item Determination of the Strong Coupling Constant αₛ from Transverse Energy-Energy Correlations in Multijet Events at √S=8 TeV Using the ATLAS Detector(Springer, 2018-09-24) Izen, Joseph M.; Meirose, Bernhard; Reeves, Kendall; Aaboud, M.; Aad, G.; Abbott, B.; Abdallah, J.; Abdinov, O.; Abeloos, B.; Abidi, S. H.; ATLAS Collaboration; 250674117 (Izen, JM); Izen, Joseph M.; Meirose, Bernhard; Reeves, KendallMeasurements of transverse energy-energy correlations and their associated asymmetries in multi-jet events using the ATLAS detector at the LHC are presented. The data used correspond to √s = 8 TeV proton-proton collisions with an integrated luminosity of 20.2 fb⁻¹. The results are presented in bins of the scalar sum of the transverse momenta of the two leading jets, unfolded to the particle level and compared to the predictions from Monte Carlo simulations. A comparison with next-to-leading-order perturbative QCD is also performed, showing excellent agreement within the uncertainties. From this comparison, the value of the strong coupling constant is extracted for different energy regimes, thus testing the running of αₛ(μ) predicted in QCD up to scales over 1 TeV. A global fit to the transverse energy-energy correlation distributions yields αₛ(m_Z) = 0.1162 ± 0.0011 (exp.)_{-0.0070}^{+0.0084} (theo.), while a global fit to the asymmetry distributions yields a value of αₛ(m_Z) = 0.1196 ± 0.0013 (exp.)_{-0.0045}^{+0.0075} (theo.).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 Electron and Photon Energy Calibration with the ATLAS Detector Using 2015-2016 LHC Proton-Proton Collision Data(Institute of Physics Publishing, 2019-03-18) Aaboud, M.; Aad, G.; Abbott, B.; Abbott, D. C.; Abdinov, O.; Abeloos, B.; Abhayasinghe, D. K.; Izen, Joseph M.; Meirose, Bernhard; Reeves, Kendall; 250674117 (Izen, JM); Izen, Joseph M.; Meirose, Bernhard; Reeves, KendallThis paper presents the electron and photon energy calibration obtained with the ATLAS detector using about 36 fb⁻¹ of LHC proton-proton collision data recorded at √s=13 TeV in 2015 and 2016. The different calibration steps applied to the data and the optimization of the reconstruction of electron and photon energies are discussed. The absolute energy scale is set using a large sample of Z boson decays into electron-positron pairs. The systematic uncertainty in the energy scale calibration varies between 0.03% to 0.2% in most of the detector acceptance for electrons with transverse momentum close to 45 GeV . For electrons with transverse momentum of 10 GeV the typical uncertainty is 0.3% to 0.8% and it varies between 0.25% and 1% for photons with transverse momentum around 60 GeV . Validations of the energy calibration with J/ψ → e⁺e⁻ decays and radiative Z boson decays are also presented. © 2019 CERN for the benefit of the ATLAS collaboration.Item Electron Efficiency Measurements with the ATLAS Detector Using 2012 LHC Proton–Proton Collision Data(Springer, 2018-05-15) Aaboud, M.; Aad, G.; Abbott, B.; Abdallah, J.; Abdinov, O.; Abeloos, B.; Abidi, S. H.; Izen, Joseph M.; Meirose, Bernhard; Reeves, Kendall; ATLAS Collaboration; 250674117 (Izen, JM); Izen, Joseph M.; Meirose, Bernhard; Reeves, KendallThe production of a Z boson and a photon in association with a high-mass dijet system is studied using 20.2 fb⁻¹ of proton-proton collision data at a centre-of-mass energy of √s̅ = 8 TeV recorded with the ATLAS detector in 2012 at the Large Hadron Collider. Final states with a photon and a Z boson decaying into a pair of either electrons, muons, or neutrinos are analysed. Electroweak and total pp → Zγjj cross-sections are extracted in two fiducial regions with different sensitivities to electroweak production processes. Quartic couplings of vector bosons are studied in regions of phase space with an enhanced contribution from pure electroweak production, sensitive to vector-boson scattering processes VV → Zγ. No deviations from Standard Model predictions are observed and constraints are placed on anomalous couplings parameterized by higher-dimensional operators using effective field theory.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 Femtoscopy with Identified Charged Pions in Proton-Lead Collisions at √{S_{NN}}=5.02 TeV with ATLAS(Amer Physical Soc, 2018-09-24) Aaboud, M.; Aad, G.; Abbott, B.; Abdallah, J.; Abdinov, O.; Abeloos, B.; Aben, R.; Izen, Joseph M.; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, Kendall; ATLAS Collaboration; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, Kendall; ATLAS Collaboration; 250674117 (Izen, JM); Izen, Joseph M.; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, KendallBose-Einstein correlations between identified charged pions are measured for p+Pb collisions at √{S_{NN}} = 5.02 TeV using data recorded by the ATLAS detector at the CERN Large Hadron Collider corresponding to a total integrated luminosity of 28 nb⁻¹. Pions are identified using ionization energy loss measured in the pixel detector. Two-particle correlation functions and the extracted source radii are presented as a function of collision centrality as well as the average transverse momentum k_T and rapidity (y_{ππ}^*) of the pair. Pairs are selected with a rapidity -2 < (y_{ππ}^*) < 1 and with an average transverse momentum 0.1 < k_T < 0.8 GeV. The effect of jet fragmentation on the two-particle correlation function is studied, and a method using opposite-charge pair data to constrain its contributions to the measured correlations is described. The measured source sizes are substantially larger in more central collisions and are observed to decrease with increasing pair k_T. A correlation of the radii with the local charged-particle density is demonstrated. The scaling of the extracted radii with the mean number of participating nucleons is also used to compare a selection of initial-geometry models. The cross term R_{ol} is measured as a function of rapidity, and a nonzero value is observed with 5.1 sigma combined significance for -1 < (y_{ππ}^*) < 1 in the most central events.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 and Rejection of Pile-Up Jets at High Pseudorapidity with the ATLAS Detector(Springer) Aaboud, M.; Aad, G.; Abbott, B.; Abdallah, J.; Abdinov, O.; Abeloos, B.; Abidi, S. H.; Izen, Joseph M.; Meirose, Bernhard; Reeves, Kendall; ATLAS Collaboration; 250674117 (Izen, JM); Izen, Joseph M.; Meirose, Bernhard; Reeves, KendallResults of a search for physics beyond the Standard Model in events containing an energetic photon and large missing transverse momentum with the ATLAS detector at the Large Hadron Collider are reported. As the number of events observed in data, corresponding to an integrated luminosity of 36.1 fb⁻¹ of proton-proton collisions at a centre-of-mass energy of 13 TeV, is in agreement with the Standard Model expectations, model-independent limits are set on the fiducial cross section for the production of events in this final state. Exclusion limits are also placed in models where dark-matter candidates are pair-produced. For dark-matter production via an axial-vector or a vector mediator in the s-channel, this search excludes mediator masses below 750-1200 GeV for dark-matter candidate masses below 230-480 GeV at 95% confidence level, depending on the couplings. In an effective theory of dark-matter production, the limits restrict the value of the suppression scale M⠄to be above 790 GeV at 95% confidence level. A limit is also reported on the production of a high-mass scalar resonance by processes beyond the Standard Model, in which the resonance decays to Z gamma and the Z boson subsequently decays into neutrinos.Item Jet Reconstruction and Performance Using Particle Flow with the ATLAS Detector(Amer Physical Soc) Aaboud, M.; Aad, G.; Abbott, B.; Abdallah, J.; Abdinov, O.; Abeloos, B.; Aben, R.; 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, KendallMeasurements of normalized differential cross sections of top quark pair tt̅ production are presented as a function of the mass, the transverse momentum and the rapidity of the tt̅ system in proton-proton collisions at center-of-mass energies of √s̅ = 7 and 8 TeV. The data set corresponds to an integrated luminosity of 4.6 fb⁻¹ at 7 TeV and 20.2 fb⁻¹ at 8 TeV, recorded with the ATLAS detector at the Large Hadron Collider. Events with top quark pair signatures are selected in the dilepton final state, requiring exactly two charged leptons and at least two jets with at least one of the jets identified as likely to contain a b hadron. The measured distributions are corrected for detector effects and selection efficiency to cross sections at the parton level. The differential cross sections are compared with different Monte Carlo generators and theoretical calculations of tt̅ production. The results are consistent with the majority of predictions in a wide kinematic range.Item Luminosity Determination in pp Collisions at √s̅ = 8 TeV Using the ATLAS Detector at the LHC(Springer New York LLC, 2018-05-15) Aaboud, M.; Aad, G.; Abbott, B.; Abdallah, J.; Abdinov, O.; Abeloos, B.; Aben, R.; 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 search for triboson W^±W^±W^∓ production in two decay channels (W^±W^±W^∓→ ℓ^± νℓ^± νℓ^∓ ν and W^±W^±W^∓→ ℓ^± νℓ^± νjj with ℓ = e, μ) in proton-proton collision data corresponding to an integrated luminosity of 20.3 fb⁻¹ at a centre-of-mass energy of 8 TeV with the ATLAS detector at the Large Hadron Collider. Events with exactly three charged leptons, or two leptons with the same electric charge in association with two jets, are selected. The total number of events observed in data is consistent with the Standard Model (SM) predictions. The observed 95% confidence level upper limit on the SM W^±W^±W^∓ production cross section is found to be 730 fb with an expected limit of 560 fb in the absence of SM W^±W^±W^∓ production. Limits are also set on WWWW anomalous quartic gauge couplings.Item Measurement of Angular and Momentum Distributions of Charged Particles within and Around Jets in Pb+Pb and pp Collisions at √{sₙₙ} = 5.02 TeV with ATLAS at the LHC(Elsevier Science B.V., 2019-01-22) Puri, Akshat; Aaboud, M.; Aad, G.; Abbott, B.; Abdinov, O.; Abeloos, B.; Abhayasinghe, D. K.; Abidi, S. H.; Izen, Joseph M.; Meirose, Bernhard; Reeves, Kendall; ATLAS Collaboration; 250674117 (Izen, JM); Izen, Joseph M.; Meirose, Bernhard; Reeves, KendallStudies of the fragmentation of jets into charged particles in heavy-ion collisions can help in understanding the mechanism of jet quenching by the hot and dense QCD matter created in such collisions, the quark-gluon plasma. These proceedings present a measurement of the angular distribution of charged particles around the jet axis in √{sₙₙ} = 5.02 TeV Pb+Pb and pp collisions, done using the ATLAS detector at the LHC. The measurement is performed inside jets reconstructed with the anti-kₜ algorithm with radius parameter R = 0.4, and is extended to regions outside the jet cone. Results are presented as a function of Pb+Pb collision centrality, and both jet and charged-particle transverse momenta.Item Measurement of B-Hadron Pair Production with the ATLAS Detector in Proton-Proton Collisions at √s̅ = 8 TeV(Springer Verlag, 2017-11-10) Aaboud, M.; Aad, G.; Abbott, B.; Abdallah, J.; Abdinov, O.; Abeloos, B.; Abidi, S. H.; Izen, Joseph M.; Meirose, Bernhard; Reeves, Kendall; ATLAS Collaboration; 250674117 (Izen, JM); Izen, Joseph M.; Meirose, Bernhard; Reeves, KendallA measurement of b-hadron pair production is presented, based on a data set corresponding to an integrated luminosity of 11.4 fb⁻¹ of proton-proton collisions recorded at √s̅ = 8 TeV with the ATLAS detector at the LHC. Events are selected in which a b-hadron is reconstructed in a decay channel containing J/ψ → μμ, and a second b-hadron is reconstructed in a decay channel containing a muon. Results are presented in a fiducial volume defined by kinematic requirements on three muons based on those used in the analysis. The fiducial cross section is measured to be 17.7 ± 0.1(stat.) ± 2.0(syst.) nb. A number of normalised differential cross sections are also measured, and compared to predictions from the Pythia8, Herwig++, MadGraph5_aMC@NLO+Pythia8 and Sherpa event generators, providing new constraints on heavy flavour production.Item Measurement of Charged-Particle Distributions Sensitive to the Underlying Event in √s̅ = 13 TeV Proton-Proton Collisions with the ATLAS Detector at the LHC(Springer Verlag, 2018-09-12) Aaboud, M.; Aad, G.; Abbott, B.; Abdallah, J.; Abdinov, O.; Abeloos, B.; Abidi, S. H.; Izen, Joseph M.; Leyton, Michael; Meirose, Bernhard; Reeves, Kendall; 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Meirose, Bernhard; Reeves, KendallWe present charged-particle distributions sensitive to the underlying event, measured by the ATLAS detector in proton-proton collisions at a centre-of-mass energy of 13 TeV, in low-luminosity Large Hadron Collider fills corresponding to an integrated luminosity of 1.6 nb−1. The distributions were constructed using charged particles with absolute pseudorapidity less than 2.5 and with transverse momentum greater than 500 MeV, in events with at least one such charged particle with transverse momentum above 1 GeV. These distributions characterise the angular distribution of energy and particle flows with respect to the charged particle with highest transverse momentum, as a function of both that momentum and of charged-particle multiplicity. The results have been corrected for detector effects and are compared to the predictions of various Monte Carlo event generators, experimentally establishing the level of underlying-event activity at LHC Run 2 energies and providing inputs for the development of event generator modelling. The current models in use for UE modelling typically describe this data to 5% accuracy, compared with data uncertainties of less than 1%.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.