Browsing by Author "Aad, G."

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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, Kendall
An 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.
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, Kendall
The 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.
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.
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, Kendall
Charged-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.
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, Kendall
This 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.
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, Kendall
The 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.
Combinations of Single-Top-Quark Production Cross-Section Measurements and |fLvV_{tb}| Determinations at √s = 7 and 8 TeV with the ATLAS and CMS Experiments
(Springer Verlag, 2019-05-16) Aaboud, M.; Aad, G.; Abbott, B.; Abbott, D. C.; Abdinov, O.; Abed Abud, A.; Abhayasinghe, D. K.; Izen, Joseph M.; Meirose, Bernhard; Reeves, Kendall; ATLAS Collaboration; CMS Collaboration; 250674117 (Izen, JM); Izen, Joseph M.; Meirose, Bernhard; Reeves, Kendall
This paper presents the combinations of single-top-quark production cross-section measurements by the ATLAS and CMS Collaborations, using data from LHC proton-proton collisions at √s = 7 and 8 TeV corresponding to integrated luminosities of 1.17 to 5.1 fb⁻¹ at √s = 7 TeV and 12.2 to 20.3 fb⁻¹ at √s = 8 TeV. These combinations are performed per centre-of-mass energy and for each production mode: t-channel, tW, and s-channel. The combined t-channel cross-sections are 67.5 ± 5.7 pb and 87.7 ± 5.8 pb at √s = 7 and 8 TeV respectively. The combined tW cross-sections are 16.3 ± 4.1 pb and 23.1 ± 3.6 pb at √s = 7 and 8 TeV respectively. For the s-channel cross-section, the combination yields 4.9 ± 1.4 pb at √s = 8 TeV. The square of the magnitude of the CKM matrix element V_{tb} multiplied by a form factor fLV is determined for each production mode and centre-of-mass energy, using the ratio of the measured cross-section to its theoretical prediction. It is assumed that the top-quark-related CKM matrix elements obey the relation |V_{td}|, |V_{ts}| ≪ |V_{tb}|. All the |fLVV_{tb}|² determinations, extracted from individual ratios at √s = 7 and 8 TeV, are combined, resulting in |fLVV_{tb}| = 1.02 ± 0.04 (meas.) ± 0.02 (theo.). All combined measurements are consistent with their corresponding Standard Model predictions. © 2019, The Author(s).
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, Kendall
A 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.
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, Xinchou
This 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.
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, Kendall
The 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.
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, Kendall
The 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.
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, Kendall
Measurements 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.
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, Kendall
The 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.
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, Kendall
Studies 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.
Determination of the Ratio of B -Quark Fragmentation Fractions F{s} / f{d} in pp Collisions at √s̅ =7 TeV with the ATLAS Detector
(American Physical Society, 2015-12-30) Aad, G.; Abbott, B.; Abdallah, J.; Abdinov, O.; Aben, R.; Abolins, M.; Abouzeid, O. S.; Ahsan, Mahsana; Izen; 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, Kendall
With an integrated luminosity of 2.47 fb⁻¹ recorded by the ATLAS experiment at the LHC, the exclusive decays B_s^0 → J/ψϕ and B_d^0 → J/ψK^{*0} of B mesons produced in pp collisions at √s̅ = 7 TeV are used to determine the ratio of fragmentation fractions f_s / f_d. From the observed B_s^0 → J/ψϕ and B_d^0 → J/ψK^{*0} yields, the quantity (f_s ± 0.017(th). This result is based on a new approach that provides a significant improvement of the world average / f_d) [ℬ(B_s^0 → J/ψϕ) /ℬ(B_d^0 → J/ψK^{*0})] is measured to be 0.199 ± 0.004 (stat) ± 0.008(syst). Using a recent theory prediction for [ℬ(B_s^0 → J/ψϕ) / ℬ(B_d^0 → J/ψK^{*0})] yields (f_s / f_d)=0.240 ± 0.004(stat) ± 0.010(syst).
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, Kendall
Measurements 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.).
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, Kendall
The 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.
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, Kendall
Various 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.
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, Kendall
A 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.
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, Kendall
This 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.