Browsing by Author "ATLAS collaboration"
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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.Item Measurement of the Ratio of Cross Sections for Inclusive Isolated-Photon Production in pp Collisions at √s = 13 and 8 TeV with the ATLAS Detector(Springer Verlag) Aaboud, M.; Aad, G.; Abbott, B.; Abbott, D. C.; Abdinov, O.; Abhayasinghe, D. K.; Abidi, S. H.; Izen, Joseph M.; Meirose, Bernhard; Reeves, Kendall; ATLAS collaboration; 250674117 (Izen, JM); Izen, Joseph M.; Meirose, Bernhard; Reeves, KendallThe ratio of the cross sections for inclusive isolated-photon production in pp collisions at centre-of-mass energies of 13 and 8 TeV is measured using the ATLAS detector at the LHC. The integrated luminosities of the 13 TeV and 8 TeV datasets are 3.2 fb⁻¹ and 20.2 fb⁻¹, respectively. The ratio is measured as a function of the photon transverse energy in different regions of the photon pseudorapidity. The predictions from next-to-leading-order perturbative QCD calculations are compared with the measured ratio. The experimental systematic uncertainties as well as the uncertainties affecting the predictions are evaluated taking into account the correlations between the two centre-of-mass energies, resulting in a reduction of up to a factor of 2.5 (5) in the experimental (theoretical) systematic uncertainties. The predictions based on several parameterisations of the proton parton distribution functions agree with the data within the reduced experimental and theoretical uncertainties. In addition, this ratio to that of the fiducial cross sections for Z boson production at 13 and 8 TeV using the decay channels Z → e⁺e⁻ and Z → μ⁺μ⁻ is made and compared with the theoretical predictions. In this double ratio, a further reduction of the experimental uncertainty is obtained because the uncertainties arising from the luminosity measurement cancel out. The predictions describe the measurements of the double ratio within the theoretical and experimental uncertainties. ©2019 CERN, for the ATLAS Collaboration.Item Measurement of VH, H → bb̅ Production as a Function of the Vector-Boson Transverse Momentum in 13 TeV pp Collisions with the ATLAS Detector(Springer Verlag, 2019-05-23) Aaboud, M.; Aad, G.; Abbott, B.; Abbott, D. C.; Abdinov, O.; Abed Abud, A.; Abhayasinghe, D. K.; ATLAS collaboration; Izen, Joseph M.; Meirose, Bernhard; Reeves, Kendall; 250674117 (Izen, JM); Izen, Joseph M.; Meirose, Bernhard; Reeves, KendallCross-sections of associated production of a Higgs boson decaying into bottomquark pairs and an electroweak gauge boson, W or Z, decaying into leptons are measured as a function of the gauge boson transverse momentum. The measurements are performed in kinematic fiducial volumes defined in the ‘simplified template cross-section’ framework. The results are obtained using 79.8 fb⁻¹ of proton-proton collisions recorded by the ATLAS detector at the Large Hadron Collider at a centre-of-mass energy of 13 TeV. All measurements are found to be in agreement with the Standard Model predictions, and limits are set on the parameters of an effective Lagrangian sensitive to modifications of the Higgs boson couplings to the electroweak gauge bosons. ©2019, The Author(s).Item Search for Higgs Boson Decays to Beyond-The-Standard-Model Light Bosons in Four-Lepton Events with the ATLAS Detector at √s=13 TeV(Springer Verlag) 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 is conducted for a beyond-the-Standard-Model boson using events where a Higgs boson with mass 125 GeV decays to four leptons (ℓ = e or μ). This decay is presumed to occur via an intermediate state which contains one or two on-shell, promptly decaying bosons: H → ZX/XX → 4ℓ, where X is a new vector boson Z_d or pseudoscalar α with mass between 1 and 60 GeV. The search uses pp collision data collected with the ATLAS detector at the LHC with an integrated luminosity of 36.1 fb⁻¹ at a centre-of-mass energy √s=13 TeV. No significant excess of events above Standard Model background predictions is observed; therefore, upper limits at 95% confidence level are set on modelindependent fiducial cross-sections, and on the Higgs boson decay branching ratios to vector and pseudoscalar bosons in two benchmark models. ©2018 CERN, for the ATLAS CollaborationItem Search for Higgs Boson Pair Production in the WW*WW* Decay Channel Using ATLAS Data Recorded at √s = 13 TeV(Springer Verlag, 2019-05-21) 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, KendallA search for a pair of neutral, scalar bosons with each decaying into two W bosons is presented using 36.1 fb⁻¹ of proton-proton collision data at a centre-of-mass energy of 13 TeV recorded with the ATLAS detector at the Large Hadron Collider. This search uses three production models: non-resonant and resonant Higgs boson pair production and resonant production of a pair of heavy scalar particles. Three final states, classified by the number of leptons, are analysed: two same-sign leptons, three leptons, and four leptons. No significant excess over the expected Standard Model backgrounds is observed. An observed (expected) 95% confidence-level upper limit of 160 (120) times the Standard Model prediction of non-resonant Higgs boson pair production cross-section is set from a combined analysis of the three final states. Upper limits are set on the production cross-section times branching ratio of a heavy scalar X decaying into a Higgs boson pair in the mass range of 260 GeV ≤ mχ ≤ 500 GeV and the observed (expected) limits range from 9.3 (10) pb to 2.8 (2.6) pb. Upper limits are set on the production cross-section times branching ratio of a heavy scalar Χ decaying into a pair of heavy scalars S for mass ranges of 280 GeV ≤ mχ ≤ 340 GeV and 135 GeV ≤ m_{S} ≤ 165 GeV and the observed (expected) limits range from 2.5 (2.5) pb to 0.16 (0.17) pb. © 2019, The Author(s).Item Search for Single Production of Vector-Like Quarks Decaying into Wb in pp Collisions at √s = 13 TeV with the ATLAS Detector(Springer Verlag, 2019-05-27) 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, KendallA search for singly produced vector-like quarks Q, where Q can be either a T quark with charge +2/3 or a Y quark with charge −4/3, is performed in proton–proton collision data at a centre-of-mass energy of 13 TeV corresponding to an integrated luminosity of 36.1 fb⁻¹, recorded with the ATLAS detector at the LHC in 2015 and 2016. The analysis targets Q → Wb decays where the W boson decays leptonically. No significant deviation from the expected Standard Model background is observed. Upper limits are set on the QWb coupling strength and the mixing between the Standard Model sector and a singlet T quark or a Y quark from a (B, Y) doublet or a (T, B, Y) triplet, taking into account the interference effects with the Standard Model background. The upper limits set on the mixing angle are as small as |sin θ_{L}| = 0.18 for a singlet T quark of mass 800 GeV, |sin θ_{R}| = 0.17 for a Y quark of mass 800 GeV in a (B, Y) doublet model and |sin θ_{L}| = 0.16 for a Y quark of mass 800 GeV in a (T, B, Y) triplet model. Within a (B, Y) doublet model, the limits set on the mixing parameter |sin θ_{R}| are comparable with the exclusion limits from electroweak precision observables in the mass range between about 900 GeV and 1250 GeV. © 2019, The Author(s).Item Search for Top-Quark Decays t → Hq with 36 fb⁻¹ of pp Collision Data at √s = 13 TeV with the ATLAS Detector(Springer Verlag, 2019-05-21) Aaboud, M.; Aad, G.; Abbott, B.; Abbott, D. C.; Abdinov, O.; Abeloos, B.; Abhayasinghe, D. K.; Izen, Joseph M.; Meirose, Bernhard; Reeves, Kendall; ATLAS collaboration; 250674117 (Izen, JM); Izen, Joseph M.; Meirose, Bernhard; Reeves, KendallA search for flavour-changing neutral current decays of a top quark into an up-type quark (q = u, c) and the Standard Model Higgs boson, t → Hq, is presented. The search is based on a dataset of pp collisions at s = 13 TeV recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider and corresponding to an integrated luminosity of 36.1 fb⁻¹. Two complementary analyses are performed to search for top-quark pair events in which one top quark decays into Wb and the other top quark decays into Hq, and target the H→ bb̅ and H → τ⁺τ⁻ decay modes, respectively. The high multiplicity of b-quark jets, or the presence of hadronically decaying τ-leptons, is exploited in the two analyses respectively. Multivariate techniques are used to separate the signal from the background, which is dominated by top-quark pair production. No significant excess of events above the background expectation is found, and 95% CL upper limits on the t → Hq branching ratios are derived. The combination of these searches with ATLAS searches in diphoton and multilepton final states yields observed (expected) 95% CL upper limits on the t → Hc and t → Hu branching ratios of 1.1 × 10⁻³ (8.3 × 10⁻⁴) and 1.2 × 10⁻³ (8.3 × 10⁻⁴), respectively. The corresponding combined observed (expected) upper limits on the |λ_{tcH}| and |λ_{tuH}| couplings are 0.064 (0.055) and 0.066 (0.055), respectively. ©2019, The Author(s).