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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., Cabrera Urban, S., et al. (2024). Search for top-philic heavy resonances in pp collisions at √s=13 TeV with the ATLAS detector. Eur. Phys. J. C, 84(2), 157–32pp.
Abstract: Asearch for the associated production of a heavy resonance with a top-quark or a top-antitop-quark pair, and decaying into a t (t) over bar pair is presented. The search uses the vdata recorded by the ATLAS detector in pp collisions at root s = 13 TeV at the Large Hadron Collider during the years 2015-2018, corresponding to an integrated luminosity of 139 fb(-1). Events containing exactly one electron ormuon are selected. The two hadronically decaying top quarks from the resonance decay are reconstructed using jets clustered with a large radius parameter of R = 1. The invariant mass spectrum of the two top quark candidates is used to search for a resonance signal in the range of 1.0 TeV to 3.2 TeV. The presence of a signal is examined using an approach with minimal model dependence followed by a model-dependent interpretation. No significant excess is observed over the background expectation. Upper limits on the production cross section times branching ratio at 95% confidence level are provided for a heavy Z' boson based on a simplified model, for Z' mass between 1.0 TeV and 3.0 TeV. The observed (expected) limits range from 21 (14) fb to 119 (86) fb depending on the choice of model parameters.
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ATLAS Collaboration(Aad, G. et al), Aikot, A., Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., et al. (2024). Study of High-Transverse-Momentum Higgs Boson Production in Association with a Vector Boson in the qqbb Final State with the ATLAS Detector. Phys. Rev. Lett., 132(13), 131802–23pp.
Abstract: This Letter presents the first study of Higgs boson production in association with a vector boson (V = W or Z) in the fully hadronic qqbb final state using data recorded by the ATLAS detector at the LHC in ffiffiproton-proton collisions at root root s= 13 TeV and corresponding to an integrated luminosity of 137fb(-1). The vector bosons and Higgs bosons are each reconstructed as large-radius jets and tagged using jet substructure techniques. Dedicated tagging algorithms exploiting b-tagging properties are used to identify jets consistent with Higgs bosons decaying into b (b) over bar. Dominant backgrounds from multijet production are determined directly from the data, and a likelihood fit to the jet mass distribution of Higgs boson candidates is used to extract the number of signal events. The VH production cross section is measured inclusively and differentially in several ranges of Higgs boson transverse momentum: 250-450, 450-650, and greater than 650 GeV. The inclusive signal yield relative to the standard model expectation is observed to be μ= 1.4(-0.9)(+1.0) and the corresponding cross section is 3.1 +/- 1.3(stat)(-1.4)(+1.8) (syst) pb.
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ATLAS Collaboration(Aad, G. et al), Aikot, A., Amos, K. R., Bouchhar, N., Cabrera Urban, S., Cantero, J., et al. (2024). Measurement of vector boson production cross sections and their ratios using pp collisions at √s=13.6 TeV with the ATLAS detector. Phys. Lett. B, 854, 138725–27pp.
Abstract: Fiducial and total W+ and Z boson cross sections, their ratios and the ratio of top-antitop-quark pair and.. -boson fiducial cross sections are measured in proton-proton collisions at a centre-of-mass energy of root s = 13.6 TeV, corresponding to an integrated luminosity of 29 fb(-1) of data collected in 2022 by the ATLAS experiment at the Large Hadron Collider. The measured fiducial cross-section values for W+ -> l(+) v, W- -> l(-) v- <overline>, and Z -> l(+)l(-) (l =e or mu) boson productions are 4250 +/- 150 pb, 3310 +/- 120 pb, and 744 +/- 20 pb, respectively, where the uncertainty is the total uncertainty, including that arising from the luminosity of about 2.2%. The measurements are in agreement with Standard-Model predictions calculated at next-to-next-to-leading-order in alpha(s) ,next-to-nextto-leading logarithmic accuracy and next-to-leading-order electroweak accuracy.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., Cabrera Urban, S., et al. (2023). Measurement of the Higgs boson mass in the H → Z Z* → 4l decay channel using 139 fb-1 of √s=13 TeV pp collisions recorded by the ATLAS detector at the LHC br. Phys. Lett. B, 843, 137880–23pp.
Abstract: The mass of the Higgs boson is measured in the H -> Z Z* -> 4l decay channel. The analysis uses proton-proton collision data from the Large Hadron Collider at a centre-of-mass energy of 13 TeV recorded by the ATLAS detector between 2015 and 2018, corresponding to an integrated luminosity of 139 fb(-1). The measured value of the Higgs boson mass is 124.99 0.18(stat.) +/- 0.04(syst.) GeV. In final states with muons, this measurement benefits from an improved momentum-scale calibration relative to that adopted in previous publications. The measurement also employs an analytic model that takes into account the invariant-mass resolution of the four-lepton system on a per-event basis and the output of a deep neural network discriminating signal from background events. This measurement is combined with the corresponding measurement using 7 and 8 TeV pp collision data, resulting in a Higgs boson mass of 124.94 +/- 0.17(stat.) +/- 0.03(syst.) GeV.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., Cabrera Urban, S., et al. (2023). Measurement of the charge asymmetry in top-quark pair production in association with a photon with the ATLAS experiment. Phys. Lett. B, 843, 137848–21pp.
Abstract: A measurement of the charge asymmetry in top-quark pair (t (t) over bar) production in association with a photon is presented. The measurement is performed in the single-lepton t (t) over bar decay channel using proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider at CERN at a centre-of-mass-energy of 13 TeV during the years 2015-2018, corresponding to an integrated luminosity of 139 fb(-1). The charge asymmetry is obtained from the distribution of the difference of the absolute rapidities of the top quark and antiquark using a profile likelihood unfolding approach. It is measured to be A(C) = -0.003 +/- 0.029 in agreement with the Standard Model expectation.
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ATLAS Collaboration(Aad, G. et al), Aikot, A., Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., et al. (2023). Measurement of the cross-sections of the electroweak and total production of a Zγ pair in association with two jets in pp collisions at √s=13 TeV with the ATLAS detector. Phys. Lett. B, 846, 138222–30pp.
Abstract: This Letter presents the measurement of the fiducial and differential cross-sections of the electroweak production of a Z gamma pair in association with two jets. The analysis uses 140 fb(-1)of LHC proton-proton collision data taken at root s=13 TeV recorded by the ATLAS detector during the years 2015-2018. Events with a Zboson candidate decaying into either an e(+) e(-) or mu(+) mu(-) pair, a photon and two jets are selected. The electroweak component is extracted by requiring a large dijet invariant mass and by using the information about the centrality of the system and is measured with an observed and expected significance well above five standard deviations. The fiducial pp -> Z gamma jj cross-section for the electroweak production is measured to be 3.6 +/- 0.5fb. The total fiducial cross-section that also includes contributions where the jets arise from strong interactions is measured to be 16.8(-1.8)(+2.0) fb. The results are consistent with the Standard Model predictions. Differential cross-sections are also measured using the same events and are compared with parton-shower Monte Carlo simulations. Good agreement is observed between data and predictions.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., Cabrera Urban, S., et al. (2023). Measurement of Suppression of Large-Radius Jets and Its Dependence on Substructure in Pb plus Pb Collisions at √sNN=5.02 TeV with the ATLAS Detector. Phys. Rev. Lett., 131(17), 172301–22pp.
Abstract: This letter presents a measurement of the nuclear modification factor of large-radius jets in root s(NN) = 5.02 TeV Pb thorn Pb collisions by the ATLAS experiment. The measurement is performed using 1.72 nb(-1) and 257 pb(-1) of Pb thorn Pb and pp data, respectively. The large-radius jets are reconstructed with the anti-kt algorithm using a radius parameter of R = 1.0, by reclustering anti-k(t) R = 0.2 jets, and are measured over the transverse momentum (p(T)) kinematic range of 158 < p(T) < 1000 GeV and absolute pseudorapidity |y| < 2.0. The large-radius jet constituents are further reclustered using the k(t) algorithm in order to obtain the splitting parameters, root d(12) and Delta R-12, which characterize the transverse momentum scale and angular separation for the hardest splitting in the jet, respectively. The nuclear modification factor, R-AA, obtained by comparing the Pb thorn Pb jet yields to those in pp collisions, is measured as a function of jet transverse momentum (p(T)) and root d(12) or Delta R-12. A significant difference in the quenching of large-radius jets having single subjet and those with more complex substructure is observed. Systematic comparison of jet suppression in terms of R-AA for different jet definitions is also provided. Presented results support the hypothesis that jets with hard internal splittings lose more energy through quenching and provide a new perspective for understanding the role of jet structure in jet suppression.
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ATLAS Collaboration(Aad, G. et al), Aikot, A., Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., et al. (2023). Measurement of the Higgs boson mass with H → γγ decays in 140 fb-1 of √s=13 TeV pp collisions with the ATLAS detector. Phys. Lett. B, 847, 138315–23pp.
Abstract: The mass of the Higgs boson is measured in the H -> gamma gamma decay channel, exploiting the high resolution of the invariant mass of photon pairs reconstructed from the decays of Higgs bosons produced in proton-proton collisions at a centre-of-mass energy root s = 13 TeV. The dataset was collected between 2015 and 2018 by the ATLAS detector at the Large Hadron Collider, and corresponds to an integrated luminosity of 140 fb(-1). The measured value of the Higgs boson mass is 125.17 +/- 0.11 (stat.) +/- 0.09 (syst.) GeV and is based on an improved energy scale calibration for photons, whose impact on the measurement is about four times smaller than in the previous publication. A combination with the corresponding measurement using 7 and 8 TeV pp collision ATLAS data results in a Higgs boson mass measurement of 125.22 +/- 0.11 (stat.) +/- 0.09 (syst.) GeV. With an uncertainty of 1.1 per mille, this is currently the most precise measurement of the mass of the Higgs boson from a single decay channel.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., Cabrera Urban, S., et al. (2023). Search for pair production of third-generation leptoquarks decaying into a bottom quark and a τ-lepton with the ATLAS detector. Eur. Phys. J. C, 83(11), 1075–35pp.
Abstract: A search for pair-produced scalar or vector leptoquarks decaying into a b-quark and a tau-lepton is presented using the full LHC Run 2 (2015-2018) data sample of 139 fb(-1) collected with the ATLAS detector in proton-proton collisions at a centre-of-mass energy of root s = 13TeV. Events in which at least one tau-lepton decays hadronically are considered, and multivariate discriminants are used to extract the signals. No significant deviations from the Standard Model expectation are observed and 95% confidence-level upper limits on the production cross-section are derived as a function of leptoquark mass and branching ratio B into a tau-lepton and b-quark. For scalar leptoquarks, masses below 1460GeV are excluded assuming B = 100%, while for vector leptoquarks the corresponding limit is 1650GeV (1910GeV) in the minimal-coupling (Yang-Mills) scenario.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cantero, J., et al. (2023). Measurements of the suppression and correlations of dijets in Xe+Xe collisions at √s NN=5.44 TeV. Phys. Rev. C, 108(2), 024906–25pp.
Abstract: Measurements of the suppression and correlations of dijets is performed using 3 μb(-1) of Xe+Xe data at root sNN = 5.44 TeV collected with the ATLAS detector at the CERN Large Hadron Collider. Dijets with jets reconstructed using the R = 0.4 anti-kt algorithm are measured differentially in jet p(T) over the range of 32 to 398 GeV and the centrality of the collisions. Significant dijet momentum imbalance is found in the most central Xe+Xe collisions, which decreases in more peripheral collisions. Results from the measurement of per-pair normalized and absolutely normalized dijet p(T) balance are compared with previous Pb+Pb measurements at root sNN = 5.02 TeV. The differences between the dijet suppression in Xe+Xe and Pb+Pb are further quantified by the ratio of pair nuclear-modification factors. The results are found to be consistent with those measured in Pb+Pb data when compared in classes of the same event activity and when taking into account the difference between the center-of-mass energies of the initial parton scattering process in Xe+Xe and Pb+Pb collisions. These results should provide input for a better understanding of the role of energy density, system size, path length, and fluctuations in the parton energy loss.
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