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Publication Open Access Fluctuations of anisotropic flow in Pb+Pb collisions at √sNN = 5.02 TeV with the ATLAS detector(Springer Science and Business Media Deutschland GmbH, 2020) Aaboud, M.; Aad, Georges; Abbott, Brad K.; Abbott, D. C.; Abdinov, O. B.; Abed Abud, Adam; Abhayasinghe, D. K.; Abidi, Syed Hani; AbouZeid, O. S.; Abraham, Nicola L.; Aaboud, M., Faculty of Science, Université Mohammed Premier Oujda, Oujda, Morocco; Aad, Georges, Centre de Physique des Particules de Marseille, Marseille, France; Abbott, Brad K., The University of Oklahoma, Norman, United States; Abbott, D. C., Department of Physics, University of Massachusetts Amherst, Amherst, United States; Abdinov, O. B., Azerbaijan National Academy of Sciences, Baku, Azerbaijan; Abed Abud, Adam, Istituto Nazionale di Fisica Nucleare, Sezione di Pavia, Pavia, Italy, Università degli Studi di Pavia, Pavia, Italy; Abhayasinghe, D. K., Department of Physics, Royal Holloway, University of London, Egham, United Kingdom; Abidi, Syed Hani, University of Toronto, Toronto, Canada; AbouZeid, O. S., Niels Bohr Institutet, Copenhagen, Denmark; Abraham, Nicola L., University of Sussex, Brighton, United KingdomMulti-particle azimuthal cumulants are measured as a function of centrality and transverse momentum using 470 μb−1 of Pb+Pb collisions at sNN = 5.02 TeV with the ATLAS detector at the LHC. These cumulants provide information on the event-by-event fluctuations of harmonic flow coefficients vn and correlated fluctuations between two harmonics vn and vm. For the first time, a non-zero four-particle cumulant is observed for dipolar flow, v1. The four-particle cumulants for elliptic flow, v2, and triangular flow, v3, exhibit a strong centrality dependence and change sign in ultra-central collisions. This sign change is consistent with significant non-Gaussian fluctuations in v2 and v3. The four-particle cumulant for quadrangular flow, v4, is found to change sign in mid-central collisions. Correlations between two harmonics are studied with three- and four-particle mixed-harmonic cumulants, which indicate an anti-correlation between v2 and v3, and a positive correlation between v2 and v4. These correlations decrease in strength towards central collisions and either approach zero or change sign in ultra-central collisions. To investigate the possible flow fluctuations arising from intrinsic centrality or volume fluctuations, the results are compared between two different event classes used for centrality definitions. In peripheral and mid-central collisions where the cumulant signals are large, only small differences are observed. In ultra-central collisions, the differences are much larger and transverse momentum dependent. These results provide new information to disentangle flow fluctuations from the initial and final states, as well as new insights on the influence of centrality fluctuations. [Figure not available: see fulltext.]. © 2025 Elsevier B.V., All rights reserved.Publication Open Access Performance of electron and photon triggers in ATLAS during LHC Run 2(Institute for Ionics, 2020) Aad, Georges; Abbott, Brad K.; Abbott, D. C.; Abed Abud, Adam; Abeling, Kira; Abhayasinghe, D. K.; Abidi, Syed Hani; AbouZeid, O. S.; Abraham, Nicola L.; Abramowicz, Halina; Aad, Georges, Centre de Physique des Particules de Marseille, Marseille, France; Abbott, Brad K., The University of Oklahoma, Norman, United States; Abbott, D. C., Department of Physics, University of Massachusetts Amherst, Amherst, United States; Abed Abud, Adam, Istituto Nazionale di Fisica Nucleare, Sezione di Pavia, Pavia, Italy, Università degli Studi di Pavia, Pavia, Italy; Abeling, Kira, Georg-August-Universität Göttingen, Gottingen, Germany; Abhayasinghe, D. K., Department of Physics, Royal Holloway, University of London, Egham, United Kingdom; Abidi, Syed Hani, TRIUMF, Vancouver, Canada; AbouZeid, O. S., Niels Bohr Institutet, Copenhagen, Denmark; Abraham, Nicola L., School of Physics, The University of Sydney, Sydney, Australia; Abramowicz, Halina, Aristotle University of Thessaloniki, Thessaloniki, GreeceElectron and photon triggers covering transverse energies from 5 GeV to several TeV are essential for the ATLAS experiment to record signals for a wide variety of physics: from Standard Model processes to searches for new phenomena in both proton–proton and heavy-ion collisions. To cope with a fourfold increase of peak LHC luminosity from 2015 to 2018 (Run 2), to 2.1×1034cm-2s-1, and a similar increase in the number of interactions per beam-crossing to about 60, trigger algorithms and selections were optimised to control the rates while retaining a high efficiency for physics analyses. For proton–proton collisions, the single-electron trigger efficiency relative to a single-electron offline selection is at least 75% for an offline electron of 31 GeV, and rises to 96% at 60 GeV, the trigger efficiency of a 25 GeV leg of the primary diphoton trigger relative to a tight offline photon selection is more than 96% for an offline photon of 30 GeV. For heavy-ion collisions, the primary electron and photon trigger efficiencies relative to the corresponding standard offline selections are at least 84% and 95%, respectively, at 5 GeV above the corresponding trigger threshold. © 2024 Elsevier B.V., All rights reserved.Publication Open Access Measurement of J/ψ production in association with a W ± boson with pp data at 8 TeV(Springer Science and Business Media Deutschland GmbH, 2020) Aaboud, M.; Aad, Georges; Abbott, Brad K.; Abbott, D. C.; Abdinov, O. B.; Abeloos, Baptiste; Abhayasinghe, D. K.; Abidi, Syed Hani; AbouZeid, O. S.; Abraham, Nicola L.; Aaboud, M., Faculty of Science, Université Mohammed Premier Oujda, Oujda, Morocco; Aad, Georges, Centre de Physique des Particules de Marseille, Marseille, France; Abbott, Brad K., The University of Oklahoma, Norman, United States; Abbott, D. C., Department of Physics, University of Massachusetts Amherst, Amherst, United States; Abdinov, O. B., Azerbaijan National Academy of Sciences, Baku, Azerbaijan; Abeloos, Baptiste, Laboratoire de l'Accélérateur Linéaire, Orsay, France; Abhayasinghe, D. K., Department of Physics, Royal Holloway, University of London, Egham, United Kingdom; Abidi, Syed Hani, University of Toronto, Toronto, Canada; AbouZeid, O. S., Niels Bohr Institutet, Copenhagen, Denmark; Abraham, Nicola L., University of Sussex, Brighton, United KingdomA measurement of the production of a prompt J/ψ meson in association with a W± boson with W± → μν and J/ψ → μ+μ− is presented for J/ψ transverse momenta in the range 8.5–150 GeV and rapidity |yJ/ψ| < 2.1 using ATLAS data recorded in 2012 at the LHC. The data were taken at a proton-proton centre-of-mass energy of s = 8 TeV and correspond to an integrated luminosity of 20.3 fb−1. The ratio of the prompt J/ψ plus W± cross-section to the inclusive W± cross-section is presented as a differential measurement as a function of J/ψ transverse momenta and compared with theoretical predictions using different double-parton-scattering cross-sections. [Figure not available: see fulltext.]. © 2025 Elsevier B.V., All rights reserved.Publication Open Access Measurement of long-range two-particle azimuthal correlations in Z-boson tagged pp collisions at √s=8 and 13 TeV(Springer, 2020) Aaboud, M.; Aad, Georges; Abbott, Brad K.; Abbott, D. C.; Abdinov, O. B.; Abeloos, Baptiste; Abhayasinghe, D. K.; Abidi, Syed Hani; AbouZeid, O. S.; Abraham, Nicola L.; Aaboud, M., Université Mohammed Premier Oujda, Oujda, Morocco; Aad, Georges, Aix Marseille Université, Marseille, France; Abbott, Brad K., Homer L. Dodge Department of Physics and Astronomy, The University of Oklahoma, Norman, United States; Abbott, D. C., Department of Physics, University of Massachusetts Amherst, Amherst, United States; Abdinov, O. B., Department of Physics, The University of Adelaide, Adelaide, Australia, Azerbaijan National Academy of Sciences, Baku, Azerbaijan; Abeloos, Baptiste, Laboratoire de l'Accélérateur Linéaire, Orsay, France; Abhayasinghe, D. K., Royal Holloway, University of London, Egham, United Kingdom; Abidi, Syed Hani, Department of Physics, University of Toronto, Toronto, Canada; AbouZeid, O. S., Niels Bohr Institutet, Copenhagen, Denmark; Abraham, Nicola L., University of Sussex, Brighton, United KingdomResults are presented from the measurement by ATLAS of long-range (| Δ η| > 2) dihadron angular correlations in s=8 and 13 TeV pp collisions containing a Z boson. The analysis is performed using 19.4 fb - 1 of s=8 TeV data recorded during Run 1 of the LHC and 36.1 fb - 1 of s=13 TeV data recorded during Run 2. Two-particle correlation functions are measured as a function of relative azimuthal angle over the relative pseudorapidity range 2 < | Δ η| < 5 for different intervals of charged-particle multiplicity and transverse momentum. The measurements are corrected for the presence of background charged particles generated by collisions that occur during one passage of two colliding proton bunches in the LHC. Contributions to the two-particle correlation functions from hard processes are removed using a template-fitting procedure. Sinusoidal modulation in the correlation functions is observed and quantified by the second Fourier coefficient of the correlation function, v2 , 2, which in turn is used to obtain the single-particle anisotropy coefficient v2. The v2 values in the Z-tagged events, integrated over 0.5 < pT< 5 GeV, are found to be independent of multiplicity and s, and consistent within uncertainties with previous measurements in inclusive pp collisions. As a function of charged-particle pT, the Z-tagged and inclusive v2 values are consistent within uncertainties for pT< 3 GeV. © 2020 Elsevier B.V., All rights reserved.Publication Open Access Transverse momentum and process dependent azimuthal anisotropies in √sNN=8.16 TeV p+Pb collisions with the ATLAS detector(Springer, 2020) Aad, Georges; Abbott, Brad K.; Abbott, D. C.; Abed Abud, Adam; Abeling, Kira; Abhayasinghe, D. K.; Abidi, Syed Hani; AbouZeid, O. S.; Abraham, Nicola L.; Abramowicz, Halina; Aad, Georges, Centre de Physique des Particules de Marseille, Marseille, France; Abbott, Brad K., Homer L. Dodge Department of Physics and Astronomy, The University of Oklahoma, Norman, United States; Abbott, D. C., Department of Physics, University of Massachusetts Amherst, Amherst, United States; Abed Abud, Adam, Organisation Européenne pour la Recherche Nucléaire, Geneva, Switzerland; Abeling, Kira, Institute of Physics, Georg-August-Universität Göttingen, Gottingen, Germany; Abhayasinghe, D. K., Royal Holloway, University of London, Egham, United Kingdom; Abidi, Syed Hani, Department of Physics, University of Toronto, Toronto, Canada; AbouZeid, O. S., Niels Bohr Institutet, Copenhagen, Denmark; Abraham, Nicola L., University of Sussex, Brighton, United Kingdom; Abramowicz, Halina, Raymond and Beverly Sackler School of Physics and Astronomy, Tel Aviv University, Tel Aviv-Yafo, IsraelThe azimuthal anisotropy of charged particles produced in sNN=8.16 TeV p+Pb collisions is measured with the ATLAS detector at the LHC. The data correspond to an integrated luminosity of 165 nb - 1 that was collected in 2016. Azimuthal anisotropy coefficients, elliptic v2 and triangular v3, extracted using two-particle correlations with a non-flow template fit procedure, are presented as a function of particle transverse momentum (pT) between 0.5 and 50 GeV. The v2 results are also reported as a function of centrality in three different particle pT intervals. The results are reported from minimum-bias events and jet-triggered events, where two jet pT thresholds are used. The anisotropies for particles with pT less than about 2 GeV are consistent with hydrodynamic flow expectations, while the significant non-zero anisotropies for pT in the range 9–50 GeV are not explained within current theoretical frameworks. In the pT range 2–9 GeV, the anisotropies are larger in minimum-bias than in jet-triggered events. Possible origins of these effects, such as the changing admixture of particles from hard scattering and the underlying event, are discussed. © 2020 Elsevier B.V., All rights reserved.Publication Open Access Combined measurements of Higgs boson production and decay using up to 80 fb-1 of proton-proton collision data at √s =13 TeV collected with the ATLAS experiment(American Physical Society, 2020) Aad, Georges; Abbott, Brad K.; Abbott, D. C.; Abdinov, O. B.; Abed Abud, Adam; Abeling, Kira; Abhayasinghe, D. K.; Abidi, Syed Hani; AbouZeid, O. S.; Abraham, Nicola L.; Aad, Georges, Centre de Physique des Particules de Marseille, Marseille, France; Abbott, Brad K., The University of Oklahoma, Norman, United States; Abbott, D. C., Department of Physics, University of Massachusetts Amherst, Amherst, United States; Abdinov, O. B., Azerbaijan National Academy of Sciences, Baku, Azerbaijan; Abed Abud, Adam, Istituto Nazionale di Fisica Nucleare, Sezione di Pavia, Pavia, Italy, Università degli Studi di Pavia, Pavia, Italy; Abeling, Kira, Georg-August-Universität Göttingen, Gottingen, Germany; Abhayasinghe, D. K., Department of Physics, Royal Holloway, University of London, Egham, United Kingdom; Abidi, Syed Hani, University of Toronto, Toronto, Canada; AbouZeid, O. S., Niels Bohr Institutet, Copenhagen, Denmark; Abraham, Nicola L., University of Sussex, Brighton, United KingdomCombined measurements of Higgs boson production cross sections and branching fractions are presented. The combination is based on the analyses of the Higgs boson decay modes H → γγ, ZZ∗, WW∗, τ τ, bb, μ μ, searches for decays into invisible final states, and on measurements of off-shell Higgs boson production. Up to 79.8 fb-1 of proton-proton collision data collected at √ = 13 TeV with the ATLAS detector are used. Results are presented for the gluon-gluon fusion and vector-boson fusion processes, and for associated production with vector bosons or top-quarks. The global signal strength is determined to be μ = 1.11-0.08+0.09. The combined measurement yields an observed (expected) significance for the vector-boson fusion production process of 6.5δ (5.3δ). Measurements in kinematic regions defined within the simplified template cross section framework are also shown. The results are interpreted in terms of modifiers applied to the Standard Model couplings of the Higgs boson to other particles, and are used to set exclusion limits on parameters in two-Higgs-doublet models and in the simplified minimal supersymmetric Standard Model. No significant deviations from Standard Model predictions are observed. © 2025 Elsevier B.V., All rights reserved.Publication Open Access Searches for lepton-flavour-violating decays of the Higgs boson in s=13 TeV pp collisions with the ATLAS detector(Elsevier B.V., 2020) Aad, Georges; Abbott, Brad K.; Abbott, D. C.; Abed Abud, Adam; Abeling, Kira; Abhayasinghe, D. K.; Abidi, Syed Hani; AbouZeid, O. S.; Abraham, Nicola L.; Abramowicz, Halina; Aad, Georges, Centre de Physique des Particules de Marseille, Marseille, France; Abbott, Brad K., The University of Oklahoma, Norman, United States; Abbott, D. C., Department of Physics, University of Massachusetts Amherst, Amherst, United States; Abed Abud, Adam, Istituto Nazionale di Fisica Nucleare, Sezione di Pavia, Pavia, Italy, Università degli Studi di Pavia, Pavia, Italy; Abeling, Kira, Georg-August-Universität Göttingen, Gottingen, Germany; Abhayasinghe, D. K., Department of Physics, Royal Holloway, University of London, Egham, United Kingdom; Abidi, Syed Hani, University of Toronto, Toronto, Canada; AbouZeid, O. S., Niels Bohr Institutet, Copenhagen, Denmark; Abraham, Nicola L., University of Sussex, Brighton, United Kingdom; Abramowicz, Halina, Tel Aviv University, Tel Aviv-Yafo, IsraelThis Letter presents direct searches for lepton flavour violation in Higgs boson decays, H→eτ and H→μτ, performed with the ATLAS detector at the LHC. The searches are based on a data sample of proton–proton collisions at a centre-of-mass energy s=13 TeV, corresponding to an integrated luminosity of 36.1 fb−1. No significant excess is observed above the expected background from Standard Model processes. The observed (median expected) 95% confidence-level upper limits on the lepton-flavour-violating branching ratios are 0.47% (0.34−0.10 +0.13%) and 0.28% (0.37−0.10 +0.14%) for H→eτ and H→μτ, respectively. © 2025 Elsevier B.V., All rights reserved.Publication Open Access Combination of searches for Higgs boson pairs in pp collisions at s=13TeV with the ATLAS detector(Elsevier B.V., 2020) Aad, Georges; Abbott, Brad K.; Abbott, D. C.; Abed Abud, Adam; Abeling, Kira; Abhayasinghe, D. K.; Abidi, Syed Hani; AbouZeid, O. S.; Abraham, Nicola L.; Abramowicz, Halina; Aad, Georges, Centre de Physique des Particules de Marseille, Marseille, France; Abbott, Brad K., The University of Oklahoma, Norman, United States; Abbott, D. C., Department of Physics, University of Massachusetts Amherst, Amherst, United States; Abed Abud, Adam, Istituto Nazionale di Fisica Nucleare, Sezione di Pavia, Pavia, Italy, Università degli Studi di Pavia, Pavia, Italy; Abeling, Kira, Georg-August-Universität Göttingen, Gottingen, Germany; Abhayasinghe, D. K., Department of Physics, Royal Holloway, University of London, Egham, United Kingdom; Abidi, Syed Hani, University of Toronto, Toronto, Canada; AbouZeid, O. S., Niels Bohr Institutet, Copenhagen, Denmark; Abraham, Nicola L., University of Sussex, Brighton, United Kingdom; Abramowicz, Halina, Tel Aviv University, Tel Aviv-Yafo, IsraelThis letter presents a combination of searches for Higgs boson pair production using up to 36.1 fb−1 of proton–proton collision data at a centre-of-mass energy s=13 TeV recorded with the ATLAS detector at the LHC. The combination is performed using six analyses searching for Higgs boson pairs decaying into the bb¯bb¯, bb¯W+W−, bb¯τ+τ−, W+W−W+W−, bb¯γγ and W+W−γγ final states. Results are presented for non-resonant and resonant Higgs boson pair production modes. No statistically significant excess in data above the Standard Model predictions is found. The combined observed (expected) limit at 95% confidence level on the non-resonant Higgs boson pair production cross-section is 6.9 (10) times the predicted Standard Model cross-section. Limits are also set on the ratio (κλ) of the Higgs boson self-coupling to its Standard Model value. This ratio is constrained at 95% confidence level in observation (expectation) to −5.0<κλ<12.0 (−5.8<κλ<12.0). In addition, limits are set on the production of narrow scalar resonances and spin-2 Kaluza–Klein Randall–Sundrum gravitons. Exclusion regions are also provided in the parameter space of the habemus Minimal Supersymmetric Standard Model and the Electroweak Singlet Model. © 2025 Elsevier B.V., All rights reserved.Publication Open Access Search for flavour-changing neutral currents in processes with one top quark and a photon using 81 fb−1 of pp collisions at s=13TeV with the ATLAS experiment(Elsevier B.V., 2020) Aad, Georges; Abbott, Brad K.; Abbott, D. C.; Abed Abud, Adam; Abeling, Kira; Abhayasinghe, D. K.; Abidi, Syed Hani; AbouZeid, O. S.; Abraham, Nicola L.; Abramowicz, Halina; Aad, Georges, Centre de Physique des Particules de Marseille, Marseille, France; Abbott, Brad K., The University of Oklahoma, Norman, United States; Abbott, D. C., Department of Physics, University of Massachusetts Amherst, Amherst, United States; Abed Abud, Adam, Istituto Nazionale di Fisica Nucleare, Sezione di Pavia, Pavia, Italy, Università degli Studi di Pavia, Pavia, Italy; Abeling, Kira, Georg-August-Universität Göttingen, Gottingen, Germany; Abhayasinghe, D. K., Department of Physics, Royal Holloway, University of London, Egham, United Kingdom; Abidi, Syed Hani, University of Toronto, Toronto, Canada; AbouZeid, O. S., Niels Bohr Institutet, Copenhagen, Denmark; Abraham, Nicola L., University of Sussex, Brighton, United Kingdom; Abramowicz, Halina, Tel Aviv University, Tel Aviv-Yafo, IsraelA search for flavour-changing neutral current (FCNC) events via the coupling of a top quark, a photon, and an up or charm quark is presented using 81 fb−1 of proton–proton collision data taken at a centre-of-mass energy of 13 TeV with the ATLAS detector at the LHC. Events with a photon, an electron or muon, a b-tagged jet, and missing transverse momentum are selected. A neural network based on kinematic variables differentiates between events from signal and background processes. The data are consistent with the background-only hypothesis, and limits are set on the strength of the tqγ coupling in an effective field theory. These are also interpreted as 95% CL upper limits on the cross section for FCNC tγ production via a left-handed (right-handed) tuγ coupling of 36 fb (78 fb) and on the branching ratio for t→γu of 2.8×10−5 (6.1×10−5). In addition, they are interpreted as 95% CL upper limits on the cross section for FCNC tγ production via a left-handed (right-handed) tcγ coupling of 40 fb (33 fb) and on the branching ratio for t→γc of 22×10−5 (18×10−5). © 2025 Elsevier B.V., All rights reserved.Publication Open Access Search for Magnetic Monopoles and Stable High-Electric-Charge Objects in 13 Tev Proton-Proton Collisions with the ATLAS Detector(American Physical Society, 2020) Aad, Georges; Abbott, Brad K.; Abbott, D. C.; Abdinov, O. B.; Abed Abud, Adam; Abeling, Kira; Abhayasinghe, D. K.; Abidi, Syed Hani; AbouZeid, O. S.; Abraham, Nicola L.; Aad, Georges, Centre de Physique des Particules de Marseille, Marseille, France; Abbott, Brad K., The University of Oklahoma, Norman, United States; Abbott, D. C., Department of Physics, University of Massachusetts Amherst, Amherst, United States; Abdinov, O. B., Azerbaijan National Academy of Sciences, Baku, Azerbaijan; Abed Abud, Adam, Istituto Nazionale di Fisica Nucleare, Sezione di Pavia, Pavia, Italy, Università degli Studi di Pavia, Pavia, Italy; Abeling, Kira, Faculty of Science, Ibn Tofail University, Kenitra, Morocco; Abhayasinghe, D. K., Department of Physics, Royal Holloway, University of London, Egham, United Kingdom; Abidi, Syed Hani, Novosibirsk State University, Novosibirsk, Russian Federation; AbouZeid, O. S., Niels Bohr Institutet, Copenhagen, Denmark; Abraham, Nicola L., University of Sussex, Brighton, United KingdomA search for magnetic monopoles and high-electric-charge objects is presented using 34.4 fb-1 of 13 TeV pp collision data collected by the ATLAS detector at the LHC during 2015 and 2016. The considered signature is based upon high ionization in the transition radiation tracker of the inner detector associated with a pencil-shape energy deposit in the electromagnetic calorimeter. The data were collected by a dedicated trigger based on the tracker high-threshold hit capability. The results are interpreted in models of Drell-Yan pair production of stable particles with two spin hypotheses (0 and 1/2) and masses ranging from 200 to 4000 GeV. The search improves by approximately a factor of 5 the constraints on the direct production of magnetic monopoles carrying one or two Dirac magnetic charges and stable objects with electric charge in the range 20≤|z|≤60 and extends the charge range to 60<|z|≤100. © 2025 Elsevier B.V., All rights reserved.