Browsing by Author "Aakvaag, E."

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Citation - WoS: 2
Citation - Scopus: 3
Accuracy Versus Precision in Boosted Top Tagging With the Atlas Detector
(Iop Publishing Ltd, 2024) Aad, G.; Aakvaag, E.; Abbott, B.; Abdelhameed, S.; Abeling, K.; Abicht, N. J.; Zwalinski, L.
The identification of top quark decays where the top quark has a large momentum transverse to the beam axis, known as top tagging, is a crucial component in many measurements of Standard Model processes and searches for beyond the Standard Model physics at the Large Hadron Collider. Machine learning techniques have improved the performance of top tagging algorithms, but the size of the systematic uncertainties for all proposed algorithms has not been systematically studied. This paper presents the performance of several machine learning based top tagging algorithms on a dataset constructed from simulated proton-proton collision events measured with the ATLAS detector at root s = 13 TeV. The systematic uncertainties associated with these algorithms are estimated through an approximate procedure that is not meant to be used in a physics analysis, but is appropriate for the level of precision required for this study. The most performant algorithms are found to have the largest uncertainties, motivating the development of methods to reduce these uncertainties without compromising performance. To enable such efforts in the wider scientific community, the datasets used in this paper are made publicly available.
Citation - WoS: 9
ATLAS Searches for Additional Scalars and Exotic Higgs Boson Decays with the LHC Run 2 Dataset
(Elsevier, 2025) Aad, G.; Aakvaag, E.; Abbott, B.; Abdelhameed, S.; Abeling, K.; Abicht, N. J.; Zwalinski, L.
This report reviews the published results of searches for possible additional scalar particles and exotic decays of the Higgs boson performed by the ATLAS Collaboration using up to 140 fb(-1) of 13 TeV proton-proton collision data collected during Run 2 of the Large Hadron Collider. Key results are examined, and observed excesses, while never statistically compelling, are noted. Constraints are placed on parameters of several models which extend the Standard Model, for example by adding one or more singlet or doublet fields, or offering exotic Higgs boson decay channels. Summaries of new searches as well as extensions of previous searches are discussed. These new results have a wider reach or attain stronger exclusion limits. New experimental techniques that were developed for these searches are highlighted. Search channels which have not yet been examined are also listed, as these provide insight into possible future areas of exploration. (c) 2024 CERN for the benefit of the ATLAS Collaboration. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Citation - WoS: 6
Citation - Scopus: 10
The Atlas Trigger System for Lhc Run 3 and Trigger Performance in 2022
(Iop Publishing Ltd, 2024) Aad, G.; Aakvaag, E.; Abbott, B.; Abeling, K.; Abicht, N. J.; Abidi, S. H.; Zwalinski, L.
The ATLAS trigger system is a crucial component of the ATLAS experiment at the LHC. It is responsible for selecting events in line with the ATLAS physics programme. This paper presents an overview of the changes to the trigger and data acquisition system during the second long shutdown of the LHC, and shows the performance of the trigger system and its components in the proton-proton collisions during the 2022 commissioning period as well as its expected performance in proton-proton and heavy-ion collisions for the remainder of the third LHC data-taking period (2022-2025).
Citation - Scopus: 1
Beam-Induced Backgrounds Measured in the Atlas Detector During Local Gas Injection Into the Lhc Beam Vacuum
(Iop Publishing Ltd, 2024) Aad, G.; Aakvaag, E.; Abbott, B.; Abdelhameed, S.; Abeling, K.; Abicht, N. J.; Zwalinski, L.
Inelastic beam-gas collisions at the Large Hadron Collider (LHC), within a few hundred metres of the ATLAS experiment, are known to give the dominant contribution to beam backgrounds. These are monitored by ATLAS with a dedicated Beam Conditions Monitor (BCM) and with the rate of fake jets in the calorimeters. These two methods are complementary since the BCM probes backgrounds just around the beam pipe while fake jets are observed at radii of up to several metres. In order to quantify the correlation between the residual gas density in the LHC beam vacuum and the experimental backgrounds recorded by ATLAS, several dedicated tests were performed during LHC Run 2. Local pressure bumps, with a gas density several orders of magnitude higher than during normal operation, were introduced at different locations. The changes of beam-related backgrounds, seen in ATLAS, are correlated with the local pressure variation. In addition the rates of beam-gas events are estimated from the pressure measurements and pressure bump profiles obtained from calculations. Using these rates, the efficiency of the ATLAS beam background monitors to detect beam-gas events is derived as a function of distance from the interaction point. These efficiencies and characteristic distributions of fake jets from the beam backgrounds are found to be in good agreement with results of beam-gas simulations performed with the Fluka Monte Carlo programme.
Calibration of a Soft Secondary Vertex Tagger Using Proton-Proton Collisions at √s=13 Tev With the Atlas Detector
(Amer Physical Soc, 2024) Aad, G.; Aakvaag, E.; Abbott, B.; Abdelhameed, S.; Abeling, K.; Abicht, N. J.; Zwalinski, L.
Several processes studied by the ATLAS experiment at the Large Hadron Collider produce low-momentum b-flavored hadrons in the final state. This paper describes the calibration of a dedicated tagging algorithm that identifies b-flavored hadrons outside of hadronic jets by reconstructing the soft secondary vertices originating from their decays. The calibration is based on a proton-proton collision dataset at a center-of-mass energy of 13 TeV corresponding to an integrated luminosity of 140 fb(-1). Scale factors used to correct the algorithm's performance in simulated events are extracted for the b-tagging efficiency and the mistag rate of the algorithm using a data sample enriched in t (t) over bar events. Several orthogonal measurement regions are defined, binned as a function of the multiplicities of soft secondary vertices and jets containing a b-flavored hadron in the event. The mistag rate scale factors are estimated separately for events with low and high average numbers of interactions per bunch crossing. The results, which are derived from events with low missing transverse momentum, are successfully validated in a phase space characterized by high missing transverse momentum and therefore are applicable to new physics searches carried out in either phase space regime.
Citation - WoS: 2
Characterising the Higgs Boson with ATLAS Data From the LHC Run-2
(Elsevier, 2025) Aad, G.; Aakvaag, E.; Abbott, B.; Abeling, K.; Abicht, N. J.; Abidi, S. H.; Zwalinski, L.
The Higgs boson was discovered by the ATLAS and CMS Collaborations in 2012 using data from Run 1 of the Large Hadron Collider (2010-2012). In Run 2 (2015-2018), about 140 fb(-1) of proton-proton collisions at a centre-of-mass energy of 13 TeV were collected by the ATLAS experiment. This review presents the most important Run 2 results obtained by the ATLAS Collaboration regarding the properties of the Higgs boson and its interactions with other particles. The performed studies significantly enhance the understanding of the Higgs boson, while hunting for deviations from the predictions of the Standard Model of particle physics. (c) 2024 CERN for the benefit of the ATLAS Collaboration. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Charged-Hadron and Identified-Hadron (Ks0, Λ, Ξ-) Yield Measurements in Photonuclear Pb Plus Pb and P Plus Pb Collisions at √snn=5.02 Tev with ATLAS
(Amer Physical Soc, 2025) Aad, G.; Aakvaag, E.; Abbott, B.; Abdelhameed, S.; Abeling, K.; Abicht, N. J.; Zwalinski, L.
This paper presents the measurement of charged-hadron and identified-hadron (K-S(0), Lambda, Xi(-)) yields in photonuclear collisions using 1.7 nb(-1) of root s(NN) = 5.02 TeV Pb + Pb data collected in 2018 with the ATLAS detector at the Large Hadron Collider. Candidate photonuclear events are selected using a combination of tracking and calorimeter information, including the zero-degree calorimeter. The yields as a function of transverse momentum and rapidity are measured in these photonuclear collisions as a function of charged-particle multiplicity. These photonuclear results are compared with 0.1nb(-1) of root s(NN) = 5.02 TeV p + Pb data collected in 2016 by ATLAS using similar charged-particle multiplicity selections. These photonuclear measurements shed light on potential quark-gluon plasma formation in photonuclear collisions via observables sensitive to radial flow, enhanced baryon-to-meson ratios, and strangeness enhancement. The results are also compared with the Monte Carlo DPMJET-III generator and hydrodynamic calculations to test whether such photonuclear collisions may produce small droplets of quark-gluon plasma that flow collectively.
Citation - WoS: 3
Citation - Scopus: 3
Climbing to the Top of the Atlas 13 TeV Data
(Elsevier, 2025) Aad, G.; Aakvaag, E.; Abbott, B.; Abdelhameed, S.; Abeling, K.; Abicht, N. J.; Haas, A.
The large amount of data recorded with the ATLAS detector at the Large Hadron Collider, corresponding to 140 fb(-1) of pp collisions at a centre-of-mass energy of root s = 13 TeV, has brought our knowledge of the top quark to a higher level. The measurement of the top-antitop quark pair-production cross-section has reached a precision of 1.8% and the cross-section was measured differentially up to several TeV in multiple observables including the top-quark transverse momentum and top-quark-pair invariant mass. Single-top-quark production was studied in all production modes. Rare production processes where the top quark is associated with a vector boson, and four-top-quark production, have become accessible and cross-section measurements for several of these processes have reached uncertainties of around 10% or smaller. Innovative measurements of the top-quark mass and properties have also emerged, including the observation of quantum entanglement in the top-quark sector and tests of lepton-flavour universality using top-quark decays. Searches for flavour-changing neutral currents in the top-quark sector have been significantly improved, reaching branchingratio exclusion limits ranging from 10(-3) to 10(-5). Many of these analyses have been used to set limits on Wilson coefficients within the effective field theory framework. (c) 2024 CERN for the benefit of the ATLAS Collaboration. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Citation - WoS: 5
Citation - Scopus: 6
Combination of Searches for Heavy Spin-1 Resonances Using 139 Fb-1 of Proton-Proton Collision Data at √s=13 Tev With the Atlas Detector
(Springer, 2024) Aad, G.; Aakvaag, E.; Abbott, B.; Abeling, K.; Abicht, N. J.; Abidi, S. H.; Aboelela, M.
A combination of searches for new heavy spin-1 resonances decaying into different pairings of W, Z, or Higgs bosons, as well as directly into leptons or quarks, is presented. The data sample used corresponds to 139 fb(-1) of proton-proton collisions at root s = 13 TeV collected during 2015-2018 with the ATLAS detector at the CERN Large Hadron Collider. Analyses selecting quark pairs (qq, bb, t (t) over bar, and tb) or third-generation leptons (tau nu and tau tau) are included in this kind of combination for the first time. A simplified model predicting a spin-1 heavy vector-boson triplet is used. Cross-section limits are set at the 95% confidence level and are compared with predictions for the benchmark model. These limits are also expressed in terms of constraints on couplings of the heavy vector-boson triplet to quarks, leptons, and the Higgs boson. The complementarity of the various analyses increases the sensitivity to new physics, and the resulting constraints are stronger than those from any individual analysis considered. The data exclude a heavy vector-boson triplet with mass below 5.8 TeV in a weakly coupled scenario, below 4.4 TeV in a strongly coupled scenario, and up to 1.5 TeV in the case of production via vector-boson fusion.
Citation - WoS: 1
Citation - Scopus: 2
Combination of Searches for Higgs Boson Decays Into a Photon and a Massless Dark Photon Using Pp Collisions at √s = 13 Tev With the Atlas Detector
(Springer, 2024) Aad, G.; Aakvaag, E.; Abbott, B.; Abdelhameed, S.; Abeling, K.; Abicht, N. J.; Zwalinski, L.
A combination of searches for Higgs boson decays into a visible photon and a massless dark photon (H -> gamma gamma(d)) is presented using 139 fb(-1) of proton-proton collision data at a centre-of-mass energy of root s = 13 TeV recorded by the ATLAS detector at the Large Hadron Collider. The observed (expected) 95% confidence level upper limit on the Standard Model Higgs boson decay branching ratio is determined to be B(H -> gamma gamma(d)) < 1.3% (1.5)%. The search is also sensitive to higher-mass Higgs bosons decaying into the same final state. The observed (expected) 95% confidence level limit on the cross-section times branching ratio ranges from 16 fb (20 fb) for m(H) = 400 GeV to 1.0 fb (1.5 fb) for m(H) = 3 TeV. Results are also interpreted in the context of a minimal simplified model.
Citation - WoS: 11
Combination of Searches for Higgs Boson Pair Production in pp Collisions at √s=13 Tev With the Atlas Detector
(Amer Physical Soc, 2024) Aad, G.; Aakvaag, E.; Abbott, B.; Abdelhameed, S.; Abeling, K.; Abicht, N. J.; Zwalinski, L.
This Letter presents results from a combination of searches for Higgs boson pair production using 126-140 fb(-1) of proton-proton collision data at root s = 13 TeV recorded with the ATLAS detector. At 95% confidence level (CL), the upper limit on the production rate is 2.9 times the standard model (SM) prediction, with an expected limit of 2.4 assuming no Higgs boson pair production. Constraints on the Higgs boson self-coupling modifier kappa(lambda) =lambda(HHH) /lambda(SM)(HHH), and the quartic HHVV coupling modifier kappa(2V) = g(HHVV)/g(HHVV)(SM), are derived individually, fixing the other parameter to its SM value. The observed 95% CL intervals are -1.2
Citation - WoS: 3
Citation - Scopus: 7
Combination of Searches for Pair-Produced Leptoquarks at S=13 Tev With the Atlas Detector
(Elsevier B.V., 2024) Aad, G.; Aakvaag, E.; Abbott, B.; Abeling, K.; Abicht, N.J.; Abidi, S.H.; Aboulhorma, A.
A statistical combination of various searches for pair-produced leptoquarks is presented, using the full LHC Run 2 (2015–2018) data set of 139 fb−1 collected with the ATLAS detector from proton–proton collisions at a centre-of-mass energy of s=13 TeV. All possible decays of the leptoquarks into quarks of the third generation and charged or neutral leptons of any generation are investigated. Since no significant deviations from the Standard Model expectation are observed in any of the individual analyses, combined exclusion limits are set on the production cross-sections for scalar and vector leptoquarks. The resulting lower bounds on leptoquark masses exceed those from the individual analyses by up to 100 GeV, depending on the signal hypothesis. © 2024 The Author(s)
Citation - WoS: 7
Combination of Searches for Resonant Higgs Boson Pair Production Using Pp Collisions at Sqrt[s]=13 Tev With the Atlas Detector
(2024) Aad, G.; Aakvaag, E.; Abbott, B.; Abeling, K.; Abicht, N.J.; Abidi, S.H.; Aboulhorma, A.
A combination of searches for a new resonance decaying into a Higgs boson pair is presented, using up to 139 fb^{-1} of pp collision data at sqrt[s]=13 TeV recorded with the ATLAS detector at the LHC. The combination includes searches performed in three decay channels: bb[over ¯]bb[over ¯], bb[over ¯]τ^{+}τ^{-}, and bb[over ¯]γγ. No excess above the expected Standard Model background is observed and upper limits are set at the 95% confidence level on the production cross section of Higgs boson pairs originating from the decay of a narrow scalar resonance with mass in the range 251 GeV-5 TeV. The observed (expected) limits are in the range 0.96-600 fb (1.2-390 fb). The limits are interpreted in the type-I two-Higgs-doublet model and the minimal supersymmetric standard model, and constrain parameter space not previously excluded by other searches.
Citation - WoS: 3
Citation - Scopus: 3
Combination of Searches for Singly and Doubly Charged Higgs Bosons Produced Via Vector-Boson Fusion in Proton-Proton Collisions at √s=13 Tev With the Atlas Detector
(Elsevier, 2024) Aad, G.; Aakvaag, E.; Abbott, B.; Abdelhameed, S.; Abeling, K.; Abicht, N. J.; Zwalinski, L.
A combination of searches for singly and doubly charged Higgs bosons, H-+/- and H-+/-+/-, produced via vector-boson fusion is performed using 140 fb(-1) of proton-proton collisions at a centre-of-mass energy of 13 TeV, collected with the ATLAS detector during Run 2 of the Large Hadron Collider. Searches targeting decays to massive vector bosons in leptonic final states (electrons or muons) are considered. New constraints are reported on the production crosssection times branching fraction for charged Higgs boson masses between 200 GeV and 3000 GeV. The results are interpreted in the context of the Georgi-Machacek model for which the most stringent constraints to date are set for the masses considered in the combination.
Citation - WoS: 7
Citation - Scopus: 4
Combination of Searches for Singly Produced Vectorlike Top Quarks in Pp Collisions at √sp=13 Tev With the Atlas Detector
(Amer Physical Soc, 2025) Aad, G.; Aakvaag, E.; Abbott, B.; Abdelhameed, S.; Abeling, K.; Abicht, N. J.; Wang, Z.
A combination of searches for the single production of vectorlike top quarks (T) is presented. These analyses are based on proton-proton collisions at root sp = 13 TeV recorded in 2015-2018 with the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of 139 fb(-1). The T decay modes considered in this combination are into a top quark and either a Standard Model Higgs boson or a Z boson (T -> Ht and T -> Zt). The individual searches used in the combination are differentiated by the number of leptons (e, mu) in the final state. The observed data are found to be in good agreement with the Standard Model background prediction. Interpretations are provided for a range of masses and couplings of the vectorlike top quark for benchmark models and generalized representations in terms of 95% confidence level limits. For a benchmark signal prediction of a vectorlike top quark SU(2) singlet with electroweak coupling, kappa, of 0.5, masses below 2.1 TeV are excluded, resulting in the most restrictive limits to date.
Configuration, Performance, and Commissioning of the Atlas B-Jet Triggers for the 2022 and 2023 LHC Data-Taking Periods
(Institute of Physics, 2025) Aad, G.; Aakvaag, E.; Abbott, B.; Abdelhameed, S.; Abeling, K.; Abicht, N.J.; Jeanty, L.
In 2022 and 2023, the Large Hadron Collider produced approximately two billion hadronic interactions each second from bunches of protons that collide at a rate of 40 MHz. The ATLAS trigger system is used to reduce this rate to a few kHz for recording. Selections based on hadronic jets, their energy, and event topology reduce the rate to Ô(10) kHz while maintaining high efficiencies for important signatures resulting in b-quarks, but to reach the desired recording rate of hundreds of Hz, additional real-time selections based on the identification of jets containing b-hadrons (b-jets) are employed to achieve low thresholds on the jet transverse momentum at the High-Level Trigger. The configuration, commissioning, and performance of the real-time ATLAS b-jet identification algorithms for the early LHC Run 3 collision data are presented. These recent developments provide substantial gains in signal efficiency for critical signatures; for the Standard Model production of Higgs boson pairs, a 50% improvement in selection efficiency is observed in final states with four b-quarks or two b-quarks and two hadronically decaying τ-leptons. © 2025 CERN.
Citation - WoS: 1
Citation - Scopus: 3
Constraint on the Total Width of the Higgs Boson From Higgs Boson and Four-Top Measurements in Pp Collisions at √s=13 Tev With the Atlas Detector
(Elsevier, 2025) Aad, G.; Aakvaag, E.; Abbott, B.; Abdelhameed, S.; Abeling, K.; Abicht, N. J.; Zhao, Z.
This Letter presents a constraint on the total width of the Higgs boson (Gamma(H)) using a combined measurement of on-shell Higgs boson production and the production of four top quarks, which involves contributions from off-shell Higgs boson-mediated processes. This method relies on the assumption that the tree-level Higgs-top Yukawa coupling strength is the same for on-shell and off-shell Higgs boson production processes, thereby avoiding any assumptions about the relationship between on-shell and off-shell gluon fusion Higgs production rates, which were central to previous measurements. The result is based on up to 140 fb(-1) of proton-proton collisions at a centre-of-mass energy of root s = 13 TeV collected with the ATLAS detector at the Large Hadron Collider. The observed (expected) 95% confidence level upper limit on Gamma(H) is 450 MeV (75 MeV). Additionally, considering the constraint on the Higgs-top Yukawa coupling from loop-induced Higgs boson production and decay processes further yields an observed (expected) upper limit of 160 MeV (55 MeV).
Citation - WoS: 7
Citation - Scopus: 9
Constraints on Simplified Dark Matter Models Involving an s-channel Mediator With the Atlas Detector in pp Collisions at √s=13 Tev
(Springer, 2024) Aad, G.; Aakvaag, E.; Abbott, B.; Abdelhameed, S.; Abeling, K.; Abicht, N. J.; Zwalinski, L.; Sultansoy, Saleh
This paper reports a summary of searches for a fermionic dark matter candidate in the context of theoretical models characterised by a mediator particle exchange in the s-channel. The data sample considered consists of pp collisions delivered by the Large Hadron Collider during its Run 2 at a centre-of-mass energy of root s = 13 TeV and recorded by the ATLAS detector, corresponding to up to 140 fb(-1). The interpretations of the results are based on simplified models where the new mediator particles can be spin-0, with scalar or pseudo-scalar couplings to fermions, or spin-1, with vector or axial-vector couplings to fermions. Exclusion limits are obtained from various searches characterised by final states with resonant production of Standard Model particles, or production of Standard Model particles in association with large missing transverse momentum.
Citation - Scopus: 3
Differential Cross-Section Measurements of Higgs Boson Production in the H → τ+τ− Decay Channel in pp Collisions at S = 13 TeV With the ATLAS Detector
(Springer Science and Business Media Deutschland GmbH, 2025) Aad, G.; Aakvaag, E.; Abbott, B.; Abdelhameed, S.; Abeling, K.; Abicht, N.J.; Jackson, P.
Differential measurements of Higgs boson production in the τ-lepton-pair decay channel are presented in the gluon fusion, vector-boson fusion (VBF), VH and tt¯H associated production modes, with particular focus on the VBF production mode. The data used to perform the measurements correspond to 140 fb−1 of proton-proton collisions collected by the ATLAS experiment at the LHC. Two methods are used to perform the measurements: the Simplified Template Cross-Section (STXS) approach and an Unfolded Fiducial Differential measurement considering only the VBF phase space. For the STXS measurement, events are categorized by their production mode and kinematic properties such as the Higgs boson’s transverse momentum (pTH), the number of jets produced in association with the Higgs boson, or the invariant mass of the two leading jets (mjj). For the VBF production mode, the ratio of the measured cross-section to the Standard Model prediction for mjj > 1.5 TeV and pTH > 200 GeV (pTH < 200 GeV) is 1.29−0.34+0.39 (0.12−0.33+0.34). This is the first VBF measurement for the higher-pTH criteria, and the most precise for the lower-pTH criteria. The fiducial cross-section measurements, which only consider the kinematic properties of the event, are performed as functions of variables characterizing the VBF topology, such as the signed ∆ϕjj between the two leading jets. The measurements have a precision of 30%–50% and agree well with the Standard Model predictions. These results are interpreted in the SMEFT framework, and place the strongest constraints to date on the CP-odd Wilson coefficient cHW~. © The Author(s) 2025.
Citation - WoS: 5
Citation - Scopus: 7
Differential Cross-Sections for Events With Missing Transverse Momentum and Jets Measured With the Atlas Detector in 13 Tev Proton-Proton Collisions
(Springer, 2024) Aad, G.; Aakvaag, E.; Abbott, B.; Abeling, K.; Abicht, N. J.; Abidi, S. H.; Zwalinski, L.
Measurements of inclusive, differential cross-sections for the production of events with missing transverse momentum in association with jets in proton-proton collisions at root s = 13 TeV are presented. The measurements are made with the ATLAS detector using an integrated luminosity of 140 fb(-1) and include measurements of dijet distributions in a region in which vector-boson fusion processes are enhanced. They are unfolded to correct for detector resolution and efficiency within the fiducial acceptance, and are designed to allow robust comparisons with a wide range of theoretical predictions. A measurement of differential cross sections for the Z -> nu nu process is made. The measurements are generally well-described by Standard Model predictions except for the dijet invariant mass distribution. Auxiliary measurements of the hadronic system recoiling against isolated leptons, and photons, are also made in the same phase space. Ratios between the measured distributions are then derived, to take advantage of cancellations in modelling effects and some of the major systematic uncertainties. These measurements are sensitive to new phenomena, and provide a mechanism to easily set constraints on phenomenological models. To illustrate the robustness of the approach, these ratios are compared with two common Dark Matter models, where the constraints derived from the measurement are comparable to those set by dedicated detector-level searches.