Determination of the parton distribution functions of the proton using diverse ATLAS data from pp collisions at root s = 7, 8 and 13 TeV

This paper presents an analysis at next-to-next to-leading order in the theory of quantum chromodynamics for the determination of a new set of proton parton distribution functions using diverse measurements in pp collisions at √s = 7, 8 and 13 TeV, performed by the ATLAS experiment at the Large Hadron Collider, together with deep inelastic scattering data from ep collisions at the HERA collider. The ATLAS data sets considered are differential cross-section measurements of inclusive W± and Z/γ ∗ boson production, W± and Z boson production in association with jets, tt ¯ production, inclusive jet production and direct photon production. In the analysis, particular attention is paid to the correlation of systematic uncertainties within and between the various ATLAS data sets and to the impact of model, theoretical and parameterisation uncertainties. The resulting set of parton distribution functions is called ATLASpdf21.G. Aad … P. Jackson … A.X.Y. Kong … J.L. Oliver … H. Potti … T.A. Ruggeri … A.S. Sharma …E. X. L. Ting ... M.J. White … et al. (The ATLAS Collaboration

Search for new phenomena in multi-body invariant masses in events with at least one isolated lepton and two jets using root s = 13 TeV proton-proton collision data collected by the ATLAS detector

Published: July 26, 2023A search for resonances in events with at least one isolated lepton (e or μ) and two jets is performed using 139 fb⁻¹ of √s = 13 TeV proton-proton collision data recorded by the ATLAS detector at the LHC. Deviations from a smoothly falling background hypothesis are tested in three- and four-body invariant mass distributions constructed from leptons and jets, including jets identified as originating from bottom quarks. Model-independent limits on generic resonances characterised by cascade decays of particles leading to multiple jets and leptons in the final state are presented. The limits are calculated using Gaussian shapes with different widths for the invariant masses. The multi-body invariant masses are also used to set 95% confidence level upper limits on the cross-section times branching ratios for the production and subsequent decay of resonances predicted by several new physics scenarios.G. Aad ... E. K. Filmer ... P. Jackson ... A. X. Y. Kong ... H. Potti ... T. A. Ruggeri ... E. X. L. Ting ... M. J. White ... et al. (The ATLAS collaboration

Observation of electroweak production of two jets in association with an isolated photon and missing transverse momentum, and search for a Higgs boson decaying into invisible particles at 13 TeV with the ATLAS detector

This paper presents a measurement of the electroweak production of two jets in association with a Zγ pair, with the Z boson decaying into two neutrinos. It also presents a search for invisible or partially invisible decays of a Higgs boson with a mass of 125 GeV produced through vector-boson fusion with a photon in the final state. These results use data from LHC proton–proton collisions at s√ = 13 TeV collected with the ATLAS detector and corresponding to an integrated luminosity of 139 fb⁻¹. The event signature, shared by all benchmark processes considered for the measurements and searches, is characterized by a significant amount of unbalanced transverse momentum and a photon in the final state, in addition to a pair of forward jets. Electroweak Zγ production in association with two jets is observed in this final state with a significance of 5.2 (5.1 expected) standard deviations. The measured fiducial cross-section for this process is 1.31 ± 0.29 fb. An observed (expected) upper limit of 0.37 (0.34(+0.15)(−0.10)) at 95% confidence level is set on the branching ratio of a 125 GeV Higgs boson to invisible particles, assuming the Standard Model production cross-section. The signature is also interpreted in the context of decays of a Higgs boson into a photon and a dark photon. An observed (expected) 95% CL upper limit on the branching ratio for this decay is set at 0.018 (0.017(+0.007)(−0.005)), assuming the Standard Model production cross-section for a 125 GeV Higgs boson.G. Aad … P. Jackson … A.X.Y. Kong … J.L. Oliver … H. Potti … T.A. Ruggeri … A.S. Sharma … M.J. White … et al. [The ATLAS Collaboration

Measurements of the Higgs boson inclusive and differential fiducial cross-sections in the diphoton decay channel with pp collisions at s root = 13 TeV with the ATLAS detector

Published: August 2, 2022A measurement of inclusive and differential fiducial cross-sections for the production of the Higgs boson decaying into two photons is performed using 139 fb−1 of proton-proton collision data recorded at s√ = 13 TeV by the ATLAS experiment at the Large Hadron Collider. The inclusive cross-section times branching ratio, in a fiducial region closely matching the experimental selection, is measured to be 67 ± 6 fb, which is in agreement with the state-of-the-art Standard Model prediction of 64 ± 4 fb. Extrapolating this result to the full phase space and correcting for the branching ratio, the total cross-section for Higgs boson production is estimated to be 58 ± 6 pb. In addition, the cross-sections in four fiducial regions sensitive to various Higgs boson production modes and differential cross-sections as a function of either one or two of several observables are measured. All the measurements are found to be in agreement with the Standard Model predictions. The measured transverse momentum distribution of the Higgs boson is used as an indirect probe of the Yukawa coupling of the Higgs boson to the bottom and charm quarks. In addition, five differential cross-section measurements are used to constrain anomalous Higgs boson couplings to vector bosons in the Standard Model effective field theory framework.The ATLAS collaboration, G. Aad ... P.Jackson ... A.X.Y Kong ... H.Potti ... T.A. Ruggeri ... A.S.Sharma ... E.X.L.Ting ... M.J.White ... et al

Search for Higgs boson decays into a pair of pseudoscalar particles in the bb mu mu final state with the ATLAS detector in pp collisions at root s=13TeV

This paper presents a search for decays of the Higgs boson with a mass of 125 GeV into a pair of new pseudoscalar particles, H → aa, where one a-boson decays into a b-quark pair and the other into a muon pair. The search uses 139 fb−1 of proton-proton collision data at a center-of-mass energy of ffiffiffi sp¼ 13 TeVrecorded between 2015 and 2018 by the ATLAS experiment at the LHC. A narrow dimuon resonance is searched for in the invariant mass spectrum between 16 GeV and 62 GeV. The largest excess of events above the Standard Model backgrounds is observed at a dimuon invariant mass of 52 GeVand corresponds to a local (global) significance of 3.3σ (1.7σ). Upper limits at 95% confidence level are placed on the branching ratio of the Higgs boson to the bbμμ final state, BðH → aa → bbμμÞ, and are in the range 0.2–4.0 × 10−4, depending on the signal mass hypothesis.G. Aad ... P. Jackson ... H. Potti ... M.J. White ... et al. (ATLAS Collaboration

Measurement of the polarisation of single top quarks and antiquarks produced in the t-channel at s root = 13 TeV and bounds on the tWb dipole operator from the ATLAS experiment

Published: November 9, 2022A simultaneous measurement of the three components of the top-quark and top-antiquark polarisation vectors in t-channel single-top-quark production is presented. This analysis is based on data from proton–proton collisions at a centre-of-mass energy of 13 TeV corresponding to an integrated luminosity of 139 fb−1, collected with the ATLAS detector at the LHC. Selected events contain exactly one isolated electron or muon, large missing transverse momentum and exactly two jets, one being b-tagged. Stringent selection requirements are applied to discriminate t-channel single-top-quark events from the background contributions. The top-quark and top-antiquark polarisation vectors are measured from the distributions of the direction cosines of the charged-lepton momentum in the top-quark rest frame. The three components of the polarisation vector for the selected top-quark event sample are Px′ = 0.01 ± 0.18, Py′ = −0.029 ± 0.027, Pz′ = 0.91 ± 0.10 and for the top-antiquark event sample they are Px′ = −0.02 ± 0.20, Py′ = −0.007 ± 0.051, Pz′ = 0.79 ± 0.16. Normalised differential cross-sections corrected to a fiducial region at the stable-particle level are presented as a function of the charged-lepton angles for top-quark and top-antiquark events inclusively and separately. These measurements are in agreement with Standard Model predictions. The angular differential cross-sections are used to derive bounds on the complex Wilson coefficient of the dimension-six OtW operator in the framework of an effective field theory. The obtained bounds are CtW ∈ [−0.9, 1.4] and CitW ∈ [−0.8, 0.2], both at 95% confidence level.G. Aad ... P. Jackson ... A.X.Y. Kong ... H. Potti ... T.A. Ruggeri .. A.S. Sharma ... M.J. White ... et al. (The ATLAS collaboration

Search for a new Z' gauge boson in 4mu events with the ATLAS experiment

Published: July 12, 2023This paper presents a search for a new Z′ vector gauge boson with the ATLAS experiment at the Large Hadron Collider using pp collision data collected at s√ = 13 TeV, corresponding to an integrated luminosity of 139 fb⁻¹. The new gauge boson Z′ is predicted by Lμ − Lτ models to address observed phenomena that can not be explained by the Standard Model. The search examines the four-muon (4μ) final state, using a deep learning neural network classifier to separate the Z′ signal from the Standard Model background events. The di-muon invariant masses in the 4μ events are used to extract the Z′ resonance signature. No significant excess of events is observed over the predicted background. Upper limits at a 95% confidence level on the Z′ production cross-section times the decay branching fraction of pp → Z′μμ → 4μ are set from 0.31 to 4.3 fb for the Z′ mass ranging from 5 to 81 GeV. The corresponding common coupling strengths, gZ′, of the Z′ boson to the second and third generation leptons above 0.003 – 0.2 have been excluded.G. Aad ... M. Amerl ... E. K. Filmer ... P. Jackson ... A. X. Y. Kong ... H. Potti ... T. A. Ruggeri ... E. X. L. Ting ... M. J. White ... et al. (The ATLAS collaboration

Search for single production of a vectorlike T quark decaying into a Higgs boson and top quark with fully hadronic final states using the ATLAS detector

A search is made for a vectorlike T quark decaying into a Higgs boson and a top quark in 13 TeV proton-proton collisions using the ATLAS detector at the Large Hadron Collider with a data sample corresponding to an integrated luminosity of 139 fb-1. The Higgs-boson and top-quark candidates are identified in the all-hadronic decay mode, where H→bb¯ and t→bW→bqq¯′ are reconstructed as large-radius jets. The candidate Higgs boson, top quark, and associated B hadrons are identified using tagging algorithms. No significant excess is observed above the background, so limits are set on the production cross section of a singlet T quark at 95% confidence level, depending on the mass mT and coupling κT of the vectorlike T quark to Standard Model particles. In the considered mass range between 1.0 and 2.3 TeV, the upper limit on the allowed coupling values increases with mT from a minimum value of 0.35 for 1.07<mT<1.4 TeV to 1.6 for mT=2.3 TeV.G. Aad … P. Jackson … A.X.Y. Kong … J.L. Oliver … H. Potti … T.A. Ruggeri … A.S. Sharma … E. X. L. Ting ... M.J. White … et al. (ATLAS Collaboration

Operation and performance of the ATLAS semiconductor tracker in LHC Run 2

The semiconductor tracker (SCT) is one of the tracking systems for charged particles in the ATLAS detector. It consists of 4088 silicon strip sensor modules. During Run 2 (2015–2018) the Large Hadron Collider delivered an integrated luminosity of 156 fb−1 to the ATLAS experiment at a centre-of-mass proton-proton collision energy of 13 TeV. The instantaneous luminosity and pile-up conditions were far in excess of those assumed in the original design of the SCT detector. Due to improvements to the data acquisition system, the SCT operated stably throughout Run 2. It was available for 99.9% of the integrated luminosity and achieved a data-quality efficiency of 99.85%. Detailed studies have been made of the leakage current in SCT modules and the evolution of the full depletion voltage, which are used to study the impact of radiation damage to the modules.The Atlas Collaboration ... G. Aad ... Paul D. Jackson ... Albert X. Kong ... Harish Potti ... Tristan A. Ruggeri ... Martin White ... et al

Search for boosted diphoton resonances in the 10 to 70 GeV mass range using 138 fb-1 of 13 TeV pp collisions with the ATLAS detector

Published: July 20, 2023A search for diphoton resonances in the mass range between 10 and 70 GeV with the ATLAS experiment at the Large Hadron Collider (LHC) is presented. The analysis is based on pp collision data corresponding to an integrated luminosity of 138 fb⁻¹ at a centre-of-mass energy of 13 TeV recorded from 2015 to 2018. Previous searches for diphoton resonances at the LHC have explored masses down to 65 GeV, finding no evidence of new particles. This search exploits the particular kinematics of events with pairs of closely spaced photons reconstructed in the detector, allowing examination of invariant masses down to 10 GeV. The presented strategy covers a region previously unexplored at hadron colliders because of the experimental challenges of recording low-energy photons and estimating the backgrounds. No significant excess is observed and the reported limits provide the strongest bound on promptly decaying axion-like particles coupling to gluons and photons for masses between 10 and 70 GeV.G. Aad ... E.K. Filmer ... P. Jackson ... A.X.Y. Kong ... H. Potti ... T.A. Ruggeri ... E.X.L. Ting ... M.J. White ... et al. (The ATLAS collaboration