| 1. |
- Aad, G, et al.
(författare)
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Alignment of the ATLAS Inner Detector in Run 2
- 2020
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Ingår i: European Physical Journal C. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; 80:12
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Tidskriftsartikel (refereegranskat)abstract
- The performance of the ATLAS Inner Detector alignment has been studied using pp collision data at s=13TeV collected by the ATLAS experiment during Run 2 (2015–2018) of the Large Hadron Collider (LHC). The goal of the detector alignment is to determine the detector geometry as accurately as possible and correct for time-dependent movements. The Inner Detector alignment is based on the minimization of track-hit residuals in a sequence of hierarchical levels, from global mechanical assembly structures to local sensors. Subsequent levels have increasing numbers of degrees of freedom; in total there are almost 750,000. The alignment determines detector geometry on both short and long timescales, where short timescales describe movements within an LHC fill. The performance and possible track parameter biases originating from systematic detector deformations are evaluated. Momentum biases are studied using resonances decaying to muons or to electrons. The residual sagitta bias and momentum scale bias after alignment are reduced to less than ∼0.1TeV-1 and 0.9 × 10 - 3, respectively. Impact parameter biases are also evaluated using tracks within jets. © 2020, The Author(s).
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| 2. |
- Aad, G, et al.
(författare)
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Exclusive dimuon production in ultraperipheral Pb + Pb collisions at √sNN= 5.02 TeV with ATLAS
- 2021
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Ingår i: Physical Review C. - : American Physical Society. - 2469-9985 .- 2469-9993. ; 104:2
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Tidskriftsartikel (refereegranskat)abstract
- Exclusive dimuon production in ultraperipheral collisions (UPC), resulting from photon-photon interactions in the strong electromagnetic fields of colliding high-energy lead nuclei, PbPb(γγ) → μ+μ-(Pb(∗)Pb(∗)), is studied using Lint = 0.48 nb-1 of √sNN = 5.02 TeV lead-lead collision data at the LHC with the ATLAS detector. Dimuon pairs are measured in the fiducial region pT,μ > 4 GeV, |ημ| <2.4, invariant mass mμμ > 10 GeV, and pT,μμ <2 GeV. The primary background from single-dissociative processes is extracted from the data using a template fitting technique. Differential cross sections are presented as a function of mμμ, absolute pair rapidity (|yμμ|), scattering angle in the dimuon rest frame (|cos $μμ∗|), and the colliding photon energies. The total cross section of the UPC γγ → μ+μ- process in the fiducial volume is measured to be σfidμμ = 34.1±0.3(stat.)±0.7(syst.) μb. Generally good agreement is found with calculations from STARlight, which incorporate the leading-order Breit-Wheeler process with no final-state effects, albeit differences between the measurements and theoretical expectations are observed. In particular, the measured cross sections at larger |yμμ| are found to be about 10-20% larger in data than in the calculations, suggesting the presence of larger fluxes of photons in the initial state. Modification of the dimuon cross sections in the presence of forward and/or backward neutron production is also studied and is found to be associated with a harder incoming photon spectrum, consistent with expectations. © 2021 CERN.
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| 3. |
- Aad, G, et al.
(författare)
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Jet energy scale and resolution measured in proton–proton collisions at √s=13 TeV with the ATLAS detector
- 2021
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Ingår i: European Physical Journal C. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; 81:8
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Tidskriftsartikel (refereegranskat)abstract
- Jet energy scale and resolution measurements with their associated uncertainties are reported for jets using 36–81 fb−1 of proton–proton collision data with a centre-of-mass energy of s√=13 TeV collected by the ATLAS detector at the LHC. Jets are reconstructed using two different input types: topo-clusters formed from energy deposits in calorimeter cells, as well as an algorithmic combination of charged-particle tracks with those topo-clusters, referred to as the ATLAS particle-flow reconstruction method. The anti-kt jet algorithm with radius parameter R=0.4 is the primary jet definition used for both jet types. This result presents new jet energy scale and resolution measurements in the high pile-up conditions of late LHC Run 2 as well as a full calibration of particle-flow jets in ATLAS. Jets are initially calibrated using a sequence of simulation-based corrections. Next, several in situ techniques are employed to correct for differences between data and simulation and to measure the resolution of jets. The systematic uncertainties in the jet energy scale for central jets (|η|<1.2) vary from 1% for a wide range of high-pT jets (2502.5 TeV). The relative jet energy resolution is measured and ranges from (24±1.5)% at 20 GeV to (6±0.5)% at 300 GeV.
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| 4. |
- Aad, G, et al.
(författare)
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Measurement of light-by-light scattering and search for axion-like particles with 2.2 nb−1 of Pb+Pb data with the ATLAS detector
- 2021
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Ingår i: Journal of High Energy Physics. - : SPRINGER. - 1029-8479 .- 1126-6708. ; 2021:3
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Tidskriftsartikel (refereegranskat)abstract
- This paper describes a measurement of light-by-light scattering based on Pb+Pb collision data recorded by the ATLAS experiment during Run 2 of the LHC. The study uses 2.2 nb−1 of integrated luminosity collected in 2015 and 2018 at sNN = 5.02 TeV. Light-by-light scattering candidates are selected in events with two photons produced exclusively, each with transverse energy ETγ> 2.5 GeV, pseudorapidity |ηγ| < 2.37, diphoton invariant mass mγγ> 5 GeV, and with small diphoton transverse momentum and diphoton acoplanarity. The integrated and differential fiducial cross sections are measured and compared with theoretical predictions. The diphoton invariant mass distribution is used to set limits on the production of axion-like particles. This result provides the most stringent limits to date on axion-like particle production for masses in the range 6–100 GeV. Cross sections above 2 to 70 nb are excluded at the 95% CL in that mass interval. [Figure not available: see fulltext.] © 2021, The Author(s).
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| 5. |
- Aad, G, et al.
(författare)
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Measurements of the inclusive and differential production cross sections of a top-quark–antiquark pair in association with a Z boson at √s=13 TeV with the ATLAS detector
- 2021
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Ingår i: European Physical Journal C. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; 81:8
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Tidskriftsartikel (refereegranskat)abstract
- Measurements of both the inclusive and differential production cross sections of a top-quark–antiquark pair in association with a Z boson (tt¯ Z) are presented. The measurements are performed by targeting final states with three or four isolated leptons (electrons or muons) and are based on s=13 TeV proton–proton collision data with an integrated luminosity of 139 fb - 1, recorded from 2015 to 2018 with the ATLAS detector at the CERN Large Hadron Collider. The inclusive cross section is measured to be σtt¯Z= 0.99 ± 0.05 (stat.) ±0.08 (syst.) pb, in agreement with the most precise theoretical predictions. The differential measurements are presented as a function of a number of kinematic variables which probe the kinematics of the tt¯ Z system. Both absolute and normalised differential cross-section measurements are performed at particle and parton levels for specific fiducial volumes and are compared with theoretical predictions at different levels of precision, based on a χ2/ ndf and p value computation. Overall, good agreement is observed between the unfolded data and the predictions. © 2021, The Author(s).
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| 6. |
- Aad, G, et al.
(författare)
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Medium-Induced Modification of Z -Tagged Charged Particle Yields in Pb+Pb Collisions at 5.02 TeV with the ATLAS Detector
- 2021
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Ingår i: Physical Review Letters. - : American Physical Society (APS). - 1079-7114 .- 0031-9007. ; 126:7
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Tidskriftsartikel (refereegranskat)abstract
- The yield of charged particles opposite to a Z boson with large transverse momentum (pT) is measured in 260 pb-1 of pp and 1.7 nb-1 of Pb+Pb collision data at 5.02 TeV per nucleon pair recorded with the ATLAS detector at the Large Hadron Collider. The Z boson tag is used to select hard-scattered partons with specific kinematics, and to observe how their showers are modified as they propagate through the quark-gluon plasma created in Pb+Pb collisions. Compared with pp collisions, charged-particle yields in Pb+Pb collisions show significant modifications as a function of charged-particle pT in a way that depends on event centrality and Z boson pT. The data are compared with a variety of theoretical calculations and provide new information about the medium-induced energy loss of partons in a pT regime difficult to measure through other channels. © 2021 CERN.
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| 7. |
- Aad, G., et al.
(författare)
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Search for a heavy Higgs boson decaying into a Z boson and another heavy Higgs boson in the ℓℓbb and ℓℓWW final states in pp collisions at √s=13 TeV with the ATLAS detector
- 2021
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Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6052 .- 1434-6044. ; 81:5
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Tidskriftsartikel (refereegranskat)abstract
- A search for a heavy neutral Higgs boson, A, decaying into a Z boson and another heavy Higgs boson, H, is performed using a data sample corresponding to an integrated luminosity of 139 fb- 1 from proton–proton collisions at s=13 TeV recorded by the ATLAS detector at the LHC. The search considers the Z boson decaying into electrons or muons and the H boson into a pair of b-quarks or W bosons. The mass range considered is 230–800 GeV for the A boson and 130–700 GeV for the H boson. The data are in good agreement with the background predicted by the Standard Model, and therefore 95% confidence-level upper limits for σ×B(A→ZH)×B(H→bborH→WW) are set. The upper limits are in the range 0.0062–0.380 pb for the H→ bb channel and in the range 0.023–8.9 pb for the H→ WW channel. An interpretation of the results in the context of two-Higgs-doublet models is also given.
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| 8. |
- Aad, G, et al.
(författare)
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Search for bottom-squark pair production in pp collision events at s =13 TeV with hadronically decaying τ -leptons, b -jets, and missing transverse momentum using the ATLAS detector
- 2021
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Ingår i: Physical Review D. - : American Physical Society. - 2470-0010 .- 2470-0029. ; 104:3
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Tidskriftsartikel (refereegranskat)abstract
- A search for pair production of bottom squarks in events with hadronically decaying τ-leptons, b-tagged jets, and large missing transverse momentum is presented. The analyzed dataset is based on proton-proton collisions at √s=13 TeV delivered by the Large Hadron Collider and recorded by the ATLAS detector from 2015 to 2018, and corresponds to an integrated luminosity of 139 fb−1. The observed data are compatible with the expected Standard Model background. Results are interpreted in a simplified model where each bottom squark is assumed to decay into the second-lightest neutralino ˜χ02 and a bottom quark, with ˜χ02 decaying into a Higgs boson and the lightest neutralino ˜χ01. The search focuses on final states where at least one Higgs boson decays into a pair of hadronically decaying τ-leptons. This allows the acceptance and thus the sensitivity to be significantly improved relative to the previous results at low masses of the ˜χ02, where bottom-squark masses up to 850 GeV are excluded at the 95% confidence level, assuming a mass difference of 130 GeV between ˜χ02 and ˜χ01. Model-independent upper limits are also set on the cross section of processes beyond the Standard Model.
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| 9. |
- Aad, G, et al.
(författare)
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Search for charged Higgs bosons decaying into a top quark and a bottom quark at √s = 13 TeV with the ATLAS detector
- 2021
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Ingår i: Journal of High Energy Physics. - : Springer Nature. - 1029-8479 .- 1126-6708. ; 2021:6
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Tidskriftsartikel (refereegranskat)abstract
- A search for charged Higgs bosons decaying into a top quark and a bottom quark is presented. The data analysed correspond to 139 fb−1 of proton-proton collisions at s = 13 TeV, recorded with the ATLAS detector at the LHC. The production of a heavy charged Higgs boson in association with a top quark and a bottom quark, pp → tbH+ → tbtb, is explored in the H+ mass range from 200 to 2000 GeV using final states with jets and one electron or muon. Events are categorised according to the multiplicity of jets and b-tagged jets, and multivariate analysis techniques are used to discriminate between signal and background events. No significant excess above the background-only hypothesis is observed and exclusion limits are derived for the production cross-section times branching ratio of a charged Higgs boson as a function of its mass; they range from 3.6 pb at 200 GeV to 0.036 pb at 2000 GeV at 95% confidence level. The results are interpreted in the hMSSM and Mh125 scenarios.
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| 10. |
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