ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., Cabrera Urban, S., et al. (2023). Observation of four-top-quark production in the multilepton final state with the ATLAS detector. Eur. Phys. J. C, 83(6), 496–35pp.
Abstract: This paper presents the observation of four-topquark (t (t) over bart (t) over bar) production in proton-proton collisions at the LHC. The analysis is performed using an integrated luminosity of 140 fb-1 at a centre-of-mass energy of 13 TeVcollected using the ATLAS detector. Events containing two leptons with the same electric charge or at least three leptons (electrons or muons) are selected. Event kinematics are used to separate signal from background through a multivariate discriminant, and dedicated control regions are used to constrain the dominant backgrounds. The observed (expected) significance of themeasured t (t) over bart (t) over bar signal with respect to the standard model (SM) background-only hypothesis is 6.1 (4.3) standard deviations. The t (t) over bart (t) over bar production cross section is measured to be 22.5(-5.5)(+6.6), consistent with the SM prediction of 12.0 +/- 2.4 fb within 1.8 standard deviations. Data are also used to set limits on the three-top-quark production cross section, being an irreducible background not measured previously, and to constrain the top-Higgs Yukawa coupling and effective field theory operator coefficients that affect t (t) over bart (t) over bar production.
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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). Studies of the muon momentum calibration and performance of the ATLAS detector with pp collisions at √s=13 TeV. Eur. Phys. J. C, 83(8), 686–37pp.
Abstract: This paper presents the muon momentum calibration and performance studies for the ATLAS detector based on the pp collisions data sample produced at root s = 13 TeV at the LHC during Run 2 and corresponding to an integrated luminosity of 139 fb(-1). An innovative approach is used to correct for potential charge-dependent momentum biases related to the knowledge of the detector geometry, using the Z ->mu(+)mu(-) resonance. The muon momentum scale and resolution are measured using samples of J/psi ->mu(+)mu(-) and Z ->mu(+)mu(-) events. A calibration procedure is defined and applied to simulated data to match the performance measured in real data. The calibration is validated using an independent sample of Upsilon ->mu(+)mu(-) events. At the Z(J/psi) peak, the momentum scale is measured with an uncertainty at the 0.05% (0.1%) level, and the resolution is measured with an uncertainty at the 1.5% (2%) level. The charge-dependent bias is removed with a dedicated in situ correction for momenta up to 450 GeV with a precision better than 0.03 TeV-1.
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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). Charged-hadron production in pp, p+Pb, Pb+Pb, and Xe+Xe collisions at √s_NN=5 TeV with the ATLAS detector at the LHC. J. High Energy Phys., 07(7), 074–58pp.
Abstract: This paper presents measurements of charged-hadron spectra obtained in pp, p+Pb, and Pb+Pb collisions at root s or root s(NN) = 5.02TeV, and in Xe+Xe collisions at root s(NN) = 5.44TeV. The data recorded by the ATLAS detector at the LHC have total integrated luminosities of 25 pb(-1), 28 nb(-1), 0.50 nb(-1), and 3 μb(-1), respectively. The nuclear modification factors RpPb and R-AA are obtained by comparing the spectra in heavy-ion and pp collisions in a wide range of charged-particle transverse momenta and pseudorapidity. The nuclear modification factor RpPb shows a moderate enhancement above unity with a maximum at p(T) approximate to 3 GeV; the enhancement is stronger in the Pb-going direction. The nuclear modification factors in both Pb+Pb and Xe+Xe collisions feature a significant, centrality-dependent suppression. They show a similar distinct p(T)-dependence with a local maximum at p(T) approximate to 2 GeV and a local minimum at p(T) approximate to 7 GeV. This dependence is more distinguishable in more central collisions. No significant eta |-dependence is found. A comprehensive comparison with several theoretical predictions is also provided. They typically describe RAA better in central collisions and in the pT range from about 10 to 100 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). Searches for lepton-flavour-violating decays of the Higgs boson into eτ and μτ in √s=13 TeV pp collisions with the ATLAS detector. J. High Energy Phys., 07(7), 166–74pp.
Abstract: This paper presents direct searches for lepton flavour violation in Higgs boson decays, H -> e tau and H -> μtau, performed using data collected 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 root s = 13 TeV, corresponding to an integrated luminosity of 138 fb(-1). Leptonic (tau -> l nu(l)nu(tau)) and hadronic (tau -> hadrons nu(tau)) decays of the tau-lepton are considered. Two background estimation techniques are employed: the MC-template method, based on data-corrected simulation samples, and the Symmetry method, based on exploiting the symmetry between electrons and muons in the Standard Model backgrounds. No significant excess of events is observed and the results are interpreted as upper limits on lepton-flavour-violating branching ratios of the Higgs boson. The observed (expected) upper limits set on the branching ratios at 95% confidence level, B(H -> e tau) < 0.20% (0.12%) and B(H -> μtau ) < 0.18% (0.09%), are obtained with the MC-template method from a simultaneous measurement of potential H -> e tau and H -> μtau signals. The best-fit branching ratio difference, B(H -> μtau) -> B(H -> e tau), measured with the Symmetry method in the channel where the tau-lepton decays to leptons, is (0.25 0.10)%, compatible with a value of zero within 2.5 sigma.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cardillo, F., et al. (2022). Determination of the parton distribution functions of the proton using diverse ATLAS data from pp collisions at root s=7, 8 and 13 TeV. Eur. Phys. J. C, 82(5), 438–70pp.
Abstract: 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 root 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/gamma* boson production, W-+/- and Z boson production in association with jets, t (t) over bar 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.
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