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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cantero, J., et al. (2023). Measurement of the total cross section and ρ-parameter from elastic scattering in pp collisions at √s=13 TeV with the ATLAS detector. Eur. Phys. J. C, 83(5), 441–49pp.
Abstract: In a special run of the LHC with beta star=2.5 km, proton-proton elastic-scattering events were recorded at root s=13 TeV with an integrated luminosity of 340 μb(-1) using the ALFA subdetector of ATLAS in 2016. The elastic cross section was measured differentially in the Mandelstam t variable in the range from -t=2.5 center dot 10(-4) GeV2 to -t=0.46 GeV2 using 6.9 million elastic-scattering candidates. This paper presents measurements of the total cross section sigma(tot), parameters of the nuclear slope, and the rho-parameter defined as the ratio of the real part to the imaginary part of the elastic-scattering amplitude in the limit t -> 0. These parameters are determined from a fit to the differential elastic cross section using the optical theorem and different parameterizations of the t-dependence. The results for sigma(tot) and rho are sigma(tot) (PP -> X ) =104.7 +/- 1.1 mob , rho=0.098 +/- 0.011. The uncertainty in sigma(tot) is dominated by the luminosity measurement, and in rho by imperfect knowledge of the detector alignment and by modelling of the nuclear amplitude.
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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). Evidence for the charge asymmetry in pp → t(t)over-bar production at √s=13 TeV with the ATLAS detector. J. High Energy Phys., 08(8), 077–89pp.
Abstract: Inclusive and differential measurements of the top-antitop ( t (t) over bar) charge asymmetry A(C)(t (t) over bar) and the leptonic asymmetry A(C)(l (l) over bar) are presented in proton-proton collisions at root s = 13 TeV recorded by the ATLAS experiment at the CERN Large Hadron Collider. The measurement uses the complete Run 2 dataset, corresponding to an integrated luminosity of 139 fb(-1), combines data in the single-lepton and dilepton channels, and employs reconstruction techniques adapted to both the resolved and boosted topologies. A Bayesian unfolding procedure is performed to correct for detector resolution and acceptance effects. The combined inclusive t (t) over bar charge asymmetry is measured to be A(C)(t (t) over bar) = 0.0068 +/- 0.0015, which differs from zero by 4.7 standard deviations. Differential measurements are performed as a function of the invariant mass, transverse momentum and longitudinal boost of the t (t) over bar system. Both the inclusive and differential measurements are found to be compatible with the Standard Model predictions, at next-to-next-to-leading order in quantum chromodynamics perturbation theory with next-to-leading-order electroweak corrections. The measurements are interpreted in the framework of the Standard Model effective field theory, placing competitive bounds on several Wilson coefficients.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cantero, J., et al. (2023). Strong Constraints on Jet Quenching in Centrality-Dependent p plus Pb Collisions at 5.02 TeV from ATLAS. Phys. Rev. Lett., 131(7), 072301–21pp.
Abstract: Jet quenching is the process of color-charged partons losing energy via interactions with quark-gluon plasma droplets created in heavy-ion collisions. The collective expansion of such droplets is well described by viscous hydrodynamics. Similar evidence of collectivity is consistently observed in smaller collision systems, including pp and p+ Pb collisions. In contrast, while jet quenching is observed in Pb + Pb collisions, no evidence has been found in these small systems to date, raising fundamental questions about the nature of the system created in these collisions. The ATLAS experiment at the Large Hadron Collider has measured the yield of charged hadrons correlated with reconstructed jets in 0.36 nb-1 of p+ Pb and 3.6 pb-1 of pp collisions at 5.02 TeV. The yields of charged hadrons with p(T)(ch) > 0.5 GeV near and opposite in azimuth to jets with p(t)(je) T > 30 or 60 GeV, and the ratios of these yields between p+ Pb and pp collisions, IpPb, are reported. The collision centrality of p+ Pb events is categorized by the energy deposited by forward neutrons from the struck nucleus. The IpPb values are consistent with unity within a few percent for hadrons with p(T )(ch)> 4 GeV at all centralities. These data provide new, strong constraints that preclude almost any parton energy loss in central p+ Pb collisions.
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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). Combination of searches for invisible decays of the Higgs boson using 139 fb-1 of proton-proton collision data at root s=13 TeV collected with the ATLAS experiment. Phys. Lett. B, 842, 137963–19pp.
Abstract: Many extensions of the Standard Model predict the production of dark matter particles at the LHC. Sufficiently light dark matter particles may be produced in decays of the Higgs boson that would appear invisible to the detector. This Letter presents a statistical combination of searches for H & RARR; invisible decays where multiple production modes of the Standard Model Higgs boson are considered. These searches are performed with the ATLAS detector using 139 fb-1 of proton-proton collisions at a centre-of-mass energy of & RADIC;s = 13 TeV at the LHC. In combination with the results at & RADIC;s = 7 TeV and 8 TeV, an upper limit on the H & RARR; invisible branching ratio of 0.107 (0.077) at the 95% confidence level is observed (expected). These results are also interpreted in the context of models where the 125 GeV Higgs boson acts as a portal to dark matter, and limits are set on the scattering cross-section of weakly interacting massive particles and nucleons.
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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). Tools for estimating fake/non-prompt lepton backgrounds with the ATLAS detector at the LHC. J. Instrum., 18(11), T11004–61pp.
Abstract: Measurements and searches performed with the ATLAS detector at the CERN LHC often involve signatures with one or more prompt leptons. Such analyses are subject to 'fake/non-prompt' lepton backgrounds, where either a hadron or a lepton from a hadron decay or an electron from a photon conversion satisfies the prompt-lepton selection criteria. These backgrounds often arise within a hadronic jet because of particle decays in the showering process, particle misidentification or particle interactions with the detector material. As it is challenging to model these processes with high accuracy in simulation, their estimation typically uses data-driven methods. Three methods for carrying out this estimation are described, along with their implementation in ATLAS and their performance.
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