ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cantero, J., et al. (2023). Differential t(t)over-tilde cross-section measurements using boosted top quarks in the all-hadronic final state with 139 fb(-1) of ATLAS data. J. High Energy Phys., 04(4), 080–108pp.
Abstract: Measurements of single-, double-, and triple-differential cross-sections are presented for boosted top-quark pair-production in 13 TeV proton-proton collisions recorded by the ATLAS detector at the LHC. The top quarks are observed through their hadronic decay and reconstructed as large-radius jets with the leading jet having transverse momentum (p(T)) greater than 500 GeV. The observed data are unfolded to remove detector effects. The particle-level cross-section, multiplied by the t (t) over bar branching fraction and measured in a fiducial phase space defined by requiring the leading and second-leading jets to have p(T)> 500 GeV and p(T)> 350 GeV, respectively, is 331 +/- 3(stat.) +/- 39(syst.) fb. This is approximately 20% lower than the prediction of 398(-49)(+48) fb by Powheg+Pythia 8 with next-to-leading-order (NLO) accuracy but consistent within the theoretical uncertainties. Results are also presented at the parton level, where the effects of top-quark decay, parton showering, and hadronization are removed such that they can be compared with fixed-order next-to-next-to-leading-order (NNLO) calculations. The parton-level cross-section, measured in a fiducial phase space similar to that at particle level, is 1.94 +/- 0.02(stat.) +/- 0.25(syst.) pb. This agrees with the NNLO prediction of 1.96(-0.17)(+0.02) pb. Reasonable agreement with the differential cross-sections is found for most NLO models, while the NNLO calculations are generally in better agreement with the data. The differential cross-sections are interpreted using a Standard Model effective field-theory formalism and limits are set on Wilson coefficients of several four-fermion operators.
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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). Search for excited tau-leptons and leptoquarks in the final state with tau-leptons and jets in pp collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 06(6), 199–46pp.
Abstract: A search is reported for excited tau-leptons and leptoquarks in events with two hadronically decaying tau-leptons and two or more jets. The search uses proton-proton (pp) collision data at root s = 13 TeV recorded by the ATLAS experiment during the Run 2 of the Large Hadron Collider in 2015-2018. The total integrated luminosity is 139 fb(-1). The excited tau-lepton is assumed to be produced and to decay via a four-fermion contact interaction into an ordinary tau-lepton and a quark-antiquark pair. The leptoquarks are assumed to be produced in pairs via the strong interaction, and each leptoquark is assumed to couple to a charm or lighter quark and a tau-lepton. No excess over the background prediction is observed. Excited tau-leptons with masses below 2.8 TeV are excluded at 95% CL in scenarios with the contact interaction scale Lambda set to 10 TeV. At the extreme limit of model validity where Lambda is set equal to the excited tau-lepton mass, excited tau-leptons with masses below 4.6 TeV are excluded. Leptoquarks with masses below 1.3 TeV are excluded at 95% CL if their branching ratio to a charm quark and a tau-lepton equals 1. The analysis does not exploit flavour-tagging in the signal region.
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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 nuclear modification factor of b-jets in 5.02 TeV Pb+Pb collisions with the ATLAS detector. Eur. Phys. J. C, 83(5), 438–32pp.
Abstract: This paper presents a measurement of b-jet production in Pb+Pb and pp collisions at root s(NN) = 5.02 TeV with the ATLAS detector at the LHC. The measurement uses 260 pb(-1) of pp collisions collected in 2017 and 1.4 nb(-1) of Pb+Pb collisions collected in 2018. In both collision systems, jets are reconstructed via the anti-kt algorithm. The b-jets are identified from a sample of jets containing muons from the semileptonic decay of b-quarks using template fits of the muon momentum relative to the jet axis. In pp collisions, b-jets are reconstructed for radius parameters R=0.2 and R=0.4, and only R=0.2 jets are used in Pb+Pb collisions. For comparison, inclusive R=0.2 jets are also measured using 1.7 nb(-1) of Pb+Pb collisions collected in 2018 and the same pp collision data as the b-jet measurement. The nuclear modification factor, RAA, is calculated for both b-jets and inclusive jets with R=0.2 over the transverse momentum range of 80-290 GeV. The nuclear modification factor for b-jets decreases from peripheral to central collisions. The ratio of the b-jet RAA to inclusive jet RAA is also presented and suggests that the RAA for b-jets is larger than that for inclusive jets in central Pb+Pb collisions. The measurements are compared with theoretical calculations and suggest a role for mass and colour-charge effects in partonic energy loss in heavy-ion 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). Measurement of the total and differential Higgs boson production cross-sections at root s=13 TeV with the ATLAS detector by combining the H -> ZZ(*)-> 4l and H -> gamma gamma decay channels. J. High Energy Phys., 05(5), 028–42pp.
Abstract: The total and differential Higgs boson production cross-sections are measured through a combined statistical analysis of the H -> ZZ(*) -> 4l and H -> gamma gamma decay channels. The results are based on a dataset of 139 fb(-1) of proton-proton collisions at a centre-of-mass energy of 13 TeV, recorded by the ATLAS detector at the Large Hadron Collider. The measured total Higgs boson production cross-section is 55.5(-3.8)(+4.0) pb, consistent with the Standard Model prediction of 55.6 +/- 2.5 pb. All results from the two decay channels are compatible with each other, and their combination agrees with the Standard Model predictions. A combined statistical interpretation of the measured fiducial cross-sections as a function of the Higgs boson transverse momentum is performed in order to probe the Yukawa couplings to the bottom and charm quarks. A similar interpretation is performed by including also the constraints from the measurements of Higgs boson production in association with a W or Z boson in the H -> b (b) over bar and c (c) over bar decay channels.
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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). ATLAS flavour-tagging algorithms for the LHC Run 2 pp collision dataset. Eur. Phys. J. C, 83(7), 681–37pp.
Abstract: The flavour-tagging algorithms developed by the AvTLAS Collaboration and used to analyse its dataset of root s = 13 TeV pp collisions from Run 2 of the Large Hadron Collider are presented. These new tagging algorithms are based on recurrent and deep neural networks, and their performance is evaluated in simulated collision events. These developments yield considerable improvements over previous jet-flavour identification strategies. At the 77% b-jet identification efficiency operating point, light-jet (charm-jet) rejection factors of 170 (5) are achieved in a sample of simulated Standard Model t (t) over bar events; similarly, at a c-jet identification efficiency of 30%, a light-jet (b-jet) rejection factor of 70 (9) is obtained.
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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). Search for t(t)bar H/A → t(t)bar t(t)bar production in the multilepton final state in proton-proton collisions at √s=13 TeV with the ATLAS detector. J. High Energy Phys., 07(7), 203–53pp.
Abstract: A search for a new heavy scalar or pseudo-scalar Higgs boson (H/A) produced in association with a pair of top quarks, with the Higgs boson decaying into a pair of top quarks (H/A -> t (t) over bar) is reported. The search targets a final state with exactly two leptons with same-sign electric charges or at least three leptons. The analysed dataset corresponds to an integrated luminosity of 139 fb(-1) of proton-proton collisions collected at a centre-of-mass energy of 13TeV with the ATLAS detector at the LHC. Two multivariate classifiers are used to separate the signal from the background. No significant excess of events over the Standard Model expectation is observed. The results are interpreted in the context of a type-II two-Higgs-doublet model. The observed (expected) upper limits at 95% confidence level on the t (t) over barH/A production cross-section times the branching ratio of H/A -> t (t) over bar range between 14 (10) fb and 6 (5) fb for a heavy Higgs boson with mass between 400 GeV and 1000 GeV, respectively. Assuming that only one particle, either the scalar H or the pseudo-scalar A, contributes to the t (t) over bart (t) over bar final state, values of tan beta below 1.2 or 0.5 are excluded for a mass of 400 GeV or 1000 GeV, respectively. These exclusion ranges increase to tan beta below 1.6 or 0.6 when both particles are considered.
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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). Model-independent search for the presence of new physics in events including H → γγ with √s=13 TeV pp data recorded by the ATLAS detector at the LHC. J. High Energy Phys., 07(7), 176–51pp.
Abstract: A model-independent search for new physics leading to final states containing a Higgs boson, with a mass of 125.09 GeV, decaying to a pair of photons is performed with 139 fb(-1) of p root s = 13TeV pp collision data recorded by the ATLAS detector at the Large Hadron Collider at CERN. This search examines 22 final states categorized by the objects that are produced in association with the Higgs boson. These objects include isolated electrons or muons, hadronically decaying iota -leptons, additional photons, missing transverse momentum, and hadronic jets, as well as jets that are tagged as containing a b-hadron. No significant excesses above Standard Model expectations are observed and limits on the production cross section at 95% confidence level are set. Detector efficiencies are reported for all 22 signal regions, which can be used to convert detector-level cross-section limits reported in this paper to particle-level cross-section constraints.
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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., Bouchhar, N., Cabrera Urban, S., et al. (2023). Search for dark photons from Higgs boson decays via ZH production with a photon plus missing transverse momentum signature from pp collisions at √s=13 TeV with the ATLAS detector. J. High Energy Phys., 07(7), 133–51pp.
Abstract: This paper describes a search for dark photons (gamma(d)) in proton-proton collisions at root s = 13TeV at the Large Hadron Collider (LHC). The dark photons are searched for in the decay of Higgs bosons ( H -> gamma gamma(d)) produced through the ZH production mode. The transverse mass of the system, made of the photon and the missing transverse momentum from the non-interacting gamma(d), presents a distinctive signature as it peaks near the Higgs boson mass. The results presented use the total Run-2 integrated luminosity of 139 fb(-1) recorded by the ATLAS detector at the LHC. The dominant reducible background processes are estimated using data-driven techniques. A Boosted Decision Tree technique is adopted to enhance the sensitivity of the search. As no excess is observed with respect to the Standard Model prediction, an observed (expected) upper limit on the branching ratio BR( H -> gamma gamma(d)) of 2.28% (2.82(-0.84%)(+1.33)) is set at 95% CL for massless gamma(d). For massive dark photons up to 40 GeV, the observed (expected) upper limits on BR( H -> gamma gamma(d)) at 95% confidence level is found within the [2.19,2.52]% ([2.71,3.11]%) range.
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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). Search for third-generation vector-like leptons in pp collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 07(7), 118–49pp.
Abstract: A search for vector-like leptons in multilepton (two, three, or four-or-more electrons plus muons) final states with zero or more hadronic tau-lepton decays is presented. The search is performed using a dataset corresponding to an integrated luminosity of 139 fb(-1) of proton-proton collisions at a centre-of-mass energy of 13TeV recorded by the ATLAS detector at the LHC. To maximize the separation of signal and background, a machine-learning classifier is used. No excess of events is observed beyond the Standard Model expectation. Using a doublet vector-like lepton model, vector-like leptons coupling to third-generation Standard Model leptons are excluded in the mass range from 130 GeV to 900 GeV at the 95% confidence level, while the highest excluded mass is expected to be 970 GeV.
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