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ATLAS Collaboration(Aad, G. et al), Aikiot, A., Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., et al. (2024). Measurements of inclusive and differential cross-sections of t(t)over-barγ production in pp collisions at √s=13 TeV with the ATLAS detector. J. High Energy Phys., 10(10), 191–77pp.
Abstract: Inclusive and differential cross-sections are measured at particle level for the associated production of a top quark pair and a photon (t (t) over bar gamma). The analysis is performed using an integrated luminosity of 140 fb(-1) of proton-proton collisions at a centre-of-mass energy of 13TeV collected by the ATLAS detector. The measurements are performed in the single-lepton and dilepton top quark pair decay channels focusing on t (t) over bar gamma topologies where the photon is radiated from an initial-state parton or one of the top quarks. The absolute and normalised differential cross-sections are measured for several variables characterising the photon, lepton and jet kinematics as well as the angular separation between those objects. The observables are found to be in good agreement with the Monte Carlo predictions. The photon transverse momentum differential distribution is used to set limits on effective field theory parameters related to the electroweak dipole moments of the top quark. The combined limits using the photon and the Z boson transverse momentum measured in t (t) over bar production in associations with a Z boson are also set.
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ATLAS Collaboration(Aad, G. et al), Aikiot, A., Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., et al. (2024). Simultaneous energy and mass calibration of large-radius jets with the ATLAS detector using a deep neural network. Mach. Learn.-Sci. Technol., 5(3), 035051–37pp.
Abstract: The energy and mass measurements of jets are crucial tasks for the Large Hadron Collider experiments. This paper presents a new calibration method to simultaneously calibrate these quantities for large-radius jets measured with the ATLAS detector using a deep neural network (DNN). To address the specificities of the calibration problem, special loss functions and training procedures are employed, and a complex network architecture, which includes feature annotation and residual connection layers, is used. The DNN-based calibration is compared to the standard numerical approach in an extensive series of tests. The DNN approach is found to perform significantly better in almost all of the tests and over most of the relevant kinematic phase space. In particular, it consistently improves the energy and mass resolutions, with a 30% better energy resolution obtained for transverse momenta p(T) > 500 GeV.
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ATLAS Collaboration(Aad, G. et al), Aikiot, A., Amos, K. R., Aparisi Pozo, J. A., Bouchhar, N., Cabrera Urban, S., et al. (2024). Search for flavour-changing neutral-current couplings between the top quark and the Higgs boson in multi-lepton final states in 13 TeV pp collisions with the ATLAS detector. Eur. Phys. J. C, 84(7), 757–42pp.
Abstract: A search is presented for flavour-changing neutral-current interactions involving the top quark, the Higgs boson and an up-type quark (q = u, c) with the ATLAS detector at the Large Hadron Collider. The analysis considers leptonic decays of the top quark along with Higgs boson decays into two W bosons, two Z bosons or a tau(+)tau(-) pair. It focuses on final states containing either two leptons (electrons or muons) of the same charge or three leptons. The considered processes are t (t) over bar and Ht production. For the t (t) over bar production, one top quark decays via t -> Hq. The proton-proton collision data set analysed amounts to (140 fb(-1)) at (root s = 13 TeV). No significant excess beyond Standard Model expectations is observed and upper limits are set on the t -> Hq branching ratios at 95% confidence level, amounting to observed (expected) limits of B(t -> Hu) < 2.8 (3.0) x 10(-4) and B(t -> Hc) < 3.3 (3.8) x 10(-4). Combining this search with other searches for t Hq flavour-changing neutral-current interactions previously conducted by ATLAS, considering H -> b (b) over bar and H -> gamma gamma decays, as well as H -> tau(+)tau(-) decays with one or two hadronically decaying tau-leptons, yields observed (expected) upper limits on the branching ratios of B(t -> Hu) < 2.6 (1.8) x 10(-4) and B(t -> Hc) < 3.4 (2.3) x 10(-4).
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ATLAS Collaboration(Aad, G. et al), Aikiot, A., Amos, K. R., Aparisi Pozo, J. A., Bouchhar, N., Cabrera Urban, S., et al. (2024). Search for charged-lepton-flavor violating μτqt interactions in top-quark production and decay in pp collisions at √s=13 TeV with the ATLAS detector at the LHC. Phys. Rev. D, 110(1), 012014–35pp.
Abstract: A search for charged-lepton-flavor violating μtau qt (q = u, c) interactions is presented, considering both top-quark production and decay. The data analyzed correspond to 140 fb(-1) of proton-proton collisions at a center-of-mass energy of root s = 13 TeV recorded with the ATLAS detector at the Large Hadron Collider. The analysis targets events containing two muons with the same electric charge, a hadronically decaying tau-lepton and at least one jet, with exactly one b-tagged jet, produced by a μtau qt interaction. Agreement with the Standard Model expectation within 1.6 standard deviations is observed, and limits are set at the 95% confidence level (CL) on the charged-lepton-flavor violation branching ratio of B(t -> μtau q) < 8.7 x 10(-7). An effective field theory interpretation is performed yielding 95% CL limits on Wilson coefficients, dependent on the flavor of the associated light quark and the Lorentz structure of the coupling. These range from vertical bar c(lequ)(3(2313))vertical bar/Lambda(2) < 0.10 TeV-2 for μtau ut to vertical bar c(lequ)(1(2323))vertical bar/Lambda(2) < 1.8 TeV-2 for μtau ct. An additional interpretation is performed for scalar leptoquark production inducing charged-lepton-flavor violation, with fixed intergenerational couplings. Upper limits on leptoquark coupling strengths are set at the 95% CL, ranging from lambda(LQ) = 1.3 to lambda(LQ) = 3.7 for leptoquark masses between 0.5 and 2.0 TeV.
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ATLAS Collaboration(Aad, G. et al), Aikot, A., Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., et al. (2024). Search for new phenomena with top-quark pairs and large missing transverse momentum using 140 fb-1 of pp collision data at √s=13 TeV with the ATLAS detector. J. High Energy Phys., 03(3), 139–58pp.
Abstract: A search is conducted for new phenomena in events with a top quark pair and large missing transverse momentum, where the top quark pair is reconstructed in final states with one isolated electron or muon and multiple jets. The search is performed using the Large Hadron Collider proton-proton collision data sample at a centre-of-mass energy of root s = 13TeV recorded by the ATLAS detector that corresponds to an integrated luminosity of 140 fb(-1). An analysis based on neural network classifiers is optimised to search for directly produced pairs of supersymmetric partners of the top quark (stop), and to search for spin-0 mediators, produced in association with a pair of top quarks, that decay into dark-matter particles. In the stop search, the analysis is designed to target models in which the mass difference between the stop and the neutralino from the stop decay is close to the top quark mass. This new search is combined with previously published searches in final states with different lepton multiplicities. No significant excess above the Standard Model background is observed, and limits at 95% confidence level are set. Models with neutralinos with masses up to 570 GeV are excluded, while for small neutralino masses models are excluded for stop masses up to 1230 GeV. Scalar (pseudoscalar) dark matter mediator masses as large as 350 (370) GeV are excluded when the coupling strengths of the mediator to Standard Model and dark-matter particles are both set to one. At lower mediator masses, models with production cross-sections as small as 0.15 (0.16) times the nominal predictions are excluded. Results of this search are also used to set constraints on effective four-fermion contact interactions between top quarks and neutrinos.
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ATLAS Collaboration(Aad, G. et al), Aikot, A., Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., et al. (2024). Combination of searches for heavy spin-1 resonances using 139 fb-1 of proton-proton collision data at √s=13 TeV with the ATLAS detector. J. High Energy Phys., 04(4), 118–47pp.
Abstract: A combination of searches for new heavy spin-1 resonances decaying into different pairings of W, Z, or Higgs bosons, as well as directly into leptons or quarks, is presented. The data sample used corresponds to 139 fb(-1) of proton-proton collisions at root s = 13 TeV collected during 2015-2018 with the ATLAS detector at the CERN Large Hadron Collider. Analyses selecting quark pairs (qq, bb, t (t) over bar, and tb) or third-generation leptons (tau nu and tau tau) are included in this kind of combination for the first time. A simplified model predicting a spin-1 heavy vector-boson triplet is used. Cross-section limits are set at the 95% confidence level and are compared with predictions for the benchmark model. These limits are also expressed in terms of constraints on couplings of the heavy vector-boson triplet to quarks, leptons, and the Higgs boson. The complementarity of the various analyses increases the sensitivity to new physics, and the resulting constraints are stronger than those from any individual analysis considered. The data exclude a heavy vector-boson triplet with mass below 5.8 TeV in a weakly coupled scenario, below 4.4 TeV in a strongly coupled scenario, and up to 1.5 TeV in the case of production via vector-boson fusion.
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ATLAS Collaboration(Aad, G. et al), Aikot, A., Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., et al. (2024). Differential cross-sections for events with missing transverse momentum and jets measured with the ATLAS detector in 13 TeV proton-proton collisions. J. High Energy Phys., 08(8), 223–62pp.
Abstract: Measurements of inclusive, differential cross-sections for the production of events with missing transverse momentum in association with jets in proton-proton collisions at root s = 13 TeV are presented. The measurements are made with the ATLAS detector using an integrated luminosity of 140 fb(-1) and include measurements of dijet distributions in a region in which vector-boson fusion processes are enhanced. They are unfolded to correct for detector resolution and efficiency within the fiducial acceptance, and are designed to allow robust comparisons with a wide range of theoretical predictions. A measurement of differential cross sections for the Z -> nu nu process is made. The measurements are generally well-described by Standard Model predictions except for the dijet invariant mass distribution. Auxiliary measurements of the hadronic system recoiling against isolated leptons, and photons, are also made in the same phase space. Ratios between the measured distributions are then derived, to take advantage of cancellations in modelling effects and some of the major systematic uncertainties. These measurements are sensitive to new phenomena, and provide a mechanism to easily set constraints on phenomenological models. To illustrate the robustness of the approach, these ratios are compared with two common Dark Matter models, where the constraints derived from the measurement are comparable to those set by dedicated detector-level searches.
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ATLAS Collaboration(Aad, G. et al), Aikot, A., Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., et al. (2024). Search for a resonance decaying into a scalar particle and a Higgs boson in the final state with two bottom quarks and two photons in proton-proton collisions at √s=13 TeV with the ATLAS detector. J. High Energy Phys., 11(11), 047–47pp.
Abstract: A search for the resonant production of a heavy scalar X decaying into a Higgs boson and a new lighter scalar S, through the process X -> S(-> b (b) over bar )H(->gamma gamma), where the two photons are consistent with the Higgs boson decay, is performed. The search is conducted using an integrated luminosity of 140 fb(-1) of proton-proton collision data at a centre-of-mass energy of 13 TeV recorded with the ATLAS detector at the Large Hadron Collider. The search is performed over the mass range 170 <= m(X) <= 1000 GeV and 15 <= m(S) <= 500 GeV. Parameterised neural networks are used to enhance the signal purity and to achieve continuous sensitivity in a domain of the (m(X), m(S)) plane. No significant excess above the expected background is found and 95% CL upper limits are set on the cross section times branching ratio, ranging from 39 fb to 0.09 fb. The largest deviation from the background-only expectation occurs for (m(X), m(S)) = (575, 200) GeV with a local (global) significance of 3.5 (2.0) standard deviations.
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ATLAS Collaboration(Aad, G. et al), Aikot, A., Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., et al. (2024). Search for the nonresonant production of Higgs boson pairs via gluon fusion and vector-boson fusion in the b(b)over-barτ+τ- final state in proton-proton collisions at √s=13 TeV with the ATLAS detector. Phys. Rev. D, 110(3), 032012–35pp.
Abstract: A search for the nonresonant production of Higgs boson pairs in the HH -> b (b) over bar tau(+)tau(-) channel is performed using 140 fb(-1) of proton-proton collisions at a center-of-mass energy of 13 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. The analysis strategy is optimized to probe anomalous values of the Higgs boson self-coupling modifier kappa(lambda) and of the quartic HHVV (V = W, Z) coupling modifier kappa(2V). No significant excess above the expected background from Standard Model processes is observed. An observed (expected) upper limit mu(HH) < 5.9(3.3) is set at 95% confidence-level on the Higgs boson pair production cross section normalized to its Standard Model prediction. The coupling modifiers are constrained to an observed (expected) 95% confidence interval of -3.1 < kappa(lambda) < 9.0 (-2.5 < kappa(lambda) < 9.3) and -0.5 < kappa(2V) < 2.7 (-0.2 < kappa(2V) < 2.4), assuming all other Higgs boson couplings are fixed to the Standard Model prediction. The results are also interpreted in the context of effective field theories via constraints on anomalous Higgs boson couplings and Higgs boson pair production cross sections assuming different kinematic benchmark scenarios.
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ATLAS Collaboration(Aad, G. et al), Aikot, A., Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., et al. (2024). Search for low-mass resonances decaying into two jets and produced in association with a photon or a jet at √s=13 TeV with the ATLAS detector. Phys. Rev. D, 110(3), 032002–30pp.
Abstract: A search is performed for localized excesses in the low-mass dijet invariant mass distribution, targeting a hypothetical new particle decaying into two jets and produced in association with either a high transverse momentum photon or a jet. The search uses the full Run 2 data sample from LHC proton-proton collisions collected by the ATLAS experiment at a center-of-mass energy of 13 TeV during 2015-2018. Two variants of the search are presented for each type of initial-state radiation: one that makes no jet flavor requirements and one that requires both of the jets to have been identified as containing b-hadrons. No excess is observed relative to the Standard Model prediction, and the data are used to set upper limits on the production cross section for a benchmark Z' model and, separately, for generic, beyond the Standard Model scenarios which might produce a Gaussian-shaped contribution to dijet invariant mass distributions. The results extend the current constraints on dijet resonances to the mass range between 200 and 650 GeV.
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