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 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 Nearly Mass-Degenerate Higgsinos Using Low-Momentum Mildly Displaced Tracks in pp Collisions at √s=13 TeV with the ATLAS Detector. Phys. Rev. Lett., 132(22), 221801–24pp.
Abstract: Higgsinos with masses near the electroweak scale can solve the hierarchy problem and provide a dark matter candidate, while detecting them at the LHC remains challenging if their mass splitting is O(1 GeV). This Letter presents a novel search for nearly mass-degenerate Higgsinos in events with an energetic jet, missing transverse momentum, and a low-momentum track with a significant transverse impact parameter using 140 fb(-1) of proton-proton collision data at root s = 13 TeV collected by the ATLAS experiment. For the first time since LEP, a range of mass splittings between the lightest charged and neutral Higgsinos from 0.3 to 0.9 GeV is excluded at 95% confidence level, with a maximum reach of approximately 170 GeV in the Higgsino mass.
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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 Resonant Higgs Boson Pair Production Using pp Collisions at √s=13 TeV with the ATLAS Detector. Phys. Rev. Lett., 132(23), 231801–23pp.
Abstract: A combination of searches for a new resonance decaying into a Higgs boson pair is presented, using up to 139 fb(-1) of pp collision data at root s = 13 TeV recorded with the ATLAS detector at the LHC. The combination includes searches performed in three decay channels: b (b) over barb (b) over bar, (b) over barb tau(+)tau(-), and b (b) over bar gamma gamma. No excess above the expected Standard Model background is observed and upper limits are set at the 95% confidence level on the production cross section of Higgs boson pairs originating fromthe decay of a narrowscalar resonance with mass in the range 251 GeV-5 TeV. The observed (expected) limits are in the range 0.96-600 fb (1.2-390 fb). The limits are interpreted in the type-I two-Higgs-doublet model and the minimal supersymmetric standard model, and constrain parameter space not previously excluded by other 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). Azimuthal Angle Correlations of Muons Produced via Heavy-Flavor Decays in 5.02 TeV Pb + Pb and pp Collisions with the ATLAS Detector. Phys. Rev. Lett., 132(20), 202301–23pp.
Abstract: Angular correlations between heavy quarks provide a unique probe of the quark-gluon plasma created in ultrarelativistic heavy-ion collisions. Results are presented of a measurement of the azimuthal angle correlations between muons originating from semileptonic decays of heavy quarks produced in 5.02 TeV Pb + Pb and pp collisions at the LHC. The muons are measured with transverse momenta and pseudorapidities satisfying p(T)(mu) > 4 GeV and vertical bar eta(mu)vertical bar < 2.4, respectively. The distributions of azimuthal angle separation Delta Phi for muon pairs having pseudorapidity separation vertical bar Delta eta vertical bar > 0.8, are measured in different Pb + Pb centrality intervals and compared to the same distribution measured in pp collisions at the same center-of-mass energy. Results are presented separately for muon pairs with opposite-sign charges, same-sign charges, and all pairs. A clear peak is observed in all Delta Phi distributions at Delta Phi similar to Pi, consistent with the parent heavy-quark pairs being produced via hard-scattering processes. The widths of that peak, characterized using Cauchy-Lorentz fits to the Delta Phi distributions, are found to not vary significantly as a function of Pb + Pb collision centrality and are similar for pp and Pb + Pb collisions. This observation will provide important constraints on theoretical descriptions of heavy-quark interactions with the quarkgluon plasma.
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ATLAS Collaboration(Aad, G. et al), Aikot, A., Amos, K. R., Aparisi Pozo, J. A., Bouchhar, N., Cabrera Urban, S., et al. (2024). Searches for exclusive Higgs boson decays into D*γ and Z boson decays into D0γ and K0sγ in pp collisions at √s=13 TeV with the ATLAS detector. Phys. Lett. B, 855, 138762–25pp.
Abstract: Searches for exclusive decays of the Higgs boson into D-0 gamma and of the Z boson into D-0 gamma and K-s(0)gamma can probe flavour-violating Higgs boson and Z boson couplings to light quarks. Searches for these decays are performed with a pp collision data sample corresponding to an integrated luminosity of 136.3 fb(-1) collected at root s=13TeV between 20162018 with the ATLAS detector at the CERN Large Hadron Collider. In the D*gamma and D-0 gamma channels, the observed (expected) 95% confidence-level upper limits on the respective branching fractions are B(H -> D*gamma)<1.0(1.2)x10(-3), B(Z -> D-0 gamma)<4.0(3.4)x10(-6), while the corresponding results in the K-s(0)gamma channel are B(Z -> K-s(0)gamma)<3.1(3.0)x10(-6)
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ATLAS Collaboration(Aad, G. et al), Aikot, A., Amos, K. R., Bouchhar, N., Cabrera Urban, S., Cantero, J., et al. (2025). Combination of searches for singly and doubly charged Higgs bosons pvroduced via vector-boson fusion in proton-proton collisions at √s=13 TeV with the ATLAS detector. Phys. Lett. B, 860, 139137–21pp.
Abstract: A combination of searches for singly and doubly charged Higgs bosons, H-+/- and H-+/-+/-, produced via vector-boson fusion is performed using 140 fb(-1) of proton-proton collisions at a centre-of-mass energy of 13 TeV, collected with the ATLAS detector during Run 2 of the Large Hadron Collider. Searches targeting decays to massive vector bosons in leptonic final states (electrons or muons) are considered. New constraints are reported on the production cross section times branching fraction for charged Higgs boson masses between 200 GeV and 3000 GeV. The results are interpreted in the context of the Georgi-Machacek model for which the most stringent constraints to date are set for the masses considered in the combination.
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ATLAS Collaboration(Aad, G. et al), Aikot, A., Amos, K. R., Bouchhar, N., Cabrera Urban, S., Cantero, J., et al. (2024). Jet radius dependence of dijet momentum balance and suppression in Pb plus Pb collisions at 5.02 TeV with the ATLAS detector. Phys. Rev. C, 110(5), 054912–35pp.
Abstract: This paper describes a measurement of the jet radius dependence of the dijet momentum balance between leading back-to-back jets in 1.72 nb(-1) of Pb+Pb collisions collected in 2018 and 255 pb(-1) of pp collisions collected in 2017 by the ATLAS detector at the LHC. Both datasets were collected at root s(NN) = 5.02 TeV. Jets are reconstructed using the anti-k(t) algorithm with jet radius parameters R = 0.2, 0.3, 0.4, 0.5, and 0.6. The dijet momentum balance distributions are constructed for leading jets with transverse momentum p(T) from 100 to 562 GeV for R = 0.2, 0.3, and 0.4 jets, and from 158 to 562 GeV for R = 0.5 and 0.6 jets. The absolutely normalized dijet momentum balance distributions are constructed to compare measurements of the dijet yields in Pb+Pb collisions directly to the dijet cross sections in pp collisions. For all jet radii considered here, there is a suppression of more balanced dijets in Pb+Pb collisions compared with pp collisions, while for more imbalanced dijets there is an enhancement. There is a jet radius dependence to the dijet yields, being stronger for more imbalanced dijets than for more balanced dijets. Additionally, jet pair nuclear modification factors are measured. The subleading jet yields are found to be more suppressed than leading jet yields in dijets. A jet radius dependence of the pair nuclear modification factors is observed, with the suppression decreasing with increasing jet radius. These measurements provide new constraints on jet quenching scenarios in the quark-gluon plasma.
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