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). Studies of the Energy Dependence of Diboson Polarization Fractions and the Radiation-Amplitude-Zero Effect in WZ Production with the ATLAS Detector. Phys. Rev. Lett., 133(10), 101802–24pp.
Abstract: This Letter presents the first study of the energy dependence of diboson polarization fractions in WZ -> lvl ' l '(l, l ' = e, mu) production. The dataset used corresponds to an integrated luminosity of 140 fb(-1) of proton-proton collisions at a center-of-mass energy of 13 TeV recorded by the ATLAS detector. Two fiducial regions with an enhanced presence of events featuring two longitudinally polarized bosons are defined. A nonzero fraction of events with two longitudinally polarized bosons is measured with an observed significance of 5.3 standard deviations in the region with 100 < p(T)(Z) <= 200 GeV and 1.6 standard deviations in the region with p(T)(Z) > 200 GeV, where p(T)(Z) is the transverse momentum of the Z boson. This Letter also reports the first study of the radiation-amplitude-zero effect. Events with two transversely polarized bosons are analyzed for the Delta Y(l(W)Z) and Delta Y(WZ) distributions defined respectively as the rapidity difference between the lepton from the W boson decay and the Z boson and the rapidity difference between the W boson and the Z boson. Significant suppression of events near zero is observed in both distributions. Unfolded Delta Y(l(W)Z) and Delta Y(WZ) distributions are also measured and compared to theoretical predictions.
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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., Aparisi Pozo, J. A., Bouchhar, N., Cabrera Urban, S., et al. (2024). Measurements of Lund subjet multiplicities in 13 TeV proton-proton collisions with the ATLAS detector. Phys. Lett. B, 859, 139090–24pp.
Abstract: This Letter presents a differential cross-section measurement of Lund subjet multiplicities, suitable for testing current and future parton shower Monte Carlo algorithms. This measurement is made in dijet events in 140 fb(-1) of root s = 13 TeV proton-proton collision data collected with the ATLAS detector at CERN's Large Hadron Collider. The data are unfolded to account for acceptance and detector-related effects, and are then compared with several Monte Carlo models and to recent resummed analytical calculations. The experimental precision achieved in the measurement allows tests of higher-order effects in QCD predictions. Most predictions fail to accurately describe the measured data, particularly at large values of jet transverse momentum accessible at the Large Hadron Collider, indicating the measurement's utility as an input to future parton shower developments and other studies probing fundamental properties of QCD and the production of hadronic final states up to the TeV-scale.
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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. (2025). Measurement of top-quark pair production in association with charm quarks in proton-proton collisions at √s=13 TeV with the ATLAS detector. Phys. Lett. B, 860, 139177–44pp.
Abstract: Inclusive cross-sections for top-quark pair production in association with charm quarks are measured with proton-proton collision data at a center-of-mass energy of 13 TeV corresponding to an integrated luminosity of 140 fb(-1), collected with the ATLAS experiment at the LHC between 2015 and 2018. The measurements are performed by requiring one or two charged leptons (electrons and muons), two b-tagged jets, and at least one additional jet in the final state. A custom flavor-tagging algorithm is employed for the simultaneous identification of b-jets and c-jets. In a fiducial phase space that replicates the acceptance of the ATLAS detector, the cross-sections for t (t) over bar >= 2c and t (t) over bar + 1c production are measured to be 1.28(- 0.24)(+0.27) pb and 6.4(-0.9)(+1.0) pb, respectively. The measurements are primarily limited by uncertainties in the modeling of inclusive t (t) over bar and t (t) over bar + b (b) over bar production, in the calibration of the flavor-tagging algorithm, and by data statistics. Cross-section predictions from various t (t) over bar simulations are largely consistent with the measured cross-section values, though all underpredict the observed values by 0.5 to 2.0 standard deviations. In a phase-space volume without requirements on the t (t) over bar decay products and the jet multiplicity, the cross-section ratios of t (t) over bar + >= 2c and t (t) over bar + 1c to total t (t) over bar + jets production are determined to be (1.23 +/- 0.25)% and (8.8 +/- 1.3)%.
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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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ATLAS Collaboration(Aad, G. et al), Aikot, A., Amos, K. R., Bouchhar, N., Cabrera Urban, S., Cantero, J., et al. (2024). Search for pair-produced vectorlike quarks coupling to light quarks in the lepton plus jets final state using 13 TeV pp collisions with the ATLAS detector. Phys. Rev. D, 110(5), 052009–27pp.
Abstract: A search is presented for the pair production of heavy vectorlike quarks (VLQs) that each decay into a W boson and a light quark. This study focuses on events where one W boson decays into leptons and the other into hadrons. The search analyzed 140 fb(-1) of pp collision data with root s = 13 TeV, recorded by the ATLAS detector from 2015 to 2018 during run 2 of the Large Hadron Collider. The final state is characterized by a high-transverse-momentum isolated electron or muon, large missing transverse momentum, multiple small-radius jets, and a single large-radius jet identified as originating from the hadronic decay of a boosted W boson. With higher center-of-mass energy and integrated luminosity than in the run 1 search, and improved analysis tools, this analysis excludes VLQs (Q) with masses below 1530 GeVat 95% confidence level for the branching ratio B(Q -> Wq) = 1, an improvement of 840 GeV on the previous ATLAS limit.
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