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., Aparisi Pozo, J. A., Bouchhar, N., Cabrera Urban, S., et al. (2024). Fiducial and differential cross-section measurements of electroweak Wγjj production in pp collisions at √s=13 TeV with the ATLAS detector. Eur. Phys. J. C, 84(10), 1064–34pp.
Abstract: The observation of the electroweak production of a W boson and a photon in association with two jets, using pp collision data at the Large Hadron Collider at a centre of mass energy of root s =13 TeV, is reported. The data were recorded by the ATLAS experiment from 2015 to 2018 and correspond to an integrated luminosity of 140 fb(-1). This process is sensitive to the quartic gauge boson couplings via the vector boson scattering mechanism and provides a stringent test of the electroweak sector of the Standard Model. Events are selected if they contain one electron or muon, missing transverse momentum, at least one photon, and two jets. Multivariate techniques are used to distinguish the electroweak W gamma jj process from irreducible background processes. The observed significance of the electroweak W gamma jj process is well above six standard deviations, compared to an expected significance of 6.3 standard deviations. Fiducial and differential cross sections are measured in a fiducial phase space close to the detector acceptance, which are in reasonable agreement with leading order Standard Model predictions from MadGraph5+Pythia8 and Sherpa. The results are used to constrain new physics effects in the context of an effective field theory.
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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 the associated production of a top-antitop-quark pair and a Higgs boson decaying into a b(b)over-bar pair in pp collisions at √s=13 TeV using the ATLAS detector at the LHC. Eur. Phys. J. C, 85(2), 210–40pp.
Abstract: This paper reports the measurement of Higgs boson production in association with a t (t) over bar pair in the H -> b (b) over bar decay channel. The analysis uses 140 fb(-1) of 13 TeV proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider. The final states with one or two electrons or muons are employed. An excess of events over the expected background is found with an observed (expected) significance of 4.6 (5.4) standard deviations. The t (t) over barH cross-section is sigma(t (t) over barH)=411(-92)(+101)fb=411 +/- 54(stat.)(-75)(+85)(syst.)fb for a Higgs boson mass of 125.09 GeV, consistent with the prediction of the Standard Model of 507(-50)(+35) fb. The cross-section is also measured differentially in bins of the Higgs boson transverse momentum within the simplified template cross-section framework.
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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). Search for a light charged Higgs boson in t → H±b decays, with H± → cs, in pp collisions at √s=13 TeV with the ATLAS detector. Eur. Phys. J. C, 85(2), 153–34pp.
Abstract: A search for a light charged Higgs boson produced in decays of the top quark, t -> H(+/-)b with H-+/- -> cs, is presented. This search targets the production of top-quark pairs t (t) over bar. WbH(+/-)b, with W -> lv (l = e, mu), resulting in a lepton-plus-jets final state characterised by an isolated electron or muon and at least four jets. The search exploits b-quark and c-quark identification techniques as well as multivariate methods to suppress the dominant t (t) over bar background. The data analysed correspond to 140 fb(-1) of pp collisions at root s = 13 TeV recorded with the ATLAS detector at the LHC between 2015 and 2018. Observed (expected) 95% confidence-level upper limits on the branching fraction B(t -> H(+/-)b), assuming B(t -> Wb) + B(t -> H +/-(-> cs)b) = 1.0, are set between 0.066% (0.077%) and 3.6% (2.3%) for a charged Higgs boson with a mass between 60 and 168 GeV.
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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). Differential cross-section measurements of Higgs boson production in the H → τ+τ- decay channel in pp collisions at √s=13 TeV with the ATLAS detector. J. High Energy Phys., 03(3), 010–66pp.
Abstract: Differential measurements of Higgs boson production in the tau-lepton-pair decay channel are presented in the gluon fusion, vector-boson fusion (VBF), VH and t (t) over barH associated production modes, with particular focus on the VBF production mode. The data used to perform the measurements correspond to 140 fb(-1) of proton-proton collisions collected by the ATLAS experiment at the LHC. Two methods are used to perform the measurements: the Simplified Template Cross-Section (STXS) approach and an Unfolded Fiducial Differential measurement considering only the VBF phase space. For the STXS measurement, events are categorized by their production mode and kinematic properties such as the Higgs boson's transverse momentum (p(T)(H)), the number of jets produced in association with the Higgs boson, or the invariant mass of the two leading jets (m(jj)). For the VBF production mode, the ratio of the measured cross-section to the Standard Model prediction for m(jj) > 1.5 TeV and p(T)(H) > 200 GeV (p(T)(H) < 200 GeV) is 1.29(-0.34)(+0.39) (0.12(-0.33)(+0.34)). This is the first VBF measurement for the higher-p(T)(H) criteria, and the most precise for the lower-p(T)(H) criteria. The fiducial cross-section measurements, which only consider the kinematic properties of the event, are performed as functions of variables characterizing the VBF topology, such as the signed Delta phi(jj) between the two leading jets. The measurements have a precision of 30%-50% and agree well with the Standard Model predictions. These results are interpreted in the SMEFT framework, and place the strongest constraints to date on the CP-odd Wilson coefficient c(H (W) over tilde).
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