ATLAS Collaboration(Aad, G. et al), Aikot, A., Amos, K. R., Bouchhar, N., Cabrera Urban, S., Cantero, J., et al. (2024). Search for a resonance decaying into a scalar particle and a Higgs boson in final states with leptons and two photons in proton-proton collisions at √s=13 TeV with the ATLAS detector. J. High Energy Phys., 10(10), 104–46pp.
Abstract: A search for a hypothetical heavy scalar particle, X, decaying into a singlet scalar particle, S, and a Standard Model Higgs boson, H, using 140 fb(-1) of proton-proton collision data at the centre-of-mass energy of 13 TeV recorded with the ATLAS detector at the LHC is presented. The explored mass range is 300 <= m(X) <= 1000 GeV and 170 <= m(S) <= 500 GeV. The signature of this search is one or two leptons (e or mu) from the decay of vector bosons originating from the S particle, S -> (WW -/+)-W-+/-/ZZ, and two photons from the Higgs boson decay, H -> gamma gamma. No significant excess is observed above the expected Standard Model background. The observed (expected) upper limits at the 95% confidence level on the cross- section for gg -> X -> SH, assuming the same S -> WW/ZZ branching ratios as for a SM-like heavy Higgs boson, are between 530 (800) fb and 120 (170) fb.
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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 boosted low-mass resonances decaying into hadrons produced in association with a photon in pp collisions at √s=13 TeV with the ATLAS detector. J. High Energy Phys., 01(1), 099–40pp.
Abstract: Many extensions of the Standard Model, including those with dark matter particles, propose new mediator particles that decay into hadrons. This paper presents a search for such low mass narrow resonances decaying into hadrons using 140 fb(-1) of proton-proton collision data recorded with the ATLAS detector at a centre-of-mass energy of 13 TeV. The resonances are searched for in the invariant mass spectrum of large-radius jets with two-pronged substructure that are recoiling against an energetic photon from initial state radiation, which is used as a trigger to circumvent limitations on the maximum data recording rate. This technique enables the search for boosted hadronically decaying resonances in the mass range 20-100 GeV hitherto unprobed by the ATLAS Collaboration. The observed data are found to agree with Standard Model predictions and 95% confidence level upper limits are set on the coupling of a hypothetical new spin-1 Z ' resonance with Standard Model quarks as a function of the assumed Z '-boson mass in the range between 20 and 200 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. (2024). Search for dark mesons decaying to top and bottom quarks in proton-proton collisions at √s=13 TeV with the ATLAS detector. J. High Energy Phys., 09(9), 005–55pp.
Abstract: A search for dark mesons originating from strongly-coupled, SU(2) dark flavor symmetry conserving models and decaying gaugephobically to pure Standard Model final states containing top and bottom quarks is presented. The search targets fully hadronic final states and final states with exactly one electron or muon and multiple jets. The analyzed data sample corresponds to an integrated luminosity of 140 fb(-1) of proton-proton collisions collected at root s = 13 TeV with the ATLAS detector at the Large Hadron Collider. No significant excess over the Standard Model background expectation is observed and the results are used to set the first direct constraints on this type of model. The two-dimensional signal space of dark pion masses m(pi D) and dark rho-meson masses m(rho D) is scanned. For m(pi D)/m(rho D) = 0.45, dark pions with masses m(pi D)< 940 GeV are excluded at the 95% CL, while for m(pi D)/m(rho D) = 0.25 masses m(pi D)< 740 GeV are excluded.
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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 heavy neutral Higgs bosons decaying into a top quark pair in 140 fb-1 of proton-proton collision data at √s=13 TeV with the ATLAS detector. J. High Energy Phys., 08(8), 013–76pp.
Abstract: A search for heavy pseudo-scalar (A) and scalar (H) Higgs bosons decaying into a top-quark pair (t (t) over bar) has been performed with 140 fb(-1) of proton-proton collision data collected by the ATLAS experiment at the Large Hadron Collider at a centre-of-mass energy of root s = 13TeV. Interference effects between the signal process and Standard Model (SM) t (t) over bar production are taken into account. Final states with exactly one or exactly two electrons or muons are considered. No significant deviation from the SM prediction is observed. The results of the search are interpreted in the context of a two-Higgs-doublet model (2HDM) of type II in the alignment limit with mass-degenerate pseudo-scalar and scalar Higgs bosons (m(A) = m(H)) and the hMSSM parameterisation of the minimal supersymmetric extension of the Standard Model. Ratios of the two vacuum expectation values, tan beta, smaller than 3.49 (3.16) are excluded at 95% confidence level for m(A) = m(H) = 400 GeV in the 2HDM (hMSSM). Masses up to 1240 GeV are excluded for the lowest tested tan beta value of 0.4 in the 2HDM. In the hMSSM, masses up to 950 GeV are excluded for tan beta = 1.0. In addition, generic exclusion limits are derived separately for single scalar and pseudo-scalar states for different choices of their mass and total width.
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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). A search for top-squark pair production, in final states containing a top quark, a charm quark and missing transverse momentum, using the 139 fb-1 of pp collision data collected by the ATLAS detector. J. High Energy Phys., 07(7), 250–49pp.
Abstract: This paper presents a search for top-squark pair production in final states with a top quark, a charm quark and missing transverse momentum. The data were collected with the ATLAS detector during LHC Run 2 and correspond to an integrated luminosity of 139 fb(-1) of proton-proton collisions at a centre-of-mass energy of root s = 13TeV. The analysis is motivated by an extended Minimal Supersymmetric Standard Model featuring a non-minimal flavour violation in the second- and third-generation squark sector. The top squark in this model has two possible decay modes, either (t) over tilde (1) -> c (chi) over tilde (0)(1) or (t) over tilde (1) -> t (chi) over tilde (0)(1), where the (chi) over tilde (0)(1) is undetected. The analysis is optimised assuming that both of the decay modes are equally probable, leading to the most likely final state of tc + E-T(miss). Good agreement is found between the Standard Model expectation and the data in the search regions. Exclusion limits at 95% CL are obtained in the m((t) over tilde1) vs . m((chi) over tilde (0)(1)) plane and, in addition, limits on the branching ratio of the (t) over tilde -> t (chi) over tilde (0)(1)1 decay as a function of m((t) over tilde (1)) are also produced. Top-squark masses of up to 800 GeV are excluded for scenarios with light neutralinos, and top-squark masses up to 600 GeV are excluded in scenarios where the neutralino and the top squark are almost mass degenerate.
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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. (2025). Search for diphoton resonances in the 66 to 110 GeV mass range using pp collisions at √s=13 TeV with the ATLAS detector. J. High Energy Phys., 01(1), 053–45pp.
Abstract: A search is performed for light, spin-0 bosons decaying into two photons in the 66 to 110 GeV mass range, using 140 fb(-1) of proton-proton collisions at root s = 13TeV produced by the Large Hadron Collider and collected by the ATLAS detector. Multivariate analysis techniques are used to define event categories that improve the sensitivity to new resonances beyond the Standard Model. A model-independent search for a generic spin-0 particle and a model-dependent search for an additional low-mass Higgs boson are performed in the diphoton invariant mass spectrum. No significant excess is observed in either search. Mass-dependent upper limits at the 95% confidence level are set in the model-independent scenario on the fiducial cross-section times branching ratio into two photons in the range of 8 fb to 53 fb. Similarly, in the model-dependent scenario upper limits are set on the total cross-section times branching ratio into two photons as a function of the Higgs boson mass in the range of 19 fb to 102 fb.
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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 same-charge top-quark pair production in pp collisions at √s=13 TeV with the ATLAS detector. J. High Energy Phys., 02(2), 084–52pp.
Abstract: A search for the production of top-quark pairs with the same electric charge (tt or (tt) over bar) is presented. The analysis uses proton-proton collision data at root s = 13 TeV, recorded by the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of 140 fb(-1). Events with two same-charge leptons and at least two b-tagged jets are selected. Neural networks are employed to define two selections sensitive to additional couplings beyond the Standard Model that would enhance the production rate of same-sign top-quark pairs. No significant signal is observed, leading to an upper limit on the total production cross-section of same-sign top-quark pairs of 1.6 fb at 95% confidence level. Corresponding limits on the three Wilson coefficients associated with the O-tu((1)), O-Qu((1)), and O-Qu((8)) operators in the Standard Model Effective Field Theory framework are derived.
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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 t(t)over-bar production in association with additional b-jets in the eμ final state in proton-proton collisions at √s=13 TeV with the ATLAS detector. J. High Energy Phys., 01(1), 068–77pp.
Abstract: This paper presents measurements of top-antitop quark pair (t (t) over bar) production in association with additional b-jets. The analysis utilises 140 fb(-1) of proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider at a centre-of-mass energy of 13TeV. Fiducial cross-sections are extracted in a final state featuring one electron and one muon, with at least three or four b-jets. Results are presented at the particle level for both integrated cross-sections and normalised differential cross-sections, as functions of global event properties, jet kinematics, and b-jet pair properties. Observable quantities characterising b-jets originating from the top quark decay and additional b-jets are also measured at the particle level, after correcting for detector effects. The measured integrated fiducial cross-sections are consistent with t (t) over barb (b) over bar predictions from various next-to-leading-order matrix element calculations matched to a parton shower within the uncertainties of the predictions. State-of-the-art theoretical predictions are compared with the differential measurements; none of them simultaneously describes all observables. Differences between any two predictions are smaller than the measurement uncertainties for most observables.
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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 single-production of vector-like quarks decaying into Wb in the fully hadronic final state in pp collisions at √s=13 TeV with the ATLAS detector. J. High Energy Phys., 02(2), 075–44pp.
Abstract: A search for T and Y vector-like quarks produced in proton-proton collisions at a centre-of-mass energy of 13 TeV and decaying into Wb in the fully hadronic final state is presented. The search uses 139 fb(-1) of data collected by the ATLAS detector at the LHC from 2015 to 2018. The final state is characterised by a hadronically decaying W boson with large Lorentz boost and a b-tagged jet, which are used to reconstruct the invariant mass of the vector-like quark candidate. The main background is QCD multijet production, which is estimated using a data-driven method. Upon finding no significant excess in data, mass limits at 95% confidence level are obtained as a function of the global coupling parameter, kappa. The observed lower limits on the masses of Y quarks with kappa = 0.5 and kappa = 0.7 are 2.0 TeV and 2.4 TeV, respectively. For T quarks, the observed mass limits are 1.4 TeV for kappa = 0.5 and 1.9 TeV for kappa = 0.7.
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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 the associated production of charm quarks and a Higgs boson decaying into a photon pair with the ATLAS detector. J. High Energy Phys., 02(2), 045–36pp.
Abstract: A search for the production of a Higgs boson and one or more charm quarks, in which the Higgs boson decays into a photon pair, is presented. This search uses proton-proton collision data with a centre-of-mass energy of root s = 13TeV and an integrated luminosity of 140 fb(-1) recorded by the ATLAS detector at the Large Hadron Collider. The analysis relies on the identification of charm-quark-containing jets, and adopts an approach based on Gaussian process regression to model the non-resonant di-photon background. The observed (expected, assuming the Standard Model signal) upper limit at the 95% confidence level on the cross-section for producing a Higgs boson and at least one charm-quark-containing jet that passes a fiducial selection is found to be 10.6 pb (8.8 pb). The observed (expected) measured cross-section for this process is 5.3 +/- 3.2 pb (2.9 +/- 3.1 pb).
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