ATLAS Collaboration(Aad, G. et al), Amoros, G., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., et al. (2012). Search for TeV-scale gravity signatures in final states with leptons and jets with the ATLAS detector at root s=7 TeV. Phys. Lett. B, 716(1), 122–141.
Abstract: The production of events with multiple high transverse momentum particles including charged leptons and jets is measured, using 1.04 fb(-1) of proton-proton collision data recorded by the ATLAS detector during the first half of 2011 at root s = 7 TeV. No excess beyond Standard Model expectations is observed, and upper limits on the fiducial cross sections for non-Standard Model production of these final states are set. Using models for string ball and black hole production and decay, exclusion contours are determined as a function of mass threshold and the fundamental gravity scale. (C) 2012 CERN. Published by Elsevier B.V. All rights reserved.
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ATLAS Collaboration(Aad, G. et al), Amoros, G., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., et al. (2012). Search for tb Resonances in Proton-Proton Collisions at root s=7 TeV with the ATLAS Detector. Phys. Rev. Lett., 109(8), 081801–19pp.
Abstract: This Letter presents a search for tb resonances in 1.04 fb(-1) of LHC proton-proton collision data collected by the ATLAS detector at a center-of-mass energy of 7 TeV. Events with a lepton, missing transverse momentum, and two jets are selected and the invariant mass of the corresponding final state is reconstructed. The search exploits the shape of the tb invariant mass distribution compared to the expected standard model backgrounds. The model of a right-handed W'(R) with standard model-like couplings is chosen as the benchmark model for this search. No statistically significant excess of events is observed in the data, and upper limits on the cross section times the branching ratio of W'(R) resonances at 95% C.L. lie in the range of 6.1-1.0 pb for W'(R) masses ranging from 0.5 to 2.0 TeV. These limits are translated into a lower bound on the allowed right-handed W'(R) mass, giving mW'(R) > 1.13 TeV at 95% C.L.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fassi, F., Ferrer, A., et al. (2013). Search for t(t)over-bar resonances in the lepton plus jets final state with ATLAS using 4.7 fb(-1) of pp collisions at root s=7 TeV. Phys. Rev. D, 88(1), 012004–28pp.
Abstract: A search for new particles that decay into top quark pairs (t (t) over bar) is performed with the ATLAS experiment at the LHC using an integrated luminosity of 4.7 fb(-1) of proton-proton (pp) collision data collected at a center-of-mass energy root s = 7 TeV. In the t (t) over bar) -> WbWb decay, the lepton plus jets final state is used, where one W boson decays leptonically and the other hadronically. The t (t) over bar) system is reconstructed using both small-radius and large-radius jets, the latter being supplemented by a jet substructure analysis. A search for local excesses in the number of data events compared to the Standard Model expectation in the t (t) over bar) invariant mass spectrum is performed. No evidence for a t (t) over bar) resonance is found and 95% credibility-level limits on the production rate are determined for massive states predicted in two benchmark models. The upper limits on the cross section times branching ratio of a narrow Z' resonance range from 5.1 pb for a boson mass of 0.5 TeV to 0.03 pb for a mass of 3 TeV. A narrow leptophobic topcolor Z' resonance with a mass below 1.74 TeV is excluded. Limits are also derived for a broad color-octet resonance with m 15.3%. A Kaluza-Klein excitation of the gluon in a Randall-Sundrum model is excluded for masses below 2.07 TeV.
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ATLAS Collaboration(Aad, G. et al), Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Castillo, F. L., Castillo Gimenez, V., et al. (2020). Search for t(t)over-bar resonances in fully hadronic final states in pp collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 10(10), 061–43pp.
Abstract: This paper presents a search for new heavy particles decaying into a pair of top quarks using 139 fb(-1) of proton-proton collision data recorded at a centre-of-mass energy of root s = 13TeV with the ATLAS detector at the Large Hadron Collider. The search is performed using events consistent with pair production of high-transverse-momentum top quarks and their subsequent decays into the fully hadronic final states. The analysis is optimized for resonances decaying into a t (t) over bar pair with mass above 1.4TeV, exploiting a dedicated multivariate technique with jet substructure to identify hadronically decaying top quarks using large-radius jets and evaluating the background expectation from data. No significant deviation from the background prediction is observed. Limits are set on the production cross-section times branching fraction for the new Z' boson in a topcolor-assisted-technicolor model. The Z0 boson masses below 3.9 and 4.7TeV are excluded at 95% confidence level for the decay widths of 1% and 3%, respectively.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cantero, J., et al. (2023). Search for t(t)bar H/A → t(t)bar t(t)bar production in the multilepton final state in proton-proton collisions at √s=13 TeV with the ATLAS detector. J. High Energy Phys., 07(7), 203–53pp.
Abstract: A search for a new heavy scalar or pseudo-scalar Higgs boson (H/A) produced in association with a pair of top quarks, with the Higgs boson decaying into a pair of top quarks (H/A -> t (t) over bar) is reported. The search targets a final state with exactly two leptons with same-sign electric charges or at least three leptons. The analysed dataset corresponds to an integrated luminosity of 139 fb(-1) of proton-proton collisions collected at a centre-of-mass energy of 13TeV with the ATLAS detector at the LHC. Two multivariate classifiers are used to separate the signal from the background. No significant excess of events over the Standard Model expectation is observed. The results are interpreted in the context of a type-II two-Higgs-doublet model. The observed (expected) upper limits at 95% confidence level on the t (t) over barH/A production cross-section times the branching ratio of H/A -> t (t) over bar range between 14 (10) fb and 6 (5) fb for a heavy Higgs boson with mass between 400 GeV and 1000 GeV, respectively. Assuming that only one particle, either the scalar H or the pseudo-scalar A, contributes to the t (t) over bart (t) over bar final state, values of tan beta below 1.2 or 0.5 are excluded for a mass of 400 GeV or 1000 GeV, respectively. These exclusion ranges increase to tan beta below 1.6 or 0.6 when both particles are considered.
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