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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ATLAS Collaboration(Aad, G. et al), Akiot, A., Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., et al. (2023). Search for a new heavy scalar particle decaying into a Higgs boson and a new scalar singlet in final states with one or two light leptons and a pair of τ-leptons with the ATLAS detector. J. High Energy Phys., 10(10), 009–46pp.
Abstract: A search for a new heavy scalar particle X decaying into a Standard Model (SM) Higgs boson and a new singlet scalar particle S is presented. The search uses a proton-proton (pp) collision data sample with an integrated luminosity of 140 fb(-1) recorded at a centre-of-mass energy of root s = 13 TeV with the ATLAS detector at the Large Hadron Collider. The most sensitive mass parameter space is explored in X mass ranging from 500 to 1500 GeV, with the corresponding S mass in the range 200-500 GeV. The search selects events with two hadronically decaying tau-lepton candidates from H -> tau(+)tau(-) decays and one or two light leptons (l = e, mu) from S -> VV (V = W, Z) decays while the remaining V boson decays hadronically or to neutrinos. A multivariate discriminant based on event kinematics is used to separate the signal from the background. No excess is observed beyond the expected SM background and 95% confidence level upper limits between 72 fb and 542 fb are derived on the cross-section sigma(pp -> X -> SH) assuming the same SM-Higgs boson-like decay branching ratios for the S -> VV decay. Upper limits on the visible cross-sections sigma(pp -> X -> SH -> WW tau tau) and sigma(pp -> X -> SH -> ZZ tau tau) are also set in the ranges 3-26 fb and 6-33 fb, respectively.
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ATLAS Collaboration(Aad, G. et al), Akiot, A., Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., et al. (2023). Search for vector-boson resonances decaying into a top quark and a bottom quark using pp collisions at √s=13 TeV with the ATLAS detector. J. High Energy Phys., 12(12), 073–63pp.
Abstract: A search for a new massive charged gauge boson, W ', is performed with the ATLAS detector at the LHC. The dataset used in this analysis was collected from proton-proton collisions at a centre-of-mass energy of root s = 13 TeV, and corresponds to an integrated luminosity of 139 fb(-1). The reconstructed tb invariant mass is used to search for a W ' boson decaying into a top quark and a bottom quark. The result is interpreted in terms of a W ' boson with purely right-handed or left-handed chirality in a mass range of 0.5-6 TeV. Different values for the coupling of the W ' boson to the top and bottom quarks are considered, taking into account interference with single-top-quark production in the s-channel. No significant deviation from the background prediction is observed. The results are expressed as upper limits on the W ' -> tb production cross-section times branching ratio as a function of the W '-boson mass and in the plane of the coupling vs the W '-boson mass.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., et al. (2019). Search for Higgs boson pair production in the (WWWW(*))-W-(*) decay channel using ATLAS data recorded at root s=13 TeV. J. High Energy Phys., 05(5), 124–38pp.
Abstract: A search for a pair of neutral, scalar bosons with each decaying into two W bosons is presented using 36.1 fb(-1) of proton-proton collision data at a centre-of-mass energy of 13 TeV recorded with the ATLAS detector at the Large Hadron Collider. This search uses three production models: non-resonant and resonant Higgs boson pair production and resonant production of a pair of heavy scalar particles. Three final states, classified by the number of leptons, are analysed: two same-sign leptons, three leptons, and four leptons. No significant excess over the expected Standard Model backgrounds is observed. An observed (expected) 95% confidence-level upper limit of 160 (120) times the Standard Model prediction of non-resonant Higgs boson pair production cross-section is set from a combined analysis of the three final states. Upper limits are set on the production cross-section times branching ratio of a heavy scalar X decaying into a Higgs boson pair in the mass range of 260 GeV m(X) 500 GeV and the observed (expected) limits range from 9.3 (10) pb to 2.8 (2.6) pb. Upper limits are set on the production cross-section times branching ratio of a heavy scalar X decaying into a pair of heavy scalars S for mass ranges of 280 GeV m(X) 340 GeV and 135 GeV m(S) 165 GeV and the observed (expected) limits range from 2.5 (2.5) pb to 0.16 (0.17) pb.
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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. (2014). Search for microscopic black holes and string balls in final states with leptons and jets with the ATLAS detector at root s=8 TeV. J. High Energy Phys., 08(8), 103–48pp.
Abstract: A search for an excess of events with multiple high transverse momentum objects including charged leptons and jets is presented, using 20.3 fb(-1) of proton-proton collision data recorded by the ATLAS detector at the Large Hadron Collider in 2012 at a centre-of-mass energy of root s = 8 TeV. No excess of events beyond Standard Model expectations is observed. Using extra-dimensional models for black hole and string ball production and decay, exclusion contours are determined as a function of the mass threshold for production and the fundamental gravity scale for two, four and six extra dimensions. For six extra dimensions, mass thresholds of 4.8-6.2 TeV are excluded at 95 % confidence level, depending on the fundamental gravity scale and model assumptions. Upper limits on the fiducial cross-sections for non-Standard Model production of these final states are set.
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