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). Study of the hard double-parton scattering contribution to inclusive four-lepton production in pp collisions at root s=8 TeV with the ATLAS detector. Phys. Lett. B, 790, 595–614.
Abstract: The inclusive production of four isolated charged leptons in pp collisions is analysed for the presence of hard double-parton scattering, using 20.2 fb(-1) of data recorded in the ATLAS detector at the LHC at centre-of-mass energy root s = 8 TeV. In the four-lepton invariant-mass range of 80 < m(4l) < 1000 GeV, an artificial neural network is used to enhance the separation between single- and double-parton scattering based on the kinematics of the four leptons in the final state. An upper limit on the fraction of events originating from double-parton scattering is determined at 95% confidence level to be f(DPS) = 0.042, which results in an estimated lower limit on the effective cross section at 95% confidence level of 1.0 mb.
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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 flavour-changing neutral current interactions of the top quark and the Higgs boson in events with a pair of tau-leptons in pp collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 06(6), 155–57pp.
Abstract: A search for flavour-changing neutral current (FCNC) tqH interactions involving a top quark, another up-type quark (q = u, c), and a Standard Model (SM) Higgs boson decaying into a tau-lepton pair (H -> tau(+)tau(-)) is presented. The search is based on a dataset of pp collisions at root s = 13 TeV that corresponds to an integrated luminosity of 139 fb(-1) recorded with the ATLAS detector at the Large Hadron Collider. Two processes are considered: single top quark FCNC production in association with a Higgs boson (pp -> tH), and top quark pair production in which one of top quarks decays into Wb and the other decays into qH through the FCNC interactions. The search selects events with two hadronically decaying tau-lepton candidates (tau(had)) or at least one tau(had) with an additional lepton (e, mu), as well as multiple jets. Event kinematics is used to separate signal from the background through a multivariate discriminant. A slight excess of data is observed with a significance of 2.3 sigma above the expected SM background, and 95% CL upper limits on the t -> qH branching ratios are derived. The observed (expected) 95% CL upper limits set on the t -> cH and t -> uH branching ratios are 9.4x10(-4)(4.8(-1.4)(+2.2) x 10(-4)) and 6.9x10(-4) (3.5(-1.0)(+1.5) x10(-4)), respectively. The corresponding combined observed (expected) upper limits on the dimension-6 operator Wilson coefficients in the effective tqH couplings are C-c phi < 1.35 (0.97) and C-u phi < 1.16 (0.82).
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2017). Search for top quark decays t -> qH,with H -> gamma gamma, in root s=13 TeV pp collisions using the ATLAS detector. J. High Energy Phys., 10(10), 129–43pp.
Abstract: This article presents a search for flavour-changing neutral currents in the decay of a top quark into an up-type (q = c; u) quark and a Higgs boson, where the Higgs boson decays into two photons. The proton-proton collision data set analysed amounts to 36.1 fb(-1) at root s = 13TeV collected by the ATLAS experiment at the LHC. Top quark pair events are searched for, where one top quark decays into qH and the other decays into bW. Both the hadronic and leptonic decay modes of the W boson are used. No significant excess is observed and an upper limit is set on the t -> cH branching ratio of 2 : 2 x 10(-3) at the 95% confidence level, while the expected limit in the absence of signal is 1 : 6 x 10(-3). The corresponding limit on the tcH coupling is 0.090 at the 95% confidence level. The observed upper limit on the t -> uH branching ratio is 2 : 4 x 10(-3).
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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). A search for high-mass resonances decaying to tau(+)tau(-) in pp collisions at root s=7 TeV with the ATLAS detector. Phys. Lett. B, 719(4-5), 242–260.
Abstract: This Letter presents a search for high-mass resonances decaying into tau(+)tau(-) final states using proton-proton collisions at root s = 7 TeV produced by the Large Hadron Collider. The data were recorded with the ATLAS detector and correspond to an integrated luminosity of 4.6 fb(-1). No statistically significant excess above the Standard Model expectation is observed; 95% credibility upper limits are set on the cross section times branching fraction of Z' resonances decaying into tau(+)tau(-) pairs as a function of the resonance mass. As a result, Z' bosons of the Sequential Standard Model with masses less than 1.40 TeV are excluded at 95% credibility.
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ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fernandez Martinez, P., et al. (2015). Analysis of events with b-jets and a pair of leptons of the same charge in pp collisions at p root s=8 TeV with the ATLAS detector. J. High Energy Phys., 10(10), 150–51pp.
Abstract: An analysis is presented of events containing jets including at least one b tagged jet, sizeable missing transverse momentum, and at least two leptons including a pair of the same electric charge, with the scalar sum of the jet and lepton transverse momenta being large. A data sample with an integrated luminosity of 20.3 fb(-1) of p p collisions at p root s = 8TeV recorded by the ATLAS detector at the Large Hadron Collider is used. Standard Model processes rarely produce these final states, but there are several models of physics beyond the Standard Model that predict an enhanced rate of production of such events; the ones considered here are production of vector-like quarks, enhanced fourtop- quark production, pair production of chiral b'-quarks, and production of two positively charged top quarks. Eleven signal regions are defined; subsets of these regions are combined when searching for each class of models. In the three signal regions primarily sensitive to positively charged top quark pair production, the data yield is consistent with the background expectation. There are more data events than expected from background in the set of eight signal regions defined for searching for vector-like quarks and chiral b'-quarks, but the significance of the discrepancy is less than two standard deviations. The discrepancy reaches 2.5 standard deviations in the set of five signal regions defined for searching for four-top-quark production. The results are used to set 95% CL limits on various models.
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