ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fernandez Martinez, P., et al. (2015). Search for new light gauge bosons in Higgs boson decays to four-lepton final states in pp collisions at root s=8 TeV with the ATLAS detector at the LHC. Phys. Rev. D, 92(9), 092001–30pp.
Abstract: This paper presents a search for Higgs bosons decaying to four leptons, either electrons or muons, via one or two light exotic gauge bosons Z(d), H -> ZZ(d) -> 4l or H -> Z(d)Z(d) -> 4l. The search was performed using pp collision data corresponding to an integrated luminosity of about 20 fb(-1) at the center-of-mass energy of root s = 8 TeV recorded with the ATLAS detector at the Large Hadron Collider. The observed data are well described by the Standard Model prediction. Upper bounds on the branching ratio of H -> ZZ(d) -> 4l and on the kinetic mixing parameter between the Z(d) and the Standard Model hypercharge gauge boson are set in the range (1-9) x 10(-5) and (4-17) x 10(-2) respectively, at 95% confidence level assuming the Standard Model branching ratio of H -> ZZ* -> 4l, for Z(d) masses between 15 and 55 GeV. Upper bounds on the effective mass mixing parameter between the Z and the Z(d) are also set using the branching ratio limits in the H -> ZZ(d) -> 4l search, and are in the range (1.5-8.7) x 10(-4) for 15 < m(Zd) < 35 GeV. Upper bounds on the branching ratio of H -> Z(d)Z(d) -> 4l and on the Higgs portal coupling parameter, controlling the strength of the coupling of the Higgs boson to dark vector bosons are set in the range (2-3) x 10(-5) and (1-10) x 10(-4) respectively, at 95% confidence level assuming the Standard Model Higgs boson production cross sections, for Z(d) masses between 15 and 60 GeV.
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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 top squarks in events with a Higgs or Z boson using 139 fb(-1) of pp collision data at root s=13 TeV with the ATLAS detector. Eur. Phys. J. C, 80(11), 1080–33pp.
Abstract: This paper presents a search for direct top squark pair production in events with missing transverse momentum plus either a pair of jets consistent with Standard Model Higgs boson decay into b-quarks or a same-flavour opposite-sign dilepton pair with an invariant mass consistent with a Z boson. The analysis is performed using the proton-proton collision data at root s = 13 TeV collected with the ATLAS detector during the LHC Run-2, corresponding to an integrated luminosity of 139 fb(-1). No excess is observed in the data above the Standard Model predictions. The results are interpreted in simplified models featuring direct production of pairs of either the lighter top squark ((t) over tilde (1)) or the heavier top squark ((t) over tilde (2)), excluding at 95% confidence level (t) over tilde (1) and (t) over tilde2 masses up to about 1220 and 875 GeV, respectively.
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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. (2018). Search for long-lived charginos based on a disappearing-track signature in pp collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 06(6), 022–48pp.
Abstract: This paper presents a search for direct electroweak gaugino or gluino pair production with a chargino nearly mass-degenerate with a stable neutralino. It is based on an integrated luminosity of 36.1 fb(-1) of pp collisions at root s = 13 TeV collected by the ATLAS experiment at the LHC. The final state of interest is a disappearing track accompanied by at least one jet with high transverse momentum from initial-state radiation or by four jets from the gluino decay chain. The use of short track segments reconstructed from the innermost tracking layers significantly improves the sensitivity to short chargino lifetimes. The results are found to be consistent with Standard Model predictions. Exclusion limits are set at 95% confidence level on the mass of charginos and gluinos for different chargino lifetimes. For a pure wino with a lifetime of about 0.2 ns, chargino masses up to 460 GeV are excluded. For the strong production channel, gluino masses up to 1.65 TeV are excluded assuming a chargino mass of 460 GeV and lifetime of 0.2 ns.
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ATLAS Collaboration(Aad, G. et al), Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cardillo, F., Castillo Gimenez, V., et al. (2022). Search for Higgs boson decays into a pair of pseudoscalar particles in the bb μμfinal state with the ATLAS detector in pp collisions at root s=13 TeV. Phys. Rev. D, 105(1), 012006–29pp.
Abstract: This paper presents a search for decays of the Higgs boson with a mass of 125 GeV into a pair of new pseudoscalar particles, H -> aa, where one a-boson decays into a b-quark pair and the other into a muon pair. The search uses 139 fb(-1) of proton-proton collision data at a center-of-mass energy root s = 13 TeV recorded between 2015 and 2018 by the ATLAS experiment at the LHC. A narrow dimuon resonance is searched for in the invariant mass spectrum between 16 GeV and 62 GeV. The largest excess of events above the Standard Model backgrounds is observed at a dimuon invariant mass of 52 GeV and corresponds to a local (global) significance of 3.3 sigma (1.7 sigma). Upper limits at 95% confidence level are placed on the branching ratio of the Higgs boson to the bb μμfinal state, beta(H -> aa -> bb μmu), and are in the range 0.2-4.0 x 10(-4), depending on the signal mass hypothesis.
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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 dark matter produced in association with a single top quark and an energetic W boson in √s=13 TeV pp collisions with the ATLAS detector. Eur. Phys. J. C, 83(7), 603–38pp.
Abstract: This paper presents a search for dark matter, chi, using events with a single top quark and an energetic W boson. The analysis is based on proton-proton collision data collected with the ATLAS experiment at root s = 13 TeV during LHC Run 2 (2015-2018), corresponding to an integrated luminosity of 139 fb(-1). The search considers final states with zero or one charged lepton (electron or muon), at least one b-jet and large missing transverse momentum. In addition, a result from a previous search considering two-charged-lepton final states is included in the interpretation of the results. The data are found to be in good agreement with the Standard Model predictions and the results are interpreted in terms of 95% confidence-level exclusion limits in the context of a class of dark matter models involving an extended two-Higgs-doublet sector together with a pseudoscalar mediator particle. The search is particularly sensitive to on-shell production of the charged Higgs boson state, H-+/-, arising from the two-Higgs-doublet mixing, and its semi-invisible decays via the mediator particle, a: H-+/- -> W(+/-)a(-> chi chi). Signal models with H-+/- masses up to 1.5 TeV and a masses up to 350 GeV are excluded assuming a tan beta value of 1. For masses of a of 150 (250) GeV, tan beta values up to 2 are excluded for H-+/- masses between 200 (400) GeV and 1.5 TeV. Signals with tan beta values between 20 and 30 are excluded for H-+/- masses between 500 and 800 GeV.
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