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 the Dimuon Decay of the Higgs Boson in pp Collisions at root s=13 TeV with the ATLAS Detector. Phys. Rev. Lett., 119(5), 051802–20pp.
Abstract: A search for the dimuon decay of the Higgs boson was performed using data corresponding to an integrated luminosity of 36.1 fb(-1) collected with the ATLAS detector in pp collisions at root s = 13 TeV at the Large Hadron Collider. No significant excess is observed above the expected background. The observed (expected) upper limit on the cross section times branching ratio is 3.0 (3.1) times the Standard Model prediction at the 95% confidence level for a Higgs boson mass of 125 GeV. When combined with the pp collision data at root s = 7 TeV and root s = 8 TeV, the observed (expected) upper limit is 2.8 (2.9) times the Standard Model prediction.
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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 the direct production of charginos and neutralinos in final states with tau leptons in root s=13 TeV pp collisions with the ATLAS detector. Eur. Phys. J. C, 78(2), 154–33pp.
Abstract: A search for the direct production of charginos and neutralinos in final states with at least two hadronically decaying tau leptons is presented. The analysis uses a dataset of pp collisions corresponding to an integrated luminosity of 36.1 fb, recorded with the ATLAS detector at the Large Hadron Collider at a centre-of-mass energy of 13 TeV. No significant deviation from the expected Standard Model background is observed. Limits are derived in scenarios of pair production and of and production in simplified models where the neutralinos and charginos decay solely via intermediate left-handed staus and tau sneutrinos, and the mass of the state is set to be halfway between the masses of the and the (chi) over tilde (0.)(1) . Chargino masses up to 630 GeV are excluded at 95% confidence level in the scenario of direct production of for a massless (chi) over tilde (0.)(1). Common and masses up to 760 GeV are excluded in the case of production of and assuming a massless . Exclusion limits for additional benchmark scenarios with large and small mass-splitting between the and the are also studied by varying the mass between the masses of the and the (chi) over tilde (0.)(1)
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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 electroweak production of supersymmetric particles in final states with two or three leptons at root s=13 Tev with the ATLAS detector. Eur. Phys. J. C, 78(12), 995–36pp.
Abstract: A search for the electroweak production of charginos, neutralinos and sleptons decaying into final states involving two or three electrons or muons is presented. The analysis is based on 36.1 fb(-1) of root s = 13 TeV protonproton collisions recorded by the ATLAS detector at the Large Hadron Collider. Several scenarios based on simplified models are considered. These include the associated production of the next-to-lightest neutralino and the lightest chargino, followed by their decays into final states with leptons and the lightest neutralino via either sleptons or Standard Model gauge bosons; direct production of chargino pairs, which in turn decay into leptons and the lightest neutralino via intermediate sleptons; and slepton pair production, where each slepton decays directly into the lightest neutralino and a lepton. No significant deviations from the Standard Model expectation are observed and stringent limits at 95% confidence level are placed on the masses of relevant supersymmetric particles in each of these scenarios. For a massless lightest neutralino, masses up to 580 GeV are excluded for the associated production of the next-to-lightest neutralino and the lightest chargino, assuming gauge-boson mediated decays, whereas for slepton-pair production masses up to 500 GeV are excluded assuming three generations of mass-degenerate sleptons.
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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 exclusive Higgs and Z boson decays to phi gamma and rho gamma with the ATLAS detector. J. High Energy Phys., 07(7), 127–37pp.
Abstract: A search for the exclusive decays of the Higgs and Z bosons to a phi or rho meson and a photon is performed with a pp collision data sample corresponding to an integrated luminosity of up to 35.6 fb(-1) collected at root s = 13 TeV with the ATLAS detector at the CERN Large Hadron Collider. These decays have been suggested as a probe of the Higgs boson couplings to light quarks. No significant excess of events is observed above the background, as expected from the Standard Model. Upper limits at 95% confidence level were obtained on the branching fractions of the Higgs boson decays to phi gamma and rho gamma of 4.8 x 10(-4) and 8.8 x 10(-4), respectively. The corresponding 95% confidence level upper limits for the Z boson decays are 0.9 x 10(-6) and 25 x 10(-6) for phi gamma and rho gamma, respectively.
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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. (2016). Search for flavour-changing neutral current top-quark decays to q Z in pp collision data collected with the ATLAS detector at root s=8 TeV. Eur. Phys. J. C, 76(1), 12–24pp.
Abstract: A search for the flavour-changing neutral-current decay is presented. Data collected by the ATLAS detector during 2012 from proton-proton collisions at the Large Hadron Collider at a centre-of-mass energy of root s = 8 TeV, corresponding to an integrated luminosity of 20.3 fb(-1), are analysed. Top-quark pair-production events with one top quark decaying through the t -> qZ (q = u,c) channel and the other through the dominant Standard Model mode t -> bW are considered as signal. Only the decays of the Z boson to charged leptons and leptonic W boson decays are used. No evidence for a signal is found and an observed (expected) upper limit on the t -> qZ branching ratio of 7 x 10(-4) (8 x 10(-4)) is set at the 95 % confidence level.
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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 a Structure in the B-s(0) pi(+/-) Invariant Mass Spectrum with the ATLAS Experiment. Phys. Rev. Lett., 120(20), 202007–19pp.
Abstract: A search for the narrow structure, X(5568), reported by the DO Collaboration in the decay sequence X -> B-s(0) pi +/-, B-s(0) -> J/psi phi, is presented. The analysis is based on a data sample recorded with the ATLAS detector at the LHC corresponding to 4.9 fb(-1) of pp collisions at 7 TeV and 19.5 fb(-1)at 8 TeV. No significant signal was found. Upper limits on the number of signal events, with properties corresponding to those reported by DO, and on the A production rate relative to B-s(0) mesons, rho x, were determined at 95% confidence level. The results are N(X) < 382 and rho x <0.015 for B-s(0) mesons with transverse momenta above 10 GeV and N(X) < 356 and rho(x) < 0.016 for transverse momenta above 15 GeV. Limits are also set for potential B-s(0) pi(+) resonances in the mass range 5550 to 5700 MeV.
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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). Search for type-III seesaw heavy leptons in pp collisions at root s=8 TeV with the ATLAS detector. Phys. Rev. D, 92(3), 032001–20pp.
Abstract: A search for the pair production of heavy leptons (N-0, L-+/-) predicted by the type-III seesaw theory formulated to explain the origin of small neutrino masses is presented. The decay channels N-0 -> W(+/-)l(+/-) (l = e, mu, tau) and L-+/- -> W(+/-)v (v = v(e), v(mu), v(tau)) are considered. The analysis is performed using the final state that contains two leptons (electrons or muons), two jets from a hadronically decaying W boson and large missing transverse momentum. The data used in the measurement correspond to an integrated luminosity of 20.3 fb(-1) of pp collisions at root s = 8 TeV collected by the ATLAS detector at the LHC. No evidence of heavy lepton pair production is observed. Heavy leptons with masses below 325-540 GeV are excluded at the 95% confidence level, depending on the theoretical scenario considered.
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ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2016). A search for top squarks with R-parity-violating decays to all-hadronic final states with the ATLAS detector in root s=8 TeV proton-proton collisions. J. High Energy Phys., 06(6), 067–49pp.
Abstract: A search for the pair production of top squarks, each with R-parity-violating decays into two Standard Model quarks, is performed using 17.4 fb(-1) of root s = 8 TeV proton-proton collision data recorded by the ATLAS experiment at the LITC. Each top squark is assumed to decay to a b- and an 8-quark, leading to four quarks in the final state. Background discrimination is achieved with the use of b-tagging and selections on the mass and substructure of large-radius jets, providing sensitivity to top squark masses as low as 100 GeV. No evidence of an excess beyond the Standard Model background prediction is observed and top squalls decaying to bs are excluded for top squark masses in the range 100 <= m((t) over tilde) <= 315 GeV at 95% confidence level.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fernandez Martinez, P., Ferrer, A., et al. (2015). Search for invisible particles produced in association with single-top-quarks in proton-proton collisions at root s=8 TeV with the ATLAS detector. Eur. Phys. J. C, 75(2), 79–24pp.
Abstract: A search for the production of single-top-quarks in association with missing energy is performed in proton-proton collisions at a centre-of-mass energy of root s = 8 TeV with the ATLAS experiment at the large hadron collider using data collected in 2012, corresponding to an integrated luminosity of 20.3 fb(-1). In this search, the W boson from the top quark is required to decay into an electron or a muon and a neutrino. No deviation from the standard model prediction is observed, and upper limits are set on the production cross-section for resonant and non-resonant production of an invisible exotic state in association with a right-handed top quark. In the case of resonant production, for a spin-0 resonance with amass of 500 GeV, an effective coupling strength above 0.15 is excluded at 95% confidence level for the top quark and an invisible spin-1/2 state with mass between 0 and 100 GeV. In the case of non-resonant production, an effective coupling strength above 0.2 is excluded at 95% confidence level for the top quark and an invisible spin-1 state with mass between 0 and 657 GeV.
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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). Search for the Standard Model Higgs boson produced in association with top quarks and decaying into in collisions at with the ATLAS detector. Eur. Phys. J. C, 75(7), 349–50pp.
Abstract: A search for the Standard Model Higgs boson produced in association with a top-quark pair, , is presented. The analysis uses 20.3 fb(-1) of pp collision data at , collected with the ATLAS detector at the Large Hadron Collider during 2012. The search is designed for the decay mode and uses events containing one or two electrons or muons. In order to improve the sensitivity of the search, events are categorised according to their jet and b-tagged jet multiplicities. A neural network is used to discriminate between signal and background events, the latter being dominated by +jets production. In the single-lepton channel, variables calculated using a matrix element method are included as inputs to the neural network to improve discrimination of the irreducible background. No significant excess of events above the background expectation is found and an observed (expected) limit of 3.4 (2.2) times the Standard Model cross section is obtained at 95 % confidence level. The ratio of the measured signal cross section to the Standard Model expectation is found to be assuming a Higgs boson mass of 125 Gev.
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