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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 Invisible Decays of a Higgs Boson Produced in Association with a Z Boson in ATLAS. Phys. Rev. Lett., 112(20), 201802–19pp.
Abstract: A search for evidence of invisible-particle decay modes of a Higgs boson produced in association with a Z boson at the Large Hadron Collider is presented. No deviation from the standard model expectation is observed in 4.5 fb(-1) (20.3 fb(-1)) of 7 (8) TeV pp collision data collected by the ATLAS experiment. Assuming the standard model rate for ZH production, an upper limit of 75%, at the 95% confidence level is set on the branching ratio to invisible-particle decay modes of the Higgs boson at a mass of 125.5 GeV. The limit on the branching ratio is also interpreted in terms of an upper limit on the allowed dark matter-nucleon scattering cross section within a Higgs-portal dark matter scenario. Within the constraints of such a scenario, the results presented in this Letter provide the strongest available limits for low-mass dark matter candidates. Limits are also set on an additional neutral Higgs boson, in the mass range 110 < m(H) < 400 GeV, produced in association with a Z boson and decaying to invisible particles.
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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). Evidence for Electroweak Production of W(+/-) W(+/-) jj in pp Collisions at root s=8 TeV with the ATLAS Detector. Phys. Rev. Lett., 113(14), 141803–19pp.
Abstract: This Letter presents the first study of W(+/-)W(+/-)jj, same-electric-charge diboson production in association with two jets, using 20.3 fb(-1) of proton-proton collision data at root s = 8 TeV recorded by the ATLAS detector at the Large Hadron Collider. Events with two reconstructed same-charge leptons (e(+/-)e(+/-), e(+/-)mu(+/-), and mu(+/-)mu(+/-)) and two or more jets are analyzed. Production cross sections are measured in two fiducial regions, with different sensitivities to the electroweak and strong production mechanisms. First evidence for W(+/-)W(+/-)jj production and electroweak-only W(+/-)W(+/-)jj production is observed with a significance of 4.5 and 3.6 standard deviations, respectively. The measured production cross sections are in agreement with standard model predictions. Limits at 95% confidence level are set on anomalous quartic gauge couplings.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., Fiorini, L., et al. (2014). Observation of an Excited B-c(+/-) Meson State with the ATLAS Detector. Phys. Rev. Lett., 113(21), 212004–18pp.
Abstract: A search for excited states of the B-c(+/-) meson is performed using 4.9 fb(-1) of 7 TeV and 19.2 fb(-1) of 8 TeV pp collision data collected by the ATLAS experiment at the LHC. A new state is observed through its hadronic transition to the ground state, with the latter detected in the decayB(c)(+/-) -> J/psi pi(+/-). The state appears in the m(B-c(+/-)pi(+)pi(-)) – m(B-c(+/-)) – 2m(pi(+/-)) mass difference distribution with a significance of 5.2 standard deviations. The mass of the observed state is 6842 +/- 4 +/- 5 MeV, where the first error is statistical and the second is systematic. The mass and decay of this state are consistent with expectations for the second S-wave state of the B-c(+/-) meson, B-c(+/-)(2S).
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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. (2015). Search for Higgs Boson Pair Production in the gamma gamma b(b)over-bar Final State Using pp Collision Data at root s=8 TeV from the ATLAS Detector. Phys. Rev. Lett., 114(8), 081802–19pp.
Abstract: Searches are performed for resonant and nonresonant Higgs boson pair production in the gamma gamma b (b) over bar final state using 20 fb(-1) of proton-proton collisions at a center-of-mass energy of 8 TeV recorded with the ATLAS detector at the CERN Large Hadron Collider. A 95% confidence level upper limit on the cross section times branching ratio of nonresonant production is set at 2.2 pb, while the expected limit is 1.0 pb. The difference derives from a modest excess of events, corresponding to 2.4 standard deviations from the background-only hypothesis. The limit observed in the search for a narrow X -> hh resonance ranges between 0.7 and 3.5 pb as a function of the resonance mass.
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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. (2012). Further search for supersymmetry at root s=7 TeV in final states with jets, missing transverse momentum, and isolated leptons with the ATLAS detector. Phys. Rev. D, 86(9), 092002–35pp.
Abstract: This work presents a new inclusive search for supersymmetry (SUSY) by the ATLAS experiment at the LHC in proton-proton collisions at a center-of-mass energy root s = 7 TeV in final states with jets, missing transverse momentum and one or more isolated electrons and/or muons. The search is based on data from the full 2011 data-taking period, corresponding to an integrated luminosity of 4.7 fb(-1). Single-lepton and multilepton channels are treated together in one analysis. An increase in sensitivity is obtained by simultaneously fitting the number of events in statistically independent signal regions, and the shapes of distributions within those regions. A dedicated signal region is introduced to be sensitive to decay cascades of SUSY particles with small mass differences (“compressed SUSY”). Background uncertainties are constrained by fitting to the jet-multiplicity distribution in background control regions. Observations are consistent with Standard Model expectations, and limits are set or extended on a number of SUSY models.
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