ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fassi, F., Ferrer, A., et al. (2014). Search for dark matter in events with a Z boson and missing transverse momentum in pp collisions at root s=8 TeV with the ATLAS detector. Phys. Rev. D, 90(1), 012004–21pp.
Abstract: A search is presented for production of dark-matter particles recoiling against a leptonically decaying Z boson in 20.3 fb(-1) of pp collisions at root s = 8 TeV with the ATLAS detector at the Large Hadron Collider. Events with large missing transverse momentum and two oppositely charged electrons or muons consistent with the decay of a Z boson are analyzed. No excess above the Standard Model prediction is observed. Limits are set on the mass scale of the contact interaction as a function of the dark-matter particle mass using an effective field theory description of the interaction of dark matter with quarks or with Z bosons. Limits are also set on the coupling and mediator mass of a model in which the interaction is mediated by a scalar particle.
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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 supersymmetry in events with four or more leptons in root s=8 TeV pp collisions with the ATLAS detector. Phys. Rev. D, 90(5), 052001–33pp.
Abstract: Results from a search for supersymmetry in events with four or more leptons including electrons, muons and taus are presented. The analysis uses a data sample corresponding to 20.3 fb(-1) of proton proton collisions delivered by the Large Hadron Collider at root s = 8 TeV and recorded by the ATLAS detector. Signal regions are designed to target supersymmetric scenarios that can be either enriched in or depleted of events involving the production of a Z boson. No significant deviations are observed in data from standard model predictions and results are used to set upper limits on the event yields from processes beyond the standard model. Exclusion limits at the 95% confidence level on the masses of relevant supersymmetric particles are obtained. In R-parity-violating simplified models with decays of the lightest supersymmetric particle to electrons and muons, limits of 1350 and 750 GeV are placed on gluino and chargino masses, respectively. In R-parity-conserving simplified models with heavy neutralinos decaying to a massless lightest supersymmetric particle, heavy neutralino masses up to 620 GeV are excluded. Limits are also placed on other supersymmetric scenarios.
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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). Measurement of event-plane correlations in root s(NN)=2.76 TeV lead-lead collisions with the ATLAS detector. Phys. Rev. C, 90(2), 024905–29pp.
Abstract: A measurement of event-plane correlations involving two or three event planes of different order is presented as a function of centrality for 7 μb(-1) Pb + Pb collision data at v root s(NN) = 2.76 TeV, recorded by the ATLAS experiment at the Large Hadron Collider. Fourteen correlators are measured using a standard event-plane method and a scalar-product method, and the latter method is found to give a systematically larger correlation signal. Several different trends in the centrality dependence of these correlators are observed. These trends are not reproduced by predictions based on the Glauber model, which includes only the correlations from the collision geometry in the initial state. Calculations that include the final-state collective dynamics are able to describe qualitatively, and in some cases also quantitatively, the centrality dependence of the measured correlators. These observations suggest that both the fluctuations in the initial geometry and the nonlinear mixing between different harmonics in the final state are important for creating these correlations in momentum space.
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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). Flavor tagged time-dependent angular analysis of the B-s(0) -> J/psi phi decay and extraction of Delta Gamma(s) and the weak phase phi(s) in ATLAS. Phys. Rev. D, 90(5), 052007–26pp.
Abstract: A measurement of the B-s(0) -> J/psi phi decay parameters, updated to include flavor tagging is reported using 4.9 fb(-1) of integrated luminosity collected by the ATLAS detector from root s = 7 TeV pp collisions recorded in 2011 at the LHC. The values measured for the physical parameters are phi(s) = 0.12 +/- 0.25(stat) +/- 0.05(syst) rad Delta Gamma(s) = 0.053 +/- 0.021(stat) +/- 0.010(syst) ps(-1) Gamma(s) = 0.677 +/- 0.007(stat) +/- 0.004(syst) ps(-1) vertical bar A(parallel to)(0)vertical bar(2) = 0.220 +/- 0.008(stat) +/- 0.009(syst) vertical bar A(0)(0)vertical bar(2) = 0.529 +/- 0.006(stat) +/- 0.012(syst) delta(perpendicular to) = 3.89 +/- 0.47(stat) +/- 0.11(syst) rad where the parameter Delta Gamma(s) is constrained to be positive. The S-wave contribution was measured and found to be compatible with zero. Results for phi(s) and Delta Gamma(s) are also presented as 68% and 95% likelihood contours, which show agreement with the Standard Model expectations.
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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 high-mass dilepton resonances in pp collisions at root s = 8 TeV with the ATLAS detector. Phys. Rev. D, 90(5), 052005–30pp.
Abstract: The ATLAS detector at the Large Hadron Collider is used to search for high-mass resonances decaying to dielectron or dimuon final states. Results are presented from an analysis of proton-proton (pp) collisions at a center-of-mass energy of 8 TeV corresponding to an integrated luminosity of 20.3 fb(-1) in the dimuon channel. A narrow resonance with Standard Model Z couplings to fermions is excluded at 95% confidence level for masses less than 2.79 TeV in the dielectron channel, 2.53 TeV in the dimuon channel, and 2.90 TeV in the two channels combined. Limits on other model interpretations are also presented, including a grand-unification model based on the E-6 gauge group, Z* bosons, minimal Z' models, a spin-2 graviton excitation from Randall-Sundrum models, quantum black holes, and a minimal walking technicolor model with a composite Higgs boson.
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