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 massive, long-lived particles using multitrack displaced vertices or displaced lepton pairs in pp collisions at root s=8 TeV with the ATLAS detector. Phys. Rev. D, 92(7), 072004–37pp.
Abstract: Many extensions of the Standard Model posit the existence of heavy particles with long lifetimes. This article presents the results of a search for events containing at least one long-lived particle that decays at a significant distance from its production point into two leptons or into five or more charged particles. This analysis uses a data sample of proton-proton collisions at root s = 8 TeV corresponding to an integrated luminosity of 20.3 fb(-1) collected in 2012 by the ATLAS detector operating at the Large Hadron Collider. No events are observed in any of the signal regions, and limits are set on model parameters within supersymmetric scenarios involving R-parity violation, split supersymmetry, and gauge mediation. In some of the search channels, the trigger and search strategy are based only on the decay products of individual long-lived particles, irrespective of the rest of the event. In these cases, the provided limits can easily be reinterpreted in different scenarios.
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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). Measurements of the top quark branching ratios into channels with leptons and quarks with the ATLAS detector. Phys. Rev. D, 92(7), 072005–31pp.
Abstract: Measurements of the branching ratios of top quark decays into leptons and jets using events with t (t) over bar ( top antitop) pairs are reported. Events were recorded with the ATLAS detector at the LHC in pp collisions at a center-of-mass energy of 7 TeV. The collected data sample corresponds to an integrated luminosity of 4.6 fb(-1). The measured top quark branching ratios agree with the Standard Model predictions within the measurement uncertainties of a few percent.
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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 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), Alvarez Piqueras, D., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fernandez Martinez, P., et al. (2015). Z boson production in p plus Pb collisions at root S-NN=5.02 TeV measured with the ATLAS detector. Phys. Rev. C, 92(4), 044915–22pp.
Abstract: The ATLAS Collaboration measures the inclusive production of Z bosons via their decays into electron and muon pairs in p + Pb collisions at root S-NN = 5.02 TeV at the Large Hadron Collider. The measurements are made using data corresponding to integrated luminosities of 29.4 and 28.1 nb(-1) for Z -> ee and Z -> μmu, respectively. The results from the two channels are consistent and combined to obtain a cross section times the Z -> ll branching ratio, integrated over the rapidity region vertical bar y(Z)*vertical bar < 3.5, of 139.8 +/- 4.8 (statistical) +/- 6.2 (systematic) +/- 3.8 (luminosity) nb. Differential cross sections are presented as functions of the Z boson rapidity and transverse momentum and compared with models based on parton distributions both with and without nuclear corrections. The centrality dependence of Z boson production in p + Pb collisions is measured and analyzed within the framework of a standard Glauber model and the model's extension for fluctuations of the underlying nucleon-nucleon scattering cross section.
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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). Searches for Higgs boson pair production in the hh -> bb tau tau, gamma gamma WW*, gamma gamma bb, bbbb channels with the ATLAS detector. Phys. Rev. D, 92(9), 092004–30pp.
Abstract: Searches for both resonant and nonresonant Higgs boson pair production are performed in the hh -> bb tau tau, gamma gamma WW* final states using 20.3 fb(-1) of pp collision data at a center-of-mass energy of 8 TeV recorded with the ATLAS detector at the Large Hadron Collider. No evidence of their production is observed and 95% confidence-level upper limits on the production cross sections are set. These results are then combined with the published results of the hh -> gamma gamma bb, bbbb analyses. An upper limit of 0.69 (0.47) pb on the nonresonant hh production is observed (expected), corresponding to 70 (48) times the SM gg -> hh cross section. For production via narrow resonances, cross-section limits of hh production from a heavy Higgs boson decay are set as a function of the heavy Higgs boson mass. The observed (expected) limits range from 2.1 (1.1) pb at 260 GeV to 0.011 (0.018) pb at 1000 GeV. These results are interpreted in the context of two simplified scenarios of the Minimal Supersymmetric Standard Model.
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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 pair production of a new heavy quark that decays into a W boson and a light quark in pp collisions at root s=8 TeV with the ATLAS detector. Phys. Rev. D, 92(11), 112007–28pp.
Abstract: A search is presented for pair production of a new heavy quark (Q) that decays into aW boson and a light quark (q) in the final state where one W boson decays leptonically (to an electron or muon plus a neutrino) and the other W boson decays hadronically. The analysis is performed using 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 Q (Q) over bar production is observed. New chiral quarks with masses below 690 GeVare excluded at 95% confidence level, assuming BR(Q -> Wq) = 1. Results are also interpreted in the context of vectorlike quark models, resulting in the limits on the mass of a vectorlike quark in the two-dimensional plane of BR(Q -> Wq) versus BR(Q -> Hq).
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ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., Costa, M. J., et al. (2016). Measurement of the correlation between the polar angles of leptons from top quark decays in the helicity basis at root s=7 TeV using the ATLAS detector. Phys. Rev. D, 93(1), 012002–23pp.
Abstract: A measurement of the correlations between the polar angles of leptons from the decay of pair-produced t and (t) over bar quarks in the helicity basis is reported, using proton-proton collision data collected by the ATLAS detector at the LHC. The dataset corresponds to an integrated luminosity of 4.6 fb(-1) at a center-of-mass energy of root s = 7 TeV collected during 2011. Candidate events are selected in the dilepton topology with large missing transverse momentum and at least two jets. The angles theta(1) and theta(2) between the charged leptons and the direction of motion of the parent quarks in the t (t) over bar rest frame are sensitive to the spin information, and the distribution of cos theta(1).cos theta(2) is sensitive to the spin correlation between the t and (t) over bar quarks. The distribution is unfolded to parton level and compared to the next-to-leading order prediction. A good agreement is observed.
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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 the electroweak production of supersymmetric particles in root s=8 TeV pp collisions with the ATLAS detector. Phys. Rev. D, 93(5), 052002–50pp.
Abstract: The ATLAS experiment has performed extensive searches for the electroweak production of charginos, neutralinos, and staus. This article summarizes and extends the search for electroweak supersymmetry with new analyses targeting scenarios not covered by previously published searches. New searches use vector-boson fusion production, initial-state radiation jets, and low-momentum lepton final states, as well as multivariate analysis techniques to improve the sensitivity to scenarios with small mass splittings and low-production cross sections. Results are based on 20 fb(-1) of proton-proton collision data at root s = 8 TeV recorded with the ATLAS experiment at the Large Hadron Collider. No significant excess beyond Standard Model expectations is observed. The new and existing searches are combined and interpreted in terms of 95% confidence-level exclusion limits in simplified models, where a single production process and decay mode is assumed, as well as within phenomenological supersymmetric models.
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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). Measurement of jet charge in dijet events from root s=8 TeV pp collisions with the ATLAS detector. Phys. Rev. D, 93(5), 052003–35pp.
Abstract: The momentum-weighted sum of the charges of tracks associated to a jet is sensitive to the charge of the initiating quark or gluon. This paper presents a measurement of the distribution of momentum-weighted sums, called jet charge, in dijet events using 20.3 fb(-1) of data recorded with the ATLAS detector at root s = 8 TeV in pp collisions at the LHC. The jet charge distribution is unfolded to remove distortions from detector effects and the resulting particle-level distribution is compared with several models. The p(T) dependence of the jet charge distribution average and standard deviation are compared to predictions obtained with several leading-order and next-to-leading-order parton distribution functions. The data are also compared to different Monte Carlo simulations of QCD dijet production using various settings of the free parameters within these models. The chosen value of the strong coupling constant used to calculate gluon radiation is found to have a significant impact on the predicted jet charge. There is evidence for a pT dependence of the jet charge distribution for a given jet flavor. In agreement with perturbative QCD predictions, the data show that the average jet charge of quark-initiated jets decreases in magnitude as the energy of the jet increases.
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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. (2016). Centrality, rapidity, and transverse momentum dependence of isolated prompt photon production in lead-lead collisions at root S-NN=2.76 TeV measured with the ATLAS detector. Phys. Rev. C, 93(3), 034914–28pp.
Abstract: Prompt photon production in root S-NN = 2.76-TeV Pb + Pb collisions has been measured by the ATLAS experiment at the Large Hadron Collider using data collected in 2011 with an integrated luminosity of 0.14 nb(-1). Inclusive photon yields, scaled by the mean nuclear thickness function, are presented as a function of collision centrality and transverse momentum in two pseudorapidity intervals, vertical bar eta vertical bar < 1.37 and 1.52 <= vertical bar eta vertical bar < 2.37. The scaled yields in the two pseudorapidity intervals, as well as the ratios of the forward yields to those at midrapidity, are compared to the expectations from next-to-leading-order perturbative QCD (pQCD) calculations. The measured cross sections agree well with the predictions for proton-proton collisions within statistical and systematic uncertainties. Both the yields and the ratios are also compared to two other pQCD calculations, one which uses the isospin content appropriate to colliding lead nuclei and another which includes nuclear modifications to the nucleon parton distribution functions.
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