ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fernandez Martinez, P., et al. (2015). A search for t(t)over-bar resonances using lepton-plus-jets events in proton-proton collisions at root s=8 TeV with the ATLAS detector. J. High Energy Phys., 08(8), 148–54pp.
Abstract: A search for new particles that decay into top quark pairs is reported. The search is performed with the ATLAS experiment at the LHC using an integrated luminosity of 20.3 fb(-1) of proton-proton collision data collected at a centre-of-mass energy of root s = 8TeV. The lepton-plus-jets final state is used, where the top pair decays to W (+) bW(-)(b) over bar, with one W boson decaying leptonically and the other hadronically. The invariant mass spectrum of top quark pairs is examined for local excesses or deficits that are inconsistent with the Standard Model predictions. No evidence for a top quark pair resonance is found, and 95% confidence-level limits on the production rate are determined for massive states in benchmark models. The upper limits on the cross-section times branching ratio of a narrow Z' boson decaying to top pairs range from 4.2 pb to 0.03 pb for resonance masses from 0.4 TeV to 3.0 TeV. A narrow leptophobic topcolour Z' boson with mass below 1.8 TeV is excluded. Upper limits are set on the cross-section times branching ratio for a broad colour-octet resonance with Gamma/m = 15% decaying to tt. These range from 4.8 pb to 0.03 pb for masses from 0.4 TeV to 3.0 TeV. A Kaluza-Klein excitation of the gluon in a Randall-Sundrum model is excluded for masses below 2.2 TeV.
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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). Study of (W/Z)H production and Higgs boson couplings using H -> WW* decays with the ATLAS detector. J. High Energy Phys., 08(8), 137–65pp.
Abstract: A search for Higgs boson production in association with a W or Z boson, in the H -> WW* decay channel, is performed with a data sample collected with the ATLAS detector at the LHC in proton-proton collisions at centre-of-mass energies root s = 7 TeV and 8TeV, corresponding to integrated luminosities of 4.5 fb(-1) and 20.3 fb(-1), respectively. The W H production mode is studied in two-lepton and three-lepton final states, while twolepton and four-lepton final states are used to search for the ZH production mode. The observed significance, for the combined WH and ZH production, is 2.5 standard deviations while a significance of 0.9 standard deviations is expected in the Standard Model Higgs boson hypothesis. The ratio of the combined W H and Z H signal yield to the Standard Model expectation, mu(VH), is found to be mu(VH) = 3.0(-1.1)(+1.3)(stat.)(-0.7)(+1.0) (sys.) for the Higgs boson mass of 125.36 GeV. The WH and ZH production modes are also combined with the gluon fusion and vector boson fusion production modes studied in the H -> WW* -> l nu l nu decay channel, resulting in an overall observed significance of 6.5 standard deviations and μF-gg+VBF+VH = 1.16(-0.15)(+0.16)(stat.)(-0.15)(+0.18)(sys.). The results are interpreted in terms of scaling factors of the Higgs boson couplings to vector bosons (kappa(V)) and fermions (kappa(F)); the combined results are: vertical bar kappa(V)vertical bar = 1.06(-0.10)(+0.10), vertical bar kappa(F)vertical bar = 0.85(-0.20)(+0.26)
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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). Measurement of the inclusive isolated prompt photon cross section in pp collisions at root s=8 TeV with the ATLAS detector. J. High Energy Phys., 08(8), 005–42pp.
Abstract: A measurement of the cross section for the inclusive production of isolated prompt photons in proton-proton collisions at a centre-of-mass energy of root s = 8 TeV is presented. The measurement covers the pseudorapidity ranges vertical bar eta(gamma)vertical bar < 1.37 and 1.56 aecurrency sign vertical bar eta(gamma)vertical bar < 2.37 in the transverse energy range 25 < E-T(gamma) < 1500 GeV. The results are based on an integrated luminosity of 20.2 fb(-1), recorded by the ATLAS detector at the LHC. Photon candidates are identified by combining information from the calorimeters and the inner tracker. The background is subtracted using a data-driven technique, based on the observed calorimeter shower-shape variables and the deposition of hadronic energy in a narrow cone around the photon candidate. The measured cross sections are compared with leading-order and next-to-leading order perturbative QCD calculations and are found to be in a good agreement over ten orders of magnitude.
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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). Measurement of the angular coefficients in Z-boson events using electron and muon pairs from data taken at root s=8 TeV with the ATLAS detector. J. High Energy Phys., 08(8), 159–101pp.
Abstract: The angular distributions of Drell-Yan charged lepton pairs in the vicinity of the Z-boson mass peak probe the underlying QCD dynamics of Z-boson production. This paper presents a measurement of the complete set of angular coefficients Lambda(0-7) describing these distributions in the Z-boson Collins-Soper frame. The data analysed correspond to 20.3 fb(-1) of pp collisions at root s = 8TeV, collected by the ATLAS detector at the CERN LHC. The measurements are compared to the most precise fixed-order calculations currently available (O (alpha(2)(s))) and with theoretical predictions embedded in Monte Carlo generators. The measurements are precise enough to probe QCD corrections beyond the formal accuracy of these calculations and to provide discrimination between different parton-shower models. A signi fi cant deviation from the O (alpha(2)(s)) predictions is observed for A(0)-A(2). Evidence is found for non-zero A(5,6, 7), consistent with expectations.
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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). Measurement of fiducial differential cross sections of gluon-fusion production of Higgs bosons decaying to WW (au)-> e nu μnu with the ATLAS detector at TeV. J. High Energy Phys., 08(8), 104–63pp.
Abstract: This paper describes a measurement of fiducial and differential cross sections of gluon-fusion Higgs boson production in the H -> W W (au)-> e nu μnu channel, using 20.3 fb(-1) of proton-proton collision data. The data were produced at a centre-of-mass energy of TeV at the CERN Large Hadron Collider and recorded by the ATLAS detector in 2012. Cross sections are measured from the observed H -> W W (au)-> e nu μnu signal yield in categories distinguished by the number of associated jets. The total cross section is measured in a fiducial region defined by the kinematic properties of the charged leptons and neutrinos. Differential cross sections are reported as a function of the number of jets, the Higgs boson transverse momentum, the dilepton rapidity, and the transverse momentum of the leading jet. The jet-veto efficiency, or fraction of events with no jets above a given transverse momentum threshold, is also reported. All measurements are compared to QCD predictions from Monte Carlo generators and fixed-order calculations, and are in agreement with the Standard Model predictions.
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