ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., Fiorini, L., et al. (2015). Measurement of the top-quark mass in the fully hadronic decay channel from ATLAS data at root s=7 TeV. Eur. Phys. J. C, 75(4), 158–26pp.
Abstract: The mass of the top quark is measured in a data set corresponding to 4.6 fb(-1) of proton-proton collisions with centre-of-mass energy root s = 7 TeV collected by the ATLAS detector at the LHC. Events consistent with hadronic decays of top-antitop quark pairs with at least six jets in the final state are selected. The substantial background from multijet production is modelled with data-driven methods that utilise the number of identified b-quark jets and the transverse momentum of the sixth leading jet, which have minimal correlation. The top-quark mass is obtained from template fits to the ratio of three-jet to dijet mass. The three-jet mass is calculated from the three jets produced in a top-quark decay. Using these three jets the dijet mass is obtained from the two jets produced in the W boson decay. The top-quark mass obtained from this fit is thus less sensitive to the uncertainty in the energy measurement of the jets. A binned likelihood fit yields a top-quark mass of m(t) = 175.1 +/- 1.4 (stat.) +/- 1.2 (syst.) GeV.
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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). Measurement of long-range pseudorapidity correlations and azimuthal harmonics in root s(NN)=5.02 TeV proton-lead collisions with the ATLAS detector. Phys. Rev. C, 90(4), 044906–29pp.
Abstract: Measurements of two-particle correlation functions and the first five azimuthal harmonics, v(1) to v(5), are presented, using 28 nb(-1) of p + Pb collisions at a nucleon-nucleon center-of-mass energy of root s(NN) = 5.02 TeV measured with the ATLAS detector at the LHC. Significant long-range “ridgelike” correlations are observed for pairs with small relative azimuthal angle (|Delta phi| < pi/3) and back-to-back pairs (|Delta phi| > 2 pi/3) over the transverse momentum range 0.4 < p(T) < 12 GeV and in different intervals of event activity. The event activity is defined by either the number of reconstructed tracks or the total transverse energy on the Pb-fragmentation side. The azimuthal structure of such long-range correlations is Fourier decomposed to obtain the harmonics v(n) as a function of p(T) and event activity. The extracted v(n) values for n = 2 to 5 decrease with n. The v(2) and v(3) values are found to be positive in the measured p(T) range. The v(1) is also measured as a function of p(T) and is observed to change sign around p(T) approximate to 1.5-2.0 GeV and then increase to about 0.1 for pT > 4 GeV. The v(2)(p(T)), v(3)(p(T)), and v(4)(p(T)) are compared to the v(n) coefficients in Pb + Pb collisions at root s(NN) = 2.76 TeV with similar event multiplicities. Reasonable agreement is observed after accounting for the difference in the average p(T) of particles produced in the two collision systems.
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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. (2015). Search for H -> gamma gamma produced in association with top quarks and constraints on the Yukawa coupling between the top quark and the Higgs boson using data taken at 7 TeV and 8 TeV with the ATLAS detector. Phys. Lett. B, 740, 222–242.
Abstract: A search is performed for Higgs bosons produced in association with top quarks using the diphoton decay mode of the Higgs boson. Selection requirements are optimized separately for leptonic and fully hadronic final states from the top quark decays. The dataset used corresponds to an integrated luminosity of 4.5 fb(-1) of proton-proton collisions at a center-of-mass energy of 7 TeV and 20.3 fb(-1) at 8 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. No significant excess over the background prediction is observed and upper limits are set on the ttH production cross section. The observed exclusion upper limit at 95% confidence level is 6.7 times the predicted Standard Model cross section value. In addition, limits are set on the strength of the Yukawa coupling between the top quark and the Higgs boson, taking into account the dependence of the ttH and tH cross sections as well as the H -> gamma gamma branching fraction on the Yukawa coupling. Lower and upper limits at 95% confidence level are set at 1.3 and +8.0 times the Yukawa coupling strength in the Standard Model.
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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). Measurement of distributions sensitive to the underlying event in inclusive Z-boson production in pp collisions at root s=7 TeV with the ATLAS detector. Eur. Phys. J. C, 74(12), 3195–33pp.
Abstract: A measurement of charged-particle distributions sensitive to the properties of the underlying event is presented for an inclusive sample of events containing a Z-boson, decaying to an electron or muon pair. The measurement is based on data collected using the ATLAS detector at the LHC in proton-proton collisions at a centre-of-mass energy of 7 TeV with an integrated luminosity of 4.6fb(-1). Distributions of the charged particle multiplicity and of the charged particle transverse momentum are measured in regions of azimuthal angle defined with respect to the Z-boson direction. The measured distributions are compared to similar distributions measured in jet events, and to the predictions of various Monte Carlo generators implementing different underlying event models.
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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). Search for pair and single production of new heavy quarks that decay to a Z boson and a third-generation quark in pp collisions at root s=8 TeV with the ATLAS detector. J. High Energy Phys., 11(11), 104–54pp.
Abstract: A search is presented for the production of new heavy quarks that decay to a Z boson and a third-generation Standard Model quark. In the case of a new charge +2/3 quark (T), the decay targeted is T -> Zt, while the decay targeted for a new charge -1/3 quark (B) is B -> Zb. The search is performed with a dataset corresponding to 20.3 fb(-1) of p p collisions at root s = 8TeV recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider. Selected events contain a high transverse momentum Z boson candidate reconstructed from a pair of oppositely charged same-flavor leptons (electrons or muons), and are analyzed in two channels defined by the absence or presence of a third lepton. Hadronic jets, in particular those with properties consistent with the decay of a b-hadron, are also required to be present in selected events. Different requirements are made on the jet activity in the event in order to enhance the sensitivity to either heavy quark pair production mediated by the strong interaction, or single production mediated by the electroweak interaction. No significant excess of events above the Standard Model expectation is observed, and lower limits are derived on the mass of vector-like T and B quarks under various branching ratio hypotheses, as well as upper limits on the magnitude of electroweak coupling parameters.
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