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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 s-channel single top-quark production in proton-proton collisions at root s=8 TeV with the ATLAS detector. Phys. Lett. B, 740, 118–136.
Abstract: This Letter presents a search at the LHC for s-channel single top-quark production in proton-proton collisions at a centre-of-mass energy of 8 TeV. The analyzed data set was recorded by the ATLAS detector and corresponds to an integrated luminosity of 20.3 fb(-1). Selected events contain one charged lepton, large missing transverse momentum and exactly two b-tagged jets. A multivariate event classifier based on boosted decision trees is developed to discriminate s-channel single top-quark events from the main background contributions. The signal extraction is based on a binned maximum-likelihood fit of the output classifier distribution. The analysis leads to an upper limit on the s-channel single topquark production cross-section of 14.6 pb at the 95% confidence level. The fit gives a cross-section of sigma(s) = 5.0 +/- 4.3 pb, consistent with the Standard Model expectation.
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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 dark matter in events with heavy quarks and missing transverse momentum in pp collisions with the ATLAS detector. Eur. Phys. J. C, 75(2), 92–22pp.
Abstract: This article reports on a search for dark matter pair production in association with bottom or top quarks in 20.3 fb(-1) of pp collisions collected at root s = 8 TeV by the ATLAS detector at the LHC. Events with large missing transverse momentum are selected when produced in association with high-momentum jets of which one or more are identified as jets containing b-quarks. Final states with top quarks are selected by requiring a high jet multiplicity and in some cases a single lepton. The data are found to be consistent with the Standard Model expectations and limits are set on the mass scale of effective field theories that describe scalar and tensor interactions between dark matter and Standard Model particles. Limits on the dark-matter-nucleon cross-section for spin-independent and spin-dependent interactions are also provided. These limits are particularly strong for low-mass dark matter. Using a simplified model, constraints are set on the mass of dark matter and of a coloured mediator suitable to explain a possible signal of annihilating dark matter.
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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 W ' -> t(b)over-bar in the lepton plus jets final state in proton-proton collisions at a centre-of-mass energy of root s=8 TeV with the ATLAS detector. Phys. Lett. B, 743, 235–255.
Abstract: A search for new charged massive gauge bosons, called W', is performed with the ATLAS detector at the LHC, in proton-proton collisions at a centre-of-mass energy of root s = 8 TeV, using a dataset corresponding to an integrated luminosity of 20.3 fb(-1). This analysis searches for W' bosons in the W' -> t (b) over tilde decay channel in final states with electrons or muons, using a multivariate method based on boosted decision trees. The search covers masses between 0.5 and 3.0 TeV, for right-handed or left-handed W' bosons. No significant deviation from the Standard Model expectation is observed and limits are set on the W' -> t (b) over bar cross-section times branching ratio and on the W'-boson effective couplings as a function of the W'-boson mass using the CLs procedure. For a left-handed (right-handed) W' boson, masses below 1.70 (1.92) TeV are excluded at 95% confidence level.
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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 the X-b and other hidden-beauty states in the pi(+)pi(-)gamma(1S) channel at ATLAS. Phys. Lett. B, 740, 199–217.
Abstract: This Letter presents a search for a hidden-beauty counterpart of the X(3872) in the mass ranges of 10.05-10.31 GeV and 10.40-11.00 GeV, in the channel X-b -> pi(+)pi(-)gamma(1S)(-> mu(+)mu(-)), using 16.2 fb(-1) of root s = 8 TeV pp collision data collected by the ATLAS detector at the LHC. No evidence for new narrow states is found, and upper limits are set on the product of the Xb cross section and branching fraction, relative to those of the gamma(25), at the 95% confidence level using the CLs approach. These limits range from 0.8% to 4.0%, depending on mass. For masses above 10.1 GeV, the expected upper limits from this analysis are the most restrictive to date. Searches for production of the gamma(1(3)D(J)), gamma(10860), and gamma(11 020) states also reveal no significant signals.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fernandez Martinez, P., Ferrer, A., et al. (2015). Search for invisible particles produced in association with single-top-quarks in proton-proton collisions at root s=8 TeV with the ATLAS detector. Eur. Phys. J. C, 75(2), 79–24pp.
Abstract: A search for the production of single-top-quarks in association with missing energy is performed in proton-proton collisions at a centre-of-mass energy of root s = 8 TeV with the ATLAS experiment at the large hadron collider using data collected in 2012, corresponding to an integrated luminosity of 20.3 fb(-1). In this search, the W boson from the top quark is required to decay into an electron or a muon and a neutrino. No deviation from the standard model prediction is observed, and upper limits are set on the production cross-section for resonant and non-resonant production of an invisible exotic state in association with a right-handed top quark. In the case of resonant production, for a spin-0 resonance with amass of 500 GeV, an effective coupling strength above 0.15 is excluded at 95% confidence level for the top quark and an invisible spin-1/2 state with mass between 0 and 100 GeV. In the case of non-resonant production, an effective coupling strength above 0.2 is excluded at 95% confidence level for the top quark and an invisible spin-1 state with mass between 0 and 657 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. (2015). Measurement of the WW plus WZ cross section and limits on anomalous triple gauge couplings using final states with one lepton, missing transverse momentum, and two jets with the ATLAS detector at root s=7 TeV. J. High Energy Phys., 01(1), 049–42pp.
Abstract: The production of a W boson decaying to ev or μv in association with a W or Z boson decaying to two jets is studied using 4.6 fb(-1) of proton-proton collision data at root s = 7 TeV recorded with the ATLAS detector at the LHC. The combined WW + WZ cross section is measured with a significance of 3.4 sigma and is found to be 68 +/- 7 (stat.) +/- 19 (syst.) pb, in agreement with the Standard Model expectation of 61.1 +/- 2.2 pb. The distribution of the transverse momentum of the dijet system is used to set limits on anomalous contributions to the triple gauge coupling vertices and on parameters of an effective-field-theory 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. (2015). Measurement of the inclusive jet cross-section in proton-proton collisions at root s=7 TeV using 4.5 fb(-1) of data with the ATLAS detector. J. High Energy Phys., 02(2), 153–54pp.
Abstract: The inclusive jet cross-section is measured in proton-proton collisions at a centre-of-mass energy of 7 TeV using a data set corresponding to an integrated luminosity of 4.5 fb(-1) collected with the ATLAS detector at the Large Hadron Collider in 2011. Jets are identified using the anti-k(t) algorithm with radius parameter values of 0.4 and 0.6. The double-differential cross-sections are presented as a function of the jet transverse momentum and the jet rapidity, covering jet transverse momenta from 100 GeV to 2 TeV. Next-to-leading-order QCD calculations corrected for non-perturbative effects and electroweak effects, as well as Monte Carlo simulations with next-to-leading-order matrix elements interfaced to parton showering, are compared to the measured cross-sections. A quantitative comparison of the measured cross-sections to the QCD calculations using several sets of parton distribution functions is performed.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fernandez Martinez, P., Ferrer, A., et al. (2015). Search for new phenomena in events with a photon and missing transverse momentum in pp collisions at root s = 8 TeV with the ATLAS detector. Phys. Rev. D, 91(1), 012008–25pp.
Abstract: Results of a search for new phenomena in events with an energetic photon and large missing transverse momentum with the ATLAS experiment at the LHC are reported. Data were collected in proton-proton collisions at a center-of-mass energy of 8 TeVand correspond to an integrated luminosity of 20.3 fb(-1). The observed data are well described by the expected Standard Model backgrounds. The expected (observed) upper limit on the fiducial cross section for the production of events with a photon and large missing transverse momentum is 6.1 (5.3) fb at 95% confidence level. Exclusion limits are presented on models of new phenomena with large extra spatial dimensions, supersymmetric quarks, and direct pair production of dark-matter candidates.
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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). Measurement of three-jet production cross-sections in pp collisions at 7 TeV centre-of-mass energy using the ATLAS detector. Eur. Phys. J. C, 75(5), 228–33pp.
Abstract: Double-differential three-jet production cross-sections are measured in proton-proton collisions at a centre-of-mass energy of root s = 7 TeV using the ATLAS detector at the large hadron collider. The measurements are presented as a function of the three-jet mass (m(jjj)), in bins of the sum of the absolute rapidity separations between the three leading jets (|Y*|). Invariant masses extending up to 5 TeV are reached for 8 < |Y*| < 10. These measurements use a sample of data recorded using the ATLAS detector in 2011, which corresponds to an integrated luminosity of 4.51 fb(-1). Jets are identified using the anti-k(t) algorithm with two different jet radius parameters, R = 0.4 and R = 0.6. The dominant uncertainty in these measurements comes from the jet energy scale. Next-to-leading-order QCD calculations corrected to account for non-perturbative effects are compared to the measurements. Good agreement is found between the data and the theoretical predictions based on most of the available sets of parton distribution functions, over the full kinematic range, covering almost seven orders of magnitude in the measured cross-section values.
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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). Measurements of the Nuclear Modification Factor for Jets in Pb plus Pb Collisions at root SNN=2.76 TeV with the ATLAS Detector. Phys. Rev. Lett., 114(7), 072302–18pp.
Abstract: Measurements of inclusive jet production are performed in pp and Pb + Pb collisions at root SNN = 2.76 TeV with the ATLAS detector at the LHC, corresponding to integrated luminosities of 4.0 and 0.14 nb(-1), respectively. The jets are identified with the anti-k(t) algorithm with R = 0.4, and the spectra are measured over the kinematic range of jet transverse momentum 32 < pT < 500 GeV and absolute rapidity |y| < 2.1 and as a function of collision centrality. The nuclear modification factor R-AA is evaluated, and jets are found to be suppressed by approximately a factor of 2 in central collisions compared to pp collisions. The RAA shows a slight increase with pT and no significant variation with rapidity.
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