ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2017). Measurement of the inclusive cross-sections of single top-quark and top-antiquark t-channel production in pp collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 04(4), 086–41pp.
Abstract: A measurement of the t-channel single-top-quark and single-top-antiquark production cross-sections in the lepton+jets channel is presented, using 3.2 fb(-1) of proton-proton collision data at a centre-of-mass energy of 13 TeV, recorded with the ATLAS detector at the LHC in 2015. Events are selected by requiring one charged lepton (electron or muon), missing transverse momentum, and two jets with high transverse momentum, exactly one of which is required to be b-tagged. Using a binned maximum-likelihood fit to the discriminant distribution of a neural network, the cross-sections are determined to be sigma(tq) = 156 +/- 5 (stat.) +/- 27 (syst.) +/- 3 (lumi.) pb for single top-quark production and sigma((t) over barq) = 91 +/- 4 (stat.) +/- 18 (syst.) +/- 2 (lumi.) pb for single top-antiquark production, assuming a top-quark mass of 172.5 GeV. The cross-section ratio is measured to be R-t = sigma(tq) / sigma((t) over barq) = 1.72 +/- 0.09 (stat.) +/- 0.18 (syst.). All results are in agreement with Standard Model predictions.
|
ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2017). Probing the W tb vertex structure in t-channel single-top-quark production and decay in pp collisions at root s=8 TeV with the ATLAS detector. J. High Energy Phys., 04(4), 124–50pp.
Abstract: To probe the Wtb vertex structure, top-quark and W-boson polarisation observables are measured from t-channel single-top-quark events produced in proton-proton collisions at a centre-of-mass energy of 8 TeV. The dataset corresponds to an integrated luminosity of 20.2 fb(-1), recorded with the ATLAS detector at the LHC. Selected events contain one isolated electron or muon, large missing transverse momentum and exactly two jets, with one of them identified as likely to contain a b-hadron. Stringent selection requirements are applied to discriminate t-channel single-top-quark events from background. The polarisation observables are extracted from asymmetries in angular distributions measured with respect to spin quantisation axes appropriately chosen for the top quark and the W boson. The asymmetry measurements are performed at parton level by correcting the observed angular distributions for detector effects and hadronisation after subtracting the background contributions. The measured top-quark and W-boson polarisation values are in agreement with the Standard Model predictions. Limits on the imaginary part of the anomalous coupling gR are also set from model-independent measurements.
|
ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fernandez Martinez, P., et al. (2015). Measurement of the t(t)over-barW and t(t)over-barZ production cross sections in pp collisions at root s=8 TeV with the ATLAS detector. J. High Energy Phys., 11(11), 172–48pp.
Abstract: The production cross sections of top-quark pairs in association with massive vector bosons have been measured using data from pp collisions at root s = 8 TeV. The dataset corresponds to an integrated luminosity of 20.3 fb(-1) collected by the ATLAS detector in 2012 at the LHC. Final states with two, three or four leptons are considered. A fit to the data considering the t (t) over barW and t (t) over barZ processes simultaneously yields a significance of 5.0 sigma (4.2 sigma) over the background-only hypothesis for t (t) over barW (t (t) over barZ) production. The measured cross sections are sigma(t (t) over barW) = 369(-91)(+100) fb and sigma(t (t) over barZ) = 176(-52)(+58) fb. The background-only hypothesis with neither t (t) over barW nor t (t) over barZ production is excluded at 7.1 sigma. All measurements are consistent with next-to-leading-order calculations for the t (t) over barW and t (t) over barZ processes.
|
ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2016). Search for new phenomena with photon plus jet events in proton-proton collisions at TeV with the ATLAS detector. J. High Energy Phys., 03(3), 041–37pp.
Abstract: A search is performed for the production of high-mass resonances decaying into a photon and a jet in 3.2 fb(-1) of proton-proton collisions at a centre-of-mass energy of TeV collected by the ATLAS detector at the Large Hadron Collider. Selected events have an isolated photon and a jet, each with transverse momentum above 150 GeV. No significant deviation of the gamma+jet invariant mass distribution from the background-only hypothesis is found. Limits are set at 95% confidence level on the cross sections of generic Gaussian-shaped signals and of a few benchmark phenomena beyond the Standard Model: excited quarks with vector-like couplings to the Standard Model particles, and non-thermal quantum black holes in two models of extra spatial dimensions. The minimum excluded visible cross sections for Gaussian-shaped resonances with width-to-mass ratios of 2% decrease from about 6 fb for a mass of 1.5 TeV to about 0.8 fb for a mass of 5 TeV. The minimum excluded visible cross sections for Gaussian-shaped resonances with width-to-mass ratios of 15% decrease from about 50 fb for a mass of 1.5 TeV to about 1.0 fb for a mass of 5 TeV. Excited quarks are excluded below masses of 4.4 TeV, and non-thermal quantum black holes are excluded below masses of 3.8 (6.2) TeV for Randall-Sundrum (Arkani-Hamed-Dimopoulous-Dvali) models with one (six) extra dimensions.
|
ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fernandez Martinez, P., et al. (2015). Summary of the ATLAS experiment's sensitivity to supersymmetry after LHC Run 1-interpreted in the phenomenological MSSM. J. High Energy Phys., 10(10), 134–76pp.
Abstract: A summary of the constraints from the ATLAS experiment on R-parity-conserving supersymmetry is presented. Results from 22 separate ATLAS searches are considered, each based on analysis of up to 20.3 fb(-1) of proton-proton collision data at centre-of-mass energies of root s = 7 and 8TeV at the Large Hadron Collider. The results are interpreted in the context of the 19-parameter phenomenological minimal supersymmetric standard model, in which the lightest supersymmetric particle is a neutralino, taking into account constraints from previous precision electroweak and flavour measurements as well as from dark matter related measurements. The results are presented in terms of constraints on supersymmetric particle masses and are compared to limits from simplified models. The impact of ATLAS searches on parameters such as the dark matter relic density, the couplings of the observed Higgs boson, and the degree of electroweak fine-tuning is also shown. Spectra for surviving supersymmetry model points with low fine-tunings are presented.
|
