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.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2016). Search for Minimal Supersymmetric Standard Model Higgs bosons H/A and for a Z ' boson in the tau tau final state produced in pp collisions at root s=13 TeV with the ATLAS Detector. Eur. Phys. J. C, 76(11), 585–30pp.
Abstract: A search for neutral Higgs bosons of the minimal supersymmetric standard model (MSSM) and for a heavneutral Z' boson is performed using a data sample corresponding to an integrated luminosity of 3.2 fb(-1) from proton-proton collisions at root s = 13 TeV recorded by the ATLAS detector at the LHC. The heavy resonance is assumed to decay to a tau(+)tau(-) pair with at least one tau lepton decaying to final states with hadrons and a neutrino. The search is performed in the mass range of 0.2-1.2 TeV for the MSSM neutral Higgs bosons and 0.5-2.5 TeV for the heavy neutral Z' boson. The data are in good agreement with the background predicted by the Standard Model. The results are interpreted in MSSMand Z' benchmark scenarios. The most stringent constraints on the MSSM m(A)-tan beta space exclude at 95% confidence level (CL) tan beta > 7.6 for m(A) = 200 GeV in the m(h)(mod+) MSSM scenario. For the Sequential Standard Model, a Z(SSM)' mass up to 1.90 TeV is excluded at 95% CL and masses up to 1.82-2.17 TeV are excluded for a Z(SFM)' of the strong flavour model.
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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). Search for single production of vector-like quarks decaying into Wb in pp collisions at root s=8 TeV with the ATLAS detector. Eur. Phys. J. C, 76(8), 442–26pp.
Abstract: A search for singly produced vector-like Q quarks, where Q can be either a T quark with charge +2/3 or a Y quark with charge -4/3, is performed in proton-proton collisions recorded with the ATLAS detector at the LHC. The dataset corresponds to an integrated luminosity of 20.3 fb(-1) and was produced with a centre-of-mass energy of root s = 8 TeV. This analysis targets Q -> Wb decays where the W boson decays leptonically. A veto on massive large-radius jets is used to reject the dominant t (t) over bar background. The reconstructed Q-candidate mass, ranging from 0.4 to 1.2 TeV, is used in the search to discriminate signal from background processes. No significant deviation from the Standard Model expectation is observed, and limits are set on the Q -> Wb cross-section times branching ratio. The results are also interpreted as limits on the QWb coupling and the mixing with the Standard Model sector for a singlet T quark or a Y quark from a doublet. T quarks with masses below 0.95 TeV are excluded at 95 % confidence level, assuming a unit coupling and a BR(T -> Wb) = 0.5, whereas the expected limit is 1.10 TeV.
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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). Search for supersymmetry at root s=13 TeV in final states with jets and two same-sign leptons or three leptons with the ATLAS detector. Eur. Phys. J. C, 76(5), 259–26pp.
Abstract: A search for strongly produced supersymmetric particles is conducted using signatures involving multiple energetic jets and either two isolated leptons (e or mu) with the same electric charge or at least three isolated leptons. The search also utilises b-tagged jets, missing transverse momentum and other observables to extend its sensitivity. The analysis uses a data sample of proton-proton collisions at root s = 13 TeV recorded with the ATLAS detector at the Large Hadron Collider in 2015 corresponding to a total integrated luminosity of 3.2 fb(-1). No significant excess over the Standard Model expectation is observed. The results are interpreted in several simplified supersymmetric models and extend the exclusion limits from previous searches. In the context of exclusive production and simplified decay modes, gluino masses are excluded at 95% confidence level up to 1.1-1.3 TeV for light neutralinos (depending on the decay channel), and bottom squark masses are also excluded up to 540 GeV. In the former scenarios, neutralino masses are also excluded up to 550-850 GeV for gluino masses around 1 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. (2016). A new method to distinguish hadronically decaying boosted Z bosons from W bosons using the ATLAS detector. Eur. Phys. J. C, 76(5), 238–33pp.
Abstract: The distribution of particles inside hadronic jets produced in the decay of boosted W and Z bosons can be used to discriminate such jets from the continuum background. Given that a jet has been identified as likely resulting from the hadronic decay of a boosted W or Z boson, this paper presents a technique for further differentiating Z bosons from W bosons. The variables used are jet mass, jet charge, and a b-tagging discriminant. A likelihood tagger is constructed from these variables and tested in the simulation of W' -> WZ for bosons in the transverse momentum range 200 GeV < p(T) < 400 GeV in root s = 8 TeV pp collisions with the ATLAS detector at the LHC. For Z-boson tagging efficiencies of epsilon(Z) = 90, 50, and 10 %, one can achieve W+-boson tagging rejection factors (1/epsilon(W+)) of 1.7, 8.3 and 1000, respectively. It is not possible to measure these efficiencies in the data due to the lack of a pure sample of high p(T), hadronically decaying Z bosons. However, the modelling of the tagger inputs for boosted W bosons is studied in data using a t (t) over bar -enriched sample of events in 20.3 fb(-1) of data at root s = 8 TeV. The inputs are well modelled within uncertainties, which builds confidence in the expected tagger performance.
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