ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fernandez Martinez, P., et al. (2016). Search for single top-quark production via flavour-changing neutral currents at 8TeV with the ATLAS detector. Eur. Phys. J. C, 76(2), 55–30pp.
Abstract: A search for single top-quark production via flavour-changing neutral current processes from gluon plus up- or charm-quark initial states in proton-proton collisions at the LHC is presented. Data collected with the ATLAS detector in 2012 at a centre-of-mass energy of 8TeV and corresponding to an integrated luminosity of 20.3 fb(-1) are used. Candidate events for a top quark decaying into a lepton, a neutrino and a jet are selected and classified into signal- and background-like candidates using a neural network. No signal is observed and an upper limit on the production cross-section multiplied by the t -> Wb branching fraction is set. The observed 95% CL limit is sigma(qg -> t) x B(t -> Wb) < 3.4 pb and the expected 95% CL limit is sigma(qg -> t) x B(t -> Wb) < 2.9 pb. The observed limit can be interpreted as upper limits on the coupling constants of the flavour-changing neutral current interactions divided by the scale of new physics kappa(ugt)/Lambda < 5.8 x 10(-3) TeV-1 and kappa(cgt)/Lambda < 13x10(-3) TeV and on the branching fractions B(t -> ug) < 4.0 x 10(-5) and B(t -> cg) < 20 x 10(-5).
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fassi, F., Ferrer, A., et al. (2013). Search for single b*-quark production with the ATLAS detector at root s=7 TeV. Phys. Lett. B, 721(4-5), 171–189.
Abstract: The results of a search for an excited bottom-quark b* in pp collisions at root s = 7 TeV, using 4.7 fb(-1) of data collected by the ATLAS detector at the LHC are presented. In the model studied, a single b*-quark is produced through a chromomagnetic interaction and subsequently decays to a W boson and a top quark. The search is performed in the dilepton and lepton + jets final states, which are combined to set limits on b*-quark couplings for a range of b*-quark masses. For a benchmark with unit size chromomagnetic and Standard Model-like electroweak b* couplings, b* quarks with masses less than 870 GeV are excluded at the 95% credibility level.
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ATLAS Collaboration(Aad, G. et al), Amoros, G., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., et al. (2012). Search for second generation scalar leptoquarks in pp collisions at root s=7 TeV with the ATLAS detector. Eur. Phys. J. C, 72(9), 2151–21pp.
Abstract: The results of a search for the production of second generation scalar leptoquarks are presented for final states consisting of either two muons and at least two jets or a muon plus missing transverse momentum and at least two jets. A total of 1.03 fb(-1) integrated luminosity of proton-proton collision data produced by the Large Hadron Collider at root s = 7 TeV and recorded by the ATLAS detector is used for the search. The event yields in the signal regions are found to be consistent with the Standard Model background expectations. The production of second generation leptoquarks is excluded for a leptoquark mass m(LQ) < 594 (685) GeV at 95 % confidence level, for a branching ratio of 0.5 (1.0) for leptoquark decay to a muon and a quark.
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ATLAS Collaboration(Aad, G. et al), Amoros, G., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., et al. (2012). Search for scalar top quark pair production in natural gauge mediated supersymmetry models with the ATLAS detector in pp collisions at root s=7 TeV. Phys. Lett. B, 715(1-3), 44–60.
Abstract: The results of a search for pair production of the lighter scalar partners of top quarks ((t) over bar (1)) in 2.05 fb(-1) of pp collisions at root s = 7 TeV using the ATLAS experiment at the LHC are reported. Scalar top quarks are searched for in events with two same flavour opposite-sign leptons (e, mu) with invariant mass consistent with the Z boson mass, large missing transverse momentum and jets in the final state. At least one of the jets is identified as originating from a b-quark. No excess over Standard Model expectations is found. The results are interpreted in the framework of R-parity conserving, gauge mediated Supersymmetry breaking 'natural' scenarios, where the neutralino ((chi) over bar (0)(1)) is the next-to-lightest supersymmetric particle. Scalar top quark masses up to 310 GeV are excluded for 115 GeV m(<(chi) over bar1)(0) 230 GeV at 95% confidence level, reaching an exclusion of m(<(t) over bar1) 330 GeV for m(<(chi) over bar1)(0) = 190 GeV. Scalar top quark masses below 240 GeV are excluded for all values of m((chi) over bar1)(0) > m(z).
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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 Scalar Diphoton Resonances in the Mass Range 65-600 GeV with the ATLAS Detector in pp Collision Data at root s=8 TeV. Phys. Rev. Lett., 113(17), 171801–18pp.
Abstract: A search for scalar particles decaying via narrow resonances into two photons in the mass range 65-600 GeV is performed using 20.3 fb(-1) of root s = 8 TeV pp collision data collected with the ATLAS detector at the Large Hadron Collider. The recently discovered Higgs boson is treated as a background. No significant evidence for an additional signal is observed. The results are presented as limits at the 95% confidence level on the production cross section of a scalar boson times branching ratio into two photons, in a fiducial volume where the reconstruction efficiency is approximately independent of the event topology. The upper limits set extend over a considerably wider mass range than previous searches.
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