Browsing by Subject "Top physics"

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    Search for FCNC single top-quark production at √s = 7 TeV with the ATLAS detector
    (2012) Aad, G; Abbott, B; Abdallah, J; Abdelalim, A A; Abdesselam, A; Abdinov, O; Abi, B; Abolins, M; AbouZeid, O S; Abramowicz, H; Abreu, H; Acerbi, E; Acharya, B S; Adamczyk, L; Adams, D L; Addy, T N; Adelman, J; Aderholz, M; Adomeit, S; Adragna, P; Adye, T; Aefsky, S; Aguilar-Saavedra, J A; Aharrouche, M; Ahlen, S P; Ahles, F; Ahmad, A; Ahsan, M; Aielli, G; Akdogan, T; Åkesson, T P A; Akimoto, G
    A search for the production of single top-quarks via flavour-changing neutral-currents is presented. Data collected with the ATLAS detector at a centre-of-mass energy of View the MathML sources=7 TeV, corresponding to an integrated luminosity of 2.05 fb−1, are used. Candidate events with a semileptonic top-quark decay signature are classified as signal- or background-like events by using several kinematic variables as input to a neural network. No signal is observed in the neural network output distribution and a Bayesian upper limit is placed on the production cross-section. The observed upper limit at 95% confidence level on the cross-section multiplied by the t→Wbt→Wb branching fraction is measured to be σqg→t×B(t→Wb)<3.9 pbσqg→t×B(t→Wb)<3.9 pb. This upper limit is converted using a model-independent approach into upper limits on the coupling strengths κugt/Λ<6.9⋅10−3 TeV−1κugt/Λ<6.9⋅10−3 TeV−1 and κcgt/Λ<1.6⋅10−2 TeV−1κcgt/Λ<1.6⋅10−2 TeV−1, where Λ is the new physics scale, and on the branching fractions B(t→ug)<5.7⋅10−5B(t→ug)<5.7⋅10−5 and B(t→cg)<2.7⋅10−4B(t→cg)<2.7⋅10−4.