/ hep-ex CERN-PH-EP-2015-172

Search for single top-quark production via flavour changing neutral currents at 8 TeV with the ATLAS detector

Pages: 29

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 8 TeV and corresponding to an integrated luminosity of 20.3 $\mbox{fb}^{-1}$ 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 $t \rightarrow Wb$ branching fraction is set. The observed 95 % CL limit is $\sigma_{qg \rightarrow t} \times B(t \rightarrow Wb)< 3.4$ pb and the expected 95% CL limit is $\sigma_{qg \rightarrow t} \times B(t \rightarrow Wb)< 2.9$ 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 $\kappa_{ugt}/\Lambda < 5.8 \times 10^{-3}$ TeV$^{-1}$ and $\kappa_{cgt}/\Lambda < 13 \times 10^{-3}$ TeV$^{-1}$ and on the branching fractions $B(t \rightarrow ug) < 4 \times 10^{-5}$ and $B(t \rightarrow cg) < 17 \times 10^{-5}$.

Note: 29 pages plus the Autors list and cover page (45 pages in total), 10 figures, 3 tables, submitted to EPJC, All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/TOPQ-2014-13/

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