Search for FCNC single top-quark production at 8 TeV with the ATLAS detector

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 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 $B(t \rightarrow cg) < 17\times 10^{-5}$.