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

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 $8\;\mbox{TeV}$ 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 $\sigma_{qg \rightarrow t} \times B(t \rightarrow Wb)< 3.4\;\mbox{pb}$ and the expected $95\,\%$ CL limit is $\sigma_{qg \rightarrow t} \times B(t \rightarrow Wb)< 2.9\;\mbox{pb}$. 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 $\kappa_{ugt}/\Lambda < 5.8 \times 10^{-3}\;\mbox{TeV}^{-1}$ and $\kappa_{cgt}/\Lambda < 13 \times 10^{-3}\;\mbox{TeV}^{-1}$ and on the branching fractions $B(t \rightarrow ug) < 4 \times 10^{-5}$ and $B(t \rightarrow cg) < 17 \times 10^{-5}$.