Search for single top-quark production via flavour-changing neutral currents 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 $$\sigma _{qg \rightarrow t} {\,\times \,} \mathcal {B}(t \rightarrow Wb)< {3.4}\,\mathrm{pb}$$ and the expected 95 % CL limit is $$\sigma _{qg \rightarrow t} \times \mathcal {B}(t \rightarrow Wb)< {2.9}\,\mathrm{pb}$$. 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 $$\kappa _{ugt}/\Lambda < 5.8 \times 10^{-3}\, \mathrm{TeV}^{-1}$$ and $$\kappa _{cgt}/\Lambda < 13 \times 10^{-3}\, \mathrm{TeV}$$ and on the branching fractions $$\mathcal {B}(t \rightarrow ug) < {4.0 \times 10^{-5}}$$ and $$\mathcal {B}(t \rightarrow cg) < {20 \times 10^{-5}}$$