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 → Wb branching fraction is set. The observed 95 % CL limit is σ_qg → t × B(t → Wb) < 3.4 pb and the expected 95 % CL limit is σ_qg → t × B(t → Wb) < 2.9pb. 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 κ_ugt / Λ < 5.8 × 10⁻³ TeV⁻¹ and κ_cgt / Λ < 13 × 10⁻³ TeV and on the branching fractions B(t → ug) < 4.0 × 10⁻⁵ and B(t→cg) < 20 × 10⁻⁵.