Search for FCNC in top-quark events in ATLAS

Searches for flavor changing neutral current (FCNC) processes in top-quark production and decays by the ATLAS Collaboration are presented. Data collected from $pp$ collisions at the LHC at a centre-of-mass energy of $\sqrt{s}=7$ GeV during 2011, corresponding to an integrated luminosity of 2.05 fb$^{-1}$, are used. In a first analysis single top-quarks produced via FCNC are searched for. Candidate events with a semileptonic top-quark decay signature are classified as signal or background-like events by using several kinematic variables as input to a neural network. No signal is observed in the neural network output distribution and a Bayesian upper limit is placed on the production cross-section. The observed upper limit is converted using a model-independent approach into upper limits on the coupling strengths $\kappa_{ugt}/\Lambda < 6.9\cdot 10^{-3}$ TeV$^{-1}$ and $\kappa_{cgt}/\Lambda < 1.6 \cdot 10^{-2}$ TeV$^{-1}$, where $\Lambda$ is the new physics scale, and on the branching fractions ${\cal B}(t \rightarrow ug) < 5.7 \cdot 10^{-5}$ and ${\cal B}(t \rightarrow cg) < 2.7\cdot 10^{-4}$. % A second search is performed for top-quark pair-production events, with one top quark decaying through the $t \to Zq$ FCNC ($q=u,c$) channel, and the other through the Standard Model dominant mode $t \to Wb$. Only the decays of the $Z$ boson to charged leptons and leptonic $W$-boson decays are considered as signal. No evidence for an FCNC signal is found and thus an upper limit on the $t \to Zq$ branching ratio of BR$(t \to Zq)<0.73$% is set at the 95% confidence level.