Search for Flavour Changing Neutral Currents in Single Top-Quark Production at $\sqrt{s}$ = 7 TeV with the ATLAS Detector

This thesis represents the search for single top-quark production through flavour changing neutral currents using data collected by the ATLAS detector in 2011, at a centre-of-mass energy of $\sqrt{s}$ = 7 TeV, corresponding to an integrated luminosity of 2.05 fb$^{-1}$. Candidate events are selected with one isolated lepton, missing transverse momentum associated to the undetected neutrino and a jet originated from the hadronisation of a $b$ quark. Given the large expected number of background events and the small number of expected signal events, a neural network classifier is developed to combine many kinematic variables to create a powerful separator in order classify the events as a signal- or a background-like events. As no sign of new physics is seen in the neural network output distribution, a Bayesian statistical method is used to set an upper limit at 95% confidence level (C.L.) on the single top-quark production cross section through FCNC processes. The observed upper limit at 95% C.L. on the cross-section multiplied by the $t \to Wb$ branching fraction is measured to be $\sigma_{qg\to t} \times {\cal B} (t\to Wb)<3.9$ pb. This upper limit is converted using a model-independent approach into upper limits on the coupling strengths $\frac{\kappa_{ugt}}{\Lambda} < 6.9\cdot 10^{-3}$ TeV$^{-1}$ and $\frac{\kappa_{cgt}}{\Lambda} < 1.6\cdot 10^{-2}$ 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 ${\cal B}(t \rightarrow cg) < 2.7\cdot 10^{-4}$. The limits on the branching fractions are the world's best limits to date and significantly improving the previous limits obtained by the DØ collaboration by a factor of 15.