Search for Flavour Changing Neutral Currents in Single Top-Quark Production at √<em>s</em> = 7 TeV with the ATLAS Detector

Abstract

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 √<em>s</em> = 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 → Wb branching fraction is measured to be σ_qg→t × <i>B</i>(t → Wb) < 3.9 pb. This upper limit is converted using a model-independent approach into upper limits on the coupling strengths Κ_ugt/Λ < 6.9·10^-3 TeV^-1 and Κ_cgt/Λ < 1.6·10^-2 TeV^-1, where Λ is the new physics scale, and on the branching fractions <i>B</i>(t→ug) < 5.7·10^-5 and <i>B</i>(t→cg) < 2.7·10^-4. The limits on the branching fractions are the world's best limits to date and significantly improving the previous limits obtained by the DO collaboration by a factor of 15.