Search for single top-quark production via flavour-changing neutral currents in strong interactions using s = 8 TeV ATLAS data

Abstract

This thesis presents results of a search for single top-quark production via flavour-changing neutral currents (FCNCs) in strong interactions using data recorded with the ATLAS experiment at the Large Hadron Collider in 2012. A dataset from proton-proton collisions at a centre-of-mass energy of $\sqrt{s}$ = 8 TeV and corresponding to an integrated luminosity of L$_{int}$ = 14.1 fb$^{−1}$ is used. It is searched for FCNC events in which a light quark (u or c) interacts with a gluon to produce a single top quark, either with or without the associated production of another light quark or gluon. Candidate events of leptonically decaying top-quarks are selected by requiring a single electron or muon, a substantial amount of missing transverse energy, and exactly one or two jets of which exactly one has to fulfil a b-tagging criterion. Multivariate techniques are employed in order to distinguish between signal- and background-like events by making use of artificial neural networks that combine multiple kinematic variables into a single classifier output. A Bayesian statistical analysis is performed on the classifier outputs in order to quantify potential signs of FCNC single top-quark production. No excess in signal events is observed and the results are in agreement with the theory predictions. Consequently a new upper limit is set on the FCNC single top-quark production cross-section multiplied by the t->Wb branching fraction. The observed 95% C.L. limit is found at σ$_{qg}$→t × B(t → W b) < 2.5 pb. Using theory calculations within a model- independent effective theory framework, this limit is converted into upper limits on the coupling constants of the FCNC interactions κ$_{ugt}$/Λ < 5.1 · 10$^{−3}$ TeV$^{−1}$ and κ$_{cgt}$/Λ < 1.1 · 10$^{−2}$ TeV$^{−1}$, as well as into respective upper limits on the branching fractions B(t → ug) < 3.1 · 10$^{−5}$ and B(t → cg) < 1.6 · 10$^{−4}$.