Search for Flavour Changing Neutral Currents in single top events

A study of top-quark anomalous couplings is performed through the search for a single top-quark produced in association with a $Z$ boson. The event selection requires the presence of three isolated leptons, electrons or muons, and of at least one jet. The signal extraction is done using kinematic variables and information related to b-tagging, combined using a Boosted Decision Tree. The search is performed in a data sample corresponding to about 5 fb$^{-1}$ of proton-proton collisions at $\sqrt{s}=7$ TeV recorded with the CMS detector. No evidence of flavor-changing neutral currents is observed and upper limits at 95\% confidence level are determined. The corresponding upper limits on the coupling strengths of an effective model are found to be $\kappa_{gut}/\Lambda < 0.10$ TeV$^{-1}$, $\kappa_{gct}/\Lambda < 0.35$ TeV$^{-1}$, $\kappa_{Zut}/\Lambda < 0.45$ TeV$^{-1}$ and $\kappa_{Zct}/\Lambda < 2.27$ TeV$^{-1}$, where $\Lambda$ is the expected scale at which new physics could appear. The equivalent top-quark-decay branching fractions are found to be $\mathcal{B}(t \to gu)\leq 0.56\%$, $\mathcal{B}(t \to gc)\leq 7.12\%$, $\mathcal{B}(t \to Zu)\leq 0.51\%$ and $\mathcal{B}(t \to Zc)\leq 11.40\%$.