Search for associated production of a Z boson with a single top quark and for tZ flavour-changing interactions in pp collisions at $\sqrt{s}=8~\mathrm{TeV}$

A search for the production of a single top quark in association with a Z boson is presented, both to identify the expected standard model (SM) process and to search for flavour changing neutral current (FCNC) interactions. The data sample corresponds to an integrated luminosity of $19.7~\mathrm{fb}^{-1}$ recorded by the CMS experiment at the LHC in proton-proton collisions at $\sqrt{s} = 8~\mathrm{TeV}$. Final states with three leptons, electrons or muons, and at least one jet are investigated. A moderate excess of events compatible with SM tZq production is observed, and the corresponding cross section is measured to be $\sigma ({\rm tZq \rightarrow \ell} \nu {\rm b \ell^+ \ell^- q}) = 10^{+8}_{-7}~\mathrm{fb}$ with a significance of $2.4~\sigma$. No presence of FCNC production of tZ(q) is observed and exclusion limits at $95\%$ confidence level on the branching ratios of a top quark decaying to a Z boson and an up or a charm quark are found to be $BR({\rm t\rightarrow Zu}) < 0.022\%$ and $BR({\rm t \rightarrow Zc}) < 0.049\%$, respectively. $$\textit{This document has been revised with respect to the version dated August 4, 2016.}$$