Single top quark production as a probe of anomalous $tq\gamma$ and $tqZ$ couplings at the FCC-ee

Abstract

In this paper, a detailed study to probe the top quark Flavour-Changing Neutral Currents (FCNC) $tq\gamma$ and $tqZ$ at the future $e^{-}e^{+}$ collider FCC-ee in three different center-of-mass energies of 240, 350 and 500 GeV is presented. A set of useful variables are proposed and used in a multivariate technique to separate signal $e^- e^+ \rightarrow Z/\gamma \rightarrow t \bar{q} ~ ( \bar{t} q )$ from standard model background processes. The study includes a fast detector simulation based on the {\sc delphes} package to consider the detector effects. The $3 \sigma$ discovery regions and the upper limits on the FCNC branching ratios at 95\% confidence level (CL) in terms of the integrated luminosity are presented. It is shown that with 300 fb$^{-1}$ of integrated luminosity of data, FCC-ee would be able to exclude the effective coupling strengths above ${\cal O} (10^{-4}-10^{-5})$ which is corresponding to branching fraction of ${\cal O}(0.01-0.001)$\%. We show that moving to a high-luminosity regime leads to a significant improvement on the upper bounds on the top quark FCNC couplings to a photon or a $Z$ boson.