Probing FCNC couplings in single top quark production associated with a neutral gauge boson at future lepton colliders

Abstract: In this paper, we consider a single top quark production in association with a neutral gauge boson (Z boson or Photon) at the future electron-positron colliders. We utilize these channels to probe the top quark Flavour Changing Neutral Currents (FCNC) interactions in $tqZ$ and $tq\gamma$ vertices. We perform two separate analyses for top-Z-jet and top-$\gamma$-jet channels. The Standard Model Effective Field Theory (SMEFT) approach is employed to search for these anomalous couplings. We consider parton showering, hadronization and fast detector simulation in our study and use a cut-and-count technique to separate the signal from the Standard Model (SM) background processes. The upper limits on the FCNC couplings at $95\%$ confidence level for the different integrated luminosities are obtained. It is shown that the future lepton collider would be able to probe the FCNC branching fractions to $\rm{Br}(t\rightarrow q \gamma)< 10^{-4}$ and $\rm{Br}(t\rightarrow q Z) < 10^{-4}$ with $3$ $\rm{ab}^{-1}$of integrated luminosity of data at a center of mass energy of $350$ GeV.