Sensitivity to top-quark FCNC interactions at future muon colliders

We investigate flavor-changing neutral current (FCNC) interactions of the top quark at a future muon collider with a center-of-mass energy of $\sqrt{s}=10\mathrm{T}\mathrm{e}\mathrm{V}$. The process ${\mu }^{+}{\mu }^{-}\to {\nu }_{\mu }{\mu }^{+}bj$ and its corresponding charge conjugate are considered as a probe of anomalous $tqZ$ and $tq\gamma$ couplings, parametrized within an effective field theory framework in terms of ${\kappa }_{tqZ}$ and ${\lambda }_{tq\gamma }$. Signal and background events are simulated using Monte Carlo techniques, including parton showering and hadronization with $\mathtt{\text{Pythia}}$ and a fast detector simulation based on $\mathtt{\text{Delphes}}$ with a dedicated 10~TeV muon collider setup. A multivariate analysis based on boosted decision trees is employed to enhance the signal discrimination. Assuming an integrated luminosity of $10{\mathrm{a}\mathrm{b}}^{-1}$, we obtain projected sensitivities to the anomalous couplings at the $\mathcal{O}({10}^{-3})$ level, corresponding to branching ratio limits of $\mathcal{O}({10}^{-6})$ for the rare $t\to qZ$ and $t\to q\gamma$ decays. These results significantly improve upon the current bounds from the CMS and ATLAS collaborations, extending the sensitivity by more than one order of magnitude. Our findings demonstrate that a multi-TeV muon collider provides a powerful and complementary platform for probing rare top-quark interactions, offering a unique opportunity to explore physics beyond the Standard Model through FCNC processes.