Probing top-quark flavor-changing couplings at the CEPC

Abstract

We propose to study the flavor properties of the top quark at the future Circular Electron Positron Collider (CEPC) in China. We systematically consider the full set of 56 real parameters that characterize the flavor-changing neutral interactions of the top quark, which can be tested at the CEPC by the single top production channel. Compared with current bounds from the LEP2 data and the projected limits at the high-luminosity LHC, we find that the CEPC could improve the limits on four-fermion flavor-changing coefficients by one to two orders of magnitude, and in the meantime providing similar sensitivities on two-fermion flavor-changing coefficients. Overall, the CEPC could explore a large fraction of currently allowed parameter space that will not be covered by the upgrade of the LHC. We show that the $c$-jet tagging capacity at the CEPC could further improve its sensitivity to top-charm flavor-changing couplings. If a signal is observed, the kinematic distribution as well as the $c$-jet tagging could be exploited to pinpoint the sizes of various flavor-changing couplings, providing valuable information on the flavor properties of the top quark.