Scotogenic top-quark FCNC decays

Abstract

Flavor-changing neutral current (FCNC) top-quark decays are highly suppressed due to the Glashow-Iliopoulos-Maiani mechanism in the standard model (SM). If $t\to q h, qV$ with $V=g,\gamma, Z$ are all induced via quantum loop levels, then we investigate the effect that can enhance the top-FCNC up to the sensitivity designed at the high-luminosity (HL) LHC. Inspired by the mechanism of the scotogenic neutrino mass, we extend the SM by including $Z_2$-odd colored fermions when a $Z_2$ discrete is imposed. The results show that by taking $BR(t\to u g) \lesssim 0.61 \times 10^{-4}$ recently measured by ATLAS as an input, $t\to q \gamma$ can be indirectly bounded to be $BR(t\to q \gamma)\lesssim 3.2 \times 10^{-6}$, which is below the expected sensitivity at the HL LHC. After taking potential constraints from various experiments into account, the obtained branching ratios for the loop-induced $t\to q h$ and $t\to qZ$ decays can be $O(10^{-4})$, which falls within the sensitivity at the HL LHC.