Large t→cZ as a sign of vectorlike quarks in light of the W mass

Abstract

The rare flavor-changing top quark decay $t\ensuremath{\rightarrow}cZ$ is a clear sign of new physics and experimentally very interesting due to the huge number of top quarks produced at the LHC. However, there are few (viable) models which can generate a sizable branching ratio for $t\ensuremath{\rightarrow}cZ$---in fact vectorlike quarks seem to be the only realistic option. In this paper, we investigate all three representations (under the Standard Model gauge group) of vectorlike quarks ($U$, ${Q}_{1}$ and ${Q}_{7}$) that can generate a sizable branching ratio for $t\ensuremath{\rightarrow}cZ$ without violating bounds from $B$ physics. Importantly, these are exactly the three vectorlike quarks which can lead to a sizable positive shift in the prediction for $W$ mass, via the couplings to the top quark also needed for a sizable $\mathrm{Br}(t\ensuremath{\rightarrow}cZ)$. Calculating and using the one-loop matching of vectorlike quarks on the Standard Model effective field theory, we find that $\mathrm{Br}(t\ensuremath{\rightarrow}cZ)$ can be of the order of ${10}^{\ensuremath{-}6}$, ${10}^{\ensuremath{-}5}$ and ${10}^{\ensuremath{-}4}$ for $U$, ${Q}_{1}$ and ${Q}_{7}$, respectively, and that in all three cases the large $W$ mass measurement can be accommodated.