The rare flavor-changing top quark decay t→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→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, Q1 and Q7) that can generate a sizable branching

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... io for t→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 Br(t→cZ). Calculating and using the one-loop matching of vectorlike quarks on the Standard Model effective field theory, we find that Br(t→cZ) can be of the order of 10−6, 10−5 and 10−4 for U, Q1 and Q7, respectively, and that in all three cases the large W mass measurement can be accommodated.