[en] In the scenario with Z mediated flavor changing neutral current occurring at the tree level due to the addition of a vector-like isosinglet down-type quark d' to the SM particle spectrum, we perform a χ² fit using the flavor physics data and obtain the best fit value along with errors of the tree level Zbs coupling, U_sb. The fit indicates that the new physics coupling is constrained to be small: we obtain vertical stroke U_sb vertical stroke ≤3.40 x 10^-4 at 3σ. Still this does allow for the possibility of new physics signals in some of the observables such as semileptonic CP asymmetry in B_s decays.