  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 chi square 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 |U_{sb}| <= 3.40 X 10^{-4} at 3 sigma. Still
this does allow for the possibility of new physics signals in some of the
observables such as semileptonic CP asymmetry in Bs decays.
