b -> s Decays in a model with Z-mediated flavor changing neutral current
  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. 
Ashutosh Kumar Alok and Shireen Gangal
hep-ph