Constraining models with vector-like fermions from FCNC in K and B physics

In this work we update the constraints on tree level FCNC couplings in the framework of a theory with n isosinglet vector-like down quarks. In this context, we emphasize the sensitivity of the B J/psi K-S CP asymmetry to the presence of new vector-like down quarks. This CP asymmetry, together with the rare decays B --> X(s,d)l (l) over bar and K --> pi nu)over bar> are the best options to further constrain the FCNC tree level couplings or even to point out, in the near future, the possible presence of vector-like quarks in the low energy spectrum, as suggested by GUT theories or models of large extra dimensions at the TeV scale