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 co ntext, 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 t he rare decays B --> X(s,d)l (l) over bar and K --> pi nu<(<nu>)over bar> a re the best options to further constrain the FCNC tree level couplings or e ven 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 o f large extra dimensions at the TeV scale. (C) 2001 Elsevier Science B.V. A ll rights reserved.