Tree-level FCNC in the B system: from CP asymmetries to rare decays

Tree-level Flavor-Changing Neutral Currents (FCNC) are characteristic of models with extra vector-like quarks. These new couplings can strongly modify the B^0 CP asymmetries without conflicting with low--energy constraints. In the light of a low CP asymmetry in B --> J/psi K_{S}, we discuss the implications of these contributions. We find that even these low values can be easily accommodated in these models. Furthermore, we show that the new data from B factories tend to favor an O(20) enhancement of the b --> d l bar{l} transition over the SM expectation.