Tree-level flavor-changing neutral currents in the B system: From CP asymmetries to rare decays

corresponding to 0.59 ≤ sin(2�) ≤ 0.82, which is cer- tainly outside the 1� Babar range but not outside the world average. This potential discrepancy is at the ori- gin of several papers (6) studying the implications of a small aJ/ in the search of new physics. In this paper, we analyze the implications of this situ- ation for a realistic model, obtained with the only addi- tion of an isosinglet down vector-like quark (7) to the SM spectrum. This model naturally arises, for instance, as the low-energy limit of an E6 grand unified theory. At a more phenomenological level, models with isosinglet quarks provide the simplest self-consistent framework to study deviations of 3 ×3 unitarity of the CKM matrix as well as flavor-changing neutral currents ( FCNC ) at tree level. In the rest of the paper, we update the strong low- energy constraints on the tree-level FCNC couplings, we show that a low CP asymmetry in B → J/ KS can be easily accommodated within the model, and we point out other observables, correlated with a low CP asymmetry,