Flavor and CP violating Z exchange and the rate asymmetry in B --> phi K_S

Recent measurements of time dependent CP asymmetry in B --> phi K_S, if confirmed, would indicate a new source of CP violation. We examine flavor violating tree-level Z currents in models with extra down-type quark singlets that arise naturally in string compactified gauge groups like E_6. We evaluate the new operators at the scale mu = m_b in NLO, and using QCD improved factorization to describe B --> phi K_S, find the allowed range of parameters for rho and psi, the magnitude and phase of the flavor violating parameter U_{bs}. This allowed range does satisfy the constraint from flavor changing process b --> s l+ l- . However, further improvement in measurement of these rates could severely constrain the model.