Z-mediated B-B-bar mixing and B-meson CP-violating asymmetries in the light of new flavor-changing neutral-current bounds.

We impose new limits on flavor-changing neutral currents (FCNC's) applied to the 4 ± 4 mixing matrix of the next heaviest down quark in a model with extra SU(2)L singlet down quarks, such as in E6. We find the new bounds still allow tree-level FCNC Z exchange to dominate Bd0-B»d0 mixing, but not Bs0-B»s0 mixing. A stronger b'd bound by a factor of 1/10 could rule out the possibility of Z mediation with FCNC accounting for Bd0-B»d0 mixing. The unitarity "d-b triangle" may still be a quadrangle with a detectable fourth side. The CP asymmetries from Bd0 decay should show a new CP-violating phase if the Z-mediated process dominates Bd0-B»d0 mixing. © 1992 The American Physical Society