Z-mediatedB−B¯mixing andB-mesonCP-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 ${\mathrm{SU}\mathrm{}\left(2\right)\mathrm{}}_L$ singlet down quarks, such as in ${\mathrm{E}}_6$. We find the new bounds still allow tree-level FCNC $Z$ exchange to dominate $B_d^0-{\overline{B}}_d^0$ mixing, but not $B_s^0-{\overline{B}}_s^0$ mixing. A stronger $b\to d$ bound by a factor of 1/10 could rule out the possibility of $Z$ mediation with FCNC accounting for $B_d^0-{\overline{B}}_d^0$ mixing. The unitarity "$d-b$ triangle" may still be a quadrangle with a detectable fourth side. The $\mathrm{CP}$ asymmetries from $B_d^0$ decay should show a new $\mathrm{CP}$-violating phase if the $Z$-mediated process dominates $B_d^0-{\overline{B}}_d^0$ mixing.