Z-mediated flavor-changing neutral currents and their implications forCPasymmetries inB0decays

In models where the quark sector is extended in a nonsequential way, there are $Z$-mediated flavor-changing neutral currents at the tree level. These may dominate mixing in neutral-$B$ systems. Unitarity of the three-generation Cabibbo-Kobayashi-Maskawa matrix is violated, and new phases take part in quark mixing. All these effects modify significantly the predictions for $\mathrm{CP}$ asymmetries in $B^0$ decays. The various aspects of new physics can be probed separately by testing specific relations among these $\mathrm{CP}$ asymmetries.