CPasymmetries inB0decays in the presence of flavor-changing neutral currents

We study in detail models with vectorlike quarks, with special emphasis on their implications for $\mathrm{CP}$ asymmetries in $B^0$ decays. In this class of models there are deviations from unitarity in the Cabibbo-Kobayashi-Maskawa matrix and flavor-changing neutral currents which, although naturally suppressed, may have important consequences. We show that even a relatively small contribution of $Z$-mediated flavor-changing neutral currents to $B^0-{\overline{B}}^0$ mixing can lead to significant departures from the predictions of the standard model for $\mathrm{CP}$ asymmetries in $B^0$ decays.