We perform an exploratory study of the allowed parameter range for the CKM-like mixing of hypothetical quarks of a fourth generation. As experimental constraints we use the tree-level determinations of the $3\times 3$ Cabibbo-Kobayashi-Maskawa (CKM) elements and flavor-changing neutral current (FCNC) processes ($K$, $D$, $B_d$, $B_s$ mixing and the decay $b\to s\gamma$) under the assumption that the $4\times 4$ CKM matrix is unitary. For the FCNCs we use some simplifying assumptions concerning the QCD corrections. Typically small mixing with the fourth family is favored; contrary to expectation, however, we find that also a quite large mixing with the fourth family is not yet excluded.

• Received 13 March 2009

DOI:https://doi.org/10.1103/PhysRevD.79.113006