How much space is left for a new family?

Understand

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 CKM elements and 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 favoured, but contrary to expectation we find that also a quite large mixing with the 4th family is not yet excluded.