Bounding CKM Mixing with a Fourth Family

TL;DR

This paper analyzes constraints on CKM mixing involving a potential fourth quark family, showing that large mixing is excluded but mixing comparable to the Cabibbo angle remains possible based on electroweak data.

Contribution

It provides a comprehensive analysis of electroweak constraints on fourth-family CKM mixing, emphasizing the role of oblique corrections over Zbb vertex corrections.

Findings

Large fourth-family mixing is excluded by data.

Mixing comparable to Cabibbo angle is still allowed.

Oblique corrections dominate constraints over vertex corrections.

Abstract

CKM mixing between third family quarks and a possible fourth family is constrained by global fits to the precision electroweak data. The dominant constraint is from nondecoupling oblique corrections rather than the Zbb vertex correction used in previous analyses. The possibility of large mixing suggested by some recent analyses of FCNC processes is excluded, but 3-4 mixing of the same order as the Cabbibo mixing of the first two families is allowed.