Constraints on the Four-Generation Quark Mixing Matrix from a Fit to Flavor-Physics Data

TL;DR

This paper performs a comprehensive fit of the four-generation quark mixing matrix using flavor-physics data, constraining the matrix elements and exploring implications for new physics signals.

Contribution

It provides the first detailed constraints on all elements of the 4x4 quark mixing matrix from current flavor-physics measurements.

Findings

|V_tb| >= 0.98 at 3sigma, close to unity

All new-physics parameters are consistent with zero

Mixing of the fourth generation with others is very small

Abstract

In the scenario with four quark generations, we perform a fit using flavor-physics data and determine the allowed values -- preferred central values and errors -- of all of the elements of the 4X4 quark mixing matrix. In addition to the direct measurements of some of the elements, we include in the fit the present measurements of several flavor-changing observables in the K and B systems that have small hadronic uncertainties, and also consider the constraints from the vertex corrections to Z -> b bbar. The values taken for the masses of the fourth-generation quarks are consistent with the measurements of the oblique parameters and perturbativity of the Yukawa couplings. We find that |{\tilde V}_{tb}| >= 0.98 at 3sigma, so that a fourth generation cannot account for any large deviation of |{\tilde V}_{tb}| from unity. The fit also indicates that all the new-physics parameters are consistent with zero, and the mixing of the fourth generation with the other three is constrained to be very small: we obtain |{\tilde V}_{ub'}| < 0.06, |{\tilde V}_{cb'}| < 0.027, and |{\tilde V}_{tb'}| < 0.31 at 3sigma. Still, this does allow for the possibility of new-physics signals in Bd, Bs and rare K decays.