Constraints on the four-generation quark mixing matrix from a fit to flavor-physics data

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 4 x 4 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 (b) over bar. b. 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 vertical bar(V) over tilde (tb)vertical bar >= 0.98 at 3 sigma, so that a fourth generation cannot account for any large deviation of vertical bar V-tb vertical bar 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 small: we obtain vertical bar(V) over tilde (ub')vertical bar < 0.06, vertical bar<(V)over tilde>(cb')vertical bar < 0.027, and vertical bar<(V)over tilde> (tb')vertical bar < 0.31 at 3 sigma. Still, this does allow for the possibility of new-physics signals in B-d, B-s and rare K decays.