The size of χ≡arg(−VtsVtb*Vcs*Vcb) and physics beyond the Standard Model

We analyse the allowed range of values of χ, both in the Standard Model and in models with new physics, pointing out that a relatively large value of χ, e.g., of order λ, is only possible in models where the unitarity of the 3×3 Cabibbo-Kobayashi-Maskawa matrix is violated through the introduction of extra Q=2/3 quarks. We study the interesting case where the extra quark is an isosinglet, determining the allowed range for χ and the effect of a large χ on various low-energy observables, such as CP asymmetries in B meson decays. We also discuss the correlated effects which would be observable at high energy colliders, like decays t→cZ, modifications of the cross section and forward-backward asymmetry in e+e−→tt¯ and the direct production of a new quark.