Small violations of unitarity, the phase in Bs0-Bs0 and visible t→cZ decays at the LHC

We show that it is possible to accommodate the observed size of the phase in Bs0-Bs0 mixing in the framework of a model with violation of 3x3 unitarity. This violation is associated to the presence of a new Q=2/3 isosinglet quark T, which mixes both with t and c and has a mass not exceeding 500 GeV. The crucial point is the fact that this framework allows for χ≡arg(-VtsVcbVtb*Vcs*) of order λ, to be contrasted with the situation in the standard model, where χ is constrained to be of order λ2. We point out that this scenario implies rare top decays t→cZ at a rate observable at the LHC and |Vtb| significantly different from unity. In this framework, one may also account for the observed size of D0-D0 mixing without having to invoke long-distance contributions. It is also shown that in the present scenario, the observed size of D0-D0 mixing constrains χ'≡arg(-VcdVusVcs*Vud*) to be of order λ4, which is significantly smaller than what is allowed in generic models with violations of 3x3 unitarity.