Small violations of 3 × 3 unitarity, the phase in B0s–B̄0s mixing and visible t → cZ decays at the LHC

Abstract

We show that it is possible to accommodate the observed size of the phase in Bs0−B̄s0 mixing in the framework of a model with violation of 3 × 3 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(−VtsVcbV*tbV*cs) 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−D̄0 mixing without having to invoke long distance contributions. It is also shown that in the present scenario, the observed size of D0−D̄0 mixing constrains χ' ≡ arg(−VcdVusVcs*V*ud) to be of order λ4, which is significantly smaller than what is allowed in generic models with violations of 3 × 3 unitarity.