Effects of mixing with quark singlets

The mixing of the known quarks with new heavy singlets can modify significantly some observables with respect to the standard model predictions. We analyze the range of deviations permitted by the constraints from precision electroweak data and flavor-changing neutral processes at low energies. We study top charged current and neutral current couplings, which will be directly tested at top factories, such as the CERN LHC and the DESY TESLA. We discuss some examples of observables from K and B physics, such as the branching ratio of K-L-->pi(0)nu(nu) over bar, the B-s(0) mass difference, or the time-dependent CP asymmetry in B-s(0)-->Ds+Ds-, which can also show large new effects.