Isosinglet down quark mixing and [Formula Presented] violation experiments

We confront the new physics models with extra isosinglet down quarks in the new (Formula presented) violation experimental era with (Formula presented) and (Formula presented) measurements, (Formula presented) events, and (Formula presented) limits. The closeness of the new experimental results to the standard model theory requires us to include full standard model (SM) amplitudes in the analysis. In models allowing mixing to a new isosinglet down quark, as in (Formula presented) flavor changing neutral currents are induced that allow a (Formula presented) mediated contribution to (Formula presented) mixing and which bring in new phases. In (Formula presented) (Formula presented) and (Formula presented) plots we still find much larger regions in the four down quark model than in the SM, reaching down to (Formula presented) (Formula presented) (Formula presented) and (Formula presented) down to zero, all at (Formula presented) We elucidate the nature of the cancellation in an order (Formula presented) four down quark mixing matrix element which satisfies the experiments and reduces the number of independent angles and phases. We also evaluate tests of unitarity for the (Formula presented) Cabibbo-Kobayashi-Maskawa submatrix. © 2002 The American Physical Society