Decoupling or nondecoupling: Is that the Rb question?

The top quark is well known for the nondecoupling e ects it implies in and Rb. The recent experimental Rb data exhibits a disagreement with the SM prediction at more than the 3 level. It is tempting to explore whether this might be due to nondecoupling New Physics e ect, opposite to those of the top. We investigate this issue in the context of models with an extra family of right or left handed, singlet or doublet quarks. It is shown that, contrary to what one might naively expect, the nondecoupling properties of a mirror t0 do not have an impact on Rb, due to a conspiracy of the mixing angles, imposed by the requirement that there be no b b0 mixing. Our analysis disagrees with an analysis performed independently, for that has ignored the charged current couplings with right-handed nontrivial multiplets. They are needed in order to extract the correct physical decoupling properties.