New physics in the third family and its effect on low-energy data

We investigate, in detail, a model in which the third family fermions are subjected to an SU(2) dynamics different from the first two families. Constrained by the precision Z-pole data, the heavy gauge boson mass is bounded from below to be about 1.7 TeV at the 2σ level. The flavor-changing neutral current (FCNC) in the lepton sector can be significant in τ↔e and τ↔μ transitions. In the latter case, the ratio $Br({{→}}{{τ}}{μ}{{{ν}}}_{{μ}}{{ν}}_{{τ}})/Br({{→}}{{τ}}e{{{ν}}}ₑ{{ν}}_{{τ}})$ and $Br({{→}}{{τ}}{μ}{μ}{μ})$ can constrain the model better than CERN LEP or SLC data in some region of the parameter space. Furthermore, FCNC's are unavoidable in the quark sector. Significant effects to the ${B}⁰{-}{B}⁰$ and ${D}⁰{-}{D}⁰$ mixing and the rare decays of the K and B mesons, such as ${K}^{±{}}{→}{{π}}^{±{}}{ν}{{ν}},$ ${{→}}{b}s{ν}{{ν}},$ ${B}ₛ{→}{{τ}}⁺{{τ}}^{{-}},$ ${{μ}}⁺{{μ}}^{{-}},$ and ${B}s,d{→}{{μ}}^{±{}}{{τ}}^{{∓}},$ are expected.