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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 $τ\leftrightarrow e$ and $τ\leftrightarrow μ$ transitions. In the latter case, the ratio $Br(τ\to μ\bar{ν_μ} ν_τ)/ Br(τ\to e \bar{ν_e} ν_τ)$ and $Br(τ\to μμμ)$ can constrain the model better than LEP/SLC data in some region of the parameter space. Furthermore, FCNCs are unavoidable in the quark sector. Significant effects to the $B^0-\bar{B^0}$ mixing and the rare decays of the K and B mesons, such as $K^\pm \to π^\pm ν{\bar ν}$, $b \to s ν{\bar ν}, B_s \to τ^+τ^-, μ^+μ^-$ and $B_{s,d} \to μ^\pm τ^\mp$, are expected.