A Vector-like Extension of the Standard Model
  A vector-like extension of the standard model for heavier quarks and leptons with $SU(2)\times U(1)$ gauge symmetry and only one Higgs doublet is examined. This scheme incorporates infinitely many fermions and avoids the appearance of a strongly interacting sector. The model is perturbatively well controllable, and constraints such as $b\rightarrow s\gamma$ and GIM suppression in general are naturally satisfied if the on-set of heavier fermion mass scale is chosen at a few TeV. In this scheme, the physical Higgs particle generally mediates leptonic as well as quark flavor-changing processes at a rate below the present experimental limit. Vector-like models, if suitably defined, provide a viable and interesting scheme for physics beyond 1 TeV. 
Kazuo Fujikawa