Precision Electroweak Constraints on Top-Color Assisted Technicolor
  Using precision electroweak data, we put limits on ``natural'' top-color assisted technicolor models. Generically the new $U(1)$ gauge bosons in these models must have masses larger than roughly 2 TeV, although in certain (seemingly unrealistic) models the bound can be much lower. 
R.S. Chivukula and J. Terning