Rs. Chivukula et J. Terning, PRECISION ELECTROWEAK CONSTRAINTS ON TOP-COLOR ASSISTED TECHNICOLOR, Physics letters. Section B, 385(1-4), 1996, pp. 209-217
Using precision electroweak data, we put limits on ''natural'' top-col
or assisted technicolor models. Generically the new U(1) gauge bosons
in these models must have masses larger than roughly 2 TeV, although i
n certain (seemingly unrealistic) models the bound can be much lower.