Reexamination of the Constraint on Topcolor-Assisted Technicolor Models   from R_b
  Recent study on the charged top-pion correction to R_b shows that it is negative and large, so that the precision experimental value of R_b gives rise to a severe constraint on the topcolor-assisted technicolor models such that the top-pion mass should be of the order of 1 TeV. In this paper, we restudy this constraint by further taking account the extended technicolor gauge boson correction which is positive. With this positive contribution to R_b, the constraint on the topcolor-assisted technicolor models from R_b changes significantly. The top-pion mass is allowed to be in the region of a few hundred GeV depending on the models. 
Chongxing Yue, Yu-Ping Kuang, Xue-Lei Wang, Weibin Li