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 top-color-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 top-color-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.

• Received 14 January 2000

DOI:https://doi.org/10.1103/PhysRevD.62.055005