Extended technicolor contribution to the Zbb vertex

The deviation of the experimental value of $R_b \equiv \Gamma_b/\Gamma_h$ from the standard-model prediction can be naturally explained in the extended technicolor scenario.The diagonal extended technicolor gauge boson which flavor-diagonally couples with both the ordinary fermions and the techni-fermions plays an important role in order to have the appropriate radiative correction to the $Zbb$ vertex. But since the diagonal extended technicolor gauge boson gives too large positive contribution to the $T$ parameter, we need some new mechanisms which generate negative contribution to the $T$ parameter.