Non--perturbative evidence for non--decoupling of heavy fermions

We investigate, using a $1/N$ expansion, the behavior of a parameter in the scalar--fermion sector of the standard model that shows perturbative non--decoupling as the fermion becomes heavy. This low energy parameter is related to the $S$ parameter defined through the $W_3-B$ vacuum polarization tensor. We obtain the leading $1/N$ contribution to this parameter that, if expanded perturbatively, collapses to its constant one--loop result; remarkably all the higher--order terms in the series vanish. Non--perturbatively, however, we find that as the mass of the fermion approaches the built--in cutoff scale of the theory --- the triviality scale --- the parameter is highly dependent on the implementation of the cutoff; it is non--universal, and shows non--decoupling.