Nonperturbative evidence for nondecoupling 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 nondecoupling as the fermion becomes heavy. This low-energy parameter is related to the S parameter defined through W3-B mixing. We obtain the leading 1/N contribution to this parameter that, if expanded perturbatively, collapses to its constant one-loop result. Nonperturbatively, we find that as the mass of the fermion approaches the triviality scale the behavior of the parameter is nonuniversal and shows nondecoupling. © 1993 The American Physical Society.