Heavy Fermions in the Standard Sequential Scheme

Some of the qualitatively new features associated with the possible existence of heavier Fermions in the standard sequential scheme are discussed. As for the heavy Fermions with masses less than the W-boson mass Mw, the weak decay of constituent quarks in the lowest 'S, state of the heavy quarkonium is expected to become appreciable for MQ<:,20 GeV and its implications are studied. We then discuss the stability of the Higgs mechanism in the presence of superheavy Fermions with masses greater than Mw. The weak interaction mediated by the W-boson in general becomes strong for such heavy objects, but some of the models such as the vector-like scheme could lead to a dynamically stable limit for superheavy Fermions.