Experimental implications of heavy, isosinglet quarks and leptons

A study is presented of low-energy family-mixing processes which would be induced by a heavy, isosinglet, negatively charged lepton or a heavy, isosinglet quark with charge (2/3) or -(1/3). With certain assumptions on the values of the mixing parameters, it is found that such isosinglets with masses in the 10--100-GeV range could exist without generating effects that would violate present low-energy experimental bounds.