Constraints on new chiral fermions in the standard model

In the light of the new experimental data on standard-model parameters, we investigate the implications of new types of chiral fermions with masses above present experimental detection capabilities. We show that new chiral fermions may exist, but only under very special circumstances; one is {ital R}-parity-violating supersymmetry. The analysis is based on the present bounds on the number of light neutrals and on the lack of flavor-changing neutral currents. No such limits exist for vectorlike fermions of the type anticipated in supersymmetric and in some grand unified extensions.