Weak-interaction models with new quarks and right-handed currents

We discuss various weak-interaction issues for a general class of models within the SU(2) × U(1) gauge-theory framework, with special emphasis on the effects of right-handed charged currents and of quarks bearing new quantum numbers. In particular we consider the restrictions on model building which are imposed by the small $K_L-K_S$ mass difference and by the $\Delta I=\frac{1}{2}$ rule, and we classify various possibilities for neutral current interactions and, in the case of heavy mesons with new quantum numbers, various possibilities for mixing effects analogous to $K_L-K_S$ mixing.