Flavor-changing neutral currents: Theory and future experiments

An SU(2) consistent introduction of particular flavor changes in the neutral current of a generalized Weinberg-Salam model for hadrons is described and examples are discussed, in particular, two $\mathrm{CP}$-violating and flavor-changing five-quark models, which are in agreement with measurements on the $K^0-{\overline{K}}^0$ and the $D^0-{\overline{D}}^0$ system and still allow $t\leftrightarrow c$ and $b\leftrightarrow s$ transitions. The evidence for the strong suppression of a neutral $u\leftrightarrow c$ transition from $D^0-{\overline{D}}^0$ measurements is briefly reviewed. The experimental signatures of the different flavor changes, to be detected or to be ruled out in the future, are discussed.