Flavor-changing neutral current

In theoretical physics, flavor changing neutral currents (FCNCs) are dangerous fermionic bilinear expressions. If they occur in the Lagrangian, they may induces processes in which the flavor of the fermions is changed in a process that has not been observed so far.

For example, a new particle (boson) $S$ may couple both to the electron as well as the tau lepton via the term

$S{\bar {\psi }}_{e}\psi _{\tau }$

The electric charge of S clearly must vanish; this is the source of the word "neutral". A Feynman diagram with S as the intermediate particle is able to convert a tau lepton into an electron (plus a photon, to conserve energy). Such a decay has not been observed. Only the processes changing the charge of the fermions are also capable to change the flavor.

FCNCs are generically predicted by theories that attempt to go beyond the Standard Model, such as the models of supersymmetry or technicolor. Their suppression that is necessary for an agreement with observations is an important task for model-building.

The reason why people focus on neutral flavor changing currents as opposed to deviations in flavor changing charged currents is because the weak neutral current doesn't change flavors whereas the weak charged currents do and it's easier to measure a tiny deviation from zero.