Flavor-changing neutral current

In particle physics, flavor-changing neutral currents or flavour-changing neutral currents (FCNCs) are a class of hypothetical interactions between elementary particles. These interactions would change a particle's flavor (i.e., its type, such as a strange quark changing into a down quark) without changing its electric charge.

In the Standard Model of particle physics, the dominant "tree-level" interactions cannot produce FCNCs. This is described by the GIM mechanism. However, FCNCs can occur through more complex, higher-order processes (so-called "loop diagrams"), but these are extremely rare. Because FCNCs are so heavily suppressed in the Standard Model, physicists consider them a "zero-background" phenomenon. Any clear observation of an FCNC would be a strong indicator of physics beyond the Standard Model.

Experiments at particle colliders, such as the Large Hadron Collider, and dedicated searches, like the MEG experiment, actively look for evidence of FCNCs. So far, the results have been largely consistent with the predictions of the Standard Model. The lack of observed FCNCs places important constraints on the development of new theories and models in physics.