Flavor-changing neutral current

Flavor-changing neutral currents (FCNCs) are processes in particle physics that involve a change in the flavor of a quark or lepton without the emission or absorption of a charged particle (such as a W or Z boson, which are responsible for charged currents). Instead, these processes are mediated by neutral particles, typically the Z boson or neutral Higgs bosons. In the Standard Model of particle physics, FCNCs are highly suppressed and can occur only at loop level (i.