A flavor-changing neutral current changes fermion flavor without changing electric charge. Such currents are absent at tree level in the Standard Model and are strongly suppressed at loop level by the Glashow-Iliopoulos-Maiani mechanism.
Unitarity of the CKM matrix cancels flavour-independent contributions to weak flavour-changing neutral amplitudes. In a quark loop, equal internal masses would give a common contribution whose off-diagonal CKM sum vanishes; unequal masses leave a suppressed nonzero amplitude. This explains why neutral kaon mixing occurs through loops rather than a tree neutral-current vertex.
The Glashow-Iliopoulos-Maiani mechanism uses the unitarity of the quark mixing matrix to cancel flavor-changing neutral-current amplitudes when equal-charge quark masses are degenerate. The remaining loop amplitudes are suppressed by quark-mass differences.
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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.