GIM mechanism
= GIM mechanism
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<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.