GIM mechanism

It is named after physicists Sheldon Glashow, John Iliopoulos and Luciano Maiani.

The mechanism was put forth in a famous paper by Glashow, Iliopoulos & Maiani (1970);[1] at that time, only three quarks (up, down, and strange) were thought to exist.

[1] The mechanism relies on the unitarity of the charged weak current flavor mixing matrix, which enters in the two vertices of a one-loop box diagram involving W boson exchanges.

The smallness of this quantity accounts for the suppressed induced FCNC, dictating a rare decay,

If that mass difference were ignorable, the minus sign between the two interfering box diagrams (itself a consequence of unitarity of the Cabibbo matrix) would lead to a complete cancellation, and thus a null effect.