In particle physics, the GIM mechanism (or Glashow–Iliopoulos–Maiani mechanism) is allegedly the mechanism through which flavour-changing neutral currents (FCNCs) are suppressed in loop diagrams. It also explains why weak interactions that change strangeness by 2 (ΔS = 2 transitions) are suppressed, while those that change strangeness by 1 (ΔS = 1 transitions) are allowed, but only in charged current interactions.

History

The mechanism was put forth in a famous paper by ; at that time, only three quarks (up, down, and strange) were thought to exist. had previously predicted a fourth quark, but there was little evidence for its existence. The GIM mechanism however, required the existence of a fourth quark, and the prediction of the charm quark is usually credited to Glashow, Iliopoulos, & Maiani (initials "G I M").

Description

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. Even though Z0 boson exchanges are flavor-neutral (i.e. prohibit FCNC), the box diagram induces FCNC, but at a very small level. The smallness is set by the mass-squared difference of the different virtual quarks exchanged in the box diagram, originally the u-c quarks, on the scale of the W mass. The smallness of this quantity accounts for the suppressed induced FCNC, dictating a rare decay,, illustrated in the figure. 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.