In particle physics, the GIM mechanism (or Glashow–Iliopoulos–Maiani mechanism) is 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.

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. Bjorken & Glashow (1964) had previously predicted a fourth quark,^[2] 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").^[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. 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, ${\displaystyle K_{L}\to \mu ^{+}\mu ^{-))$, 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.

• Das, Ashok; Ferbel, Thomas (2006) [2003]. "Chapter 14   Standard Model and confrontation with data". Introduction to Nuclear and Particle Physics (2nd ed.). Singapore: World Scientific. pp. 345ff. ISBN 981-238-744-7. OCLC 849916889. Retrieved 2024-08-20.
• Iliopoulos, J. (2010). "Glashow–Iliopoulos–Maiani mechanism". Scholarpedia. 5 (5): 7125. Bibcode:2010SchpJ...5.7125I. doi:10.4249/scholarpedia.7125.
• Popescu, Bogdan F. (February 2006). "Weak interactions (1)" (course notes). Physics 842. University of Cincinnati. pp. 45–48. weak1.ppt. Archived from the original on 11 March 2012. Retrieved 4 September 2010.