GIM Mechanism and FCNC

[maani]

Hi everyone! Can somebody briefly explain me what the GIM Mechanism (Glashow, Iliopoulos, and Maiani) is and why does it not allow FCNC transitions at tree level. Thanks in advance!

[humanino]

By introducing charm, you can compensate the d Cabibbo rotation so that the neutral current remains diagonal. Basically, now you can rotate the (c,s) doublet as well so that it remains orthogonal to the (u,d).

However, the suppression of FCNC is the rather "trivial" part of the GIM mechanism. On top of that, the GIM provides \Delta S = 2 suppression.
Glashow-Iliopoulos-Maiani mechanism (http://www.scholarpedia.org/article/Glashow-Iliopoulos-Maiani_mechanism)