GIM Mechanism & FCNC: Explained

[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

 

[sir-pinski]

The GIM Mechanism, also known as the Glashow, Iliopoulos, and Maiani Mechanism, is a theoretical framework in particle physics that explains the suppression of Flavor Changing Neutral Currents (FCNC) at the tree level. FCNC transitions involve the exchange of a neutral particle between two fermions of different flavors, which violates the conservation of flavor in the Standard Model of particle physics.

The GIM Mechanism proposes the existence of a fourth quark, known as the charm quark, which was later confirmed by experimental evidence. This fourth quark, along with the other three quarks in the Standard Model, forms a "quark box" that cancels out the FCNC contributions at the tree level. This means that FCNC transitions are only possible through higher-order loop diagrams, making them highly suppressed and rare.

In simpler terms, the GIM Mechanism introduces an additional quark to the Standard Model that helps to cancel out the FCNC transitions, ensuring that they occur at a much lower rate. This is important because FCNC transitions are considered to be highly unnatural and are not observed in nature, so the GIM Mechanism provides a theoretical explanation for their suppression.

In summary, the GIM Mechanism plays a crucial role in maintaining the validity of the Standard Model by explaining why FCNC transitions are highly suppressed at the tree level. It also serves as a foundation for future developments in particle physics, such as the search for new physics beyond the Standard Model.