# The GIM Mechanism

1. May 15, 2010

### aw7879

Can Anyone explain how the existance of more than one generation of quark means that flavour change neutral currents do not occour at tree level? How is a flavour changing neutral current deffined anyway? I understand that it has something to do with Cabbibo suppression

2. May 15, 2010

### ansgar

FCNC is that you can change flavour with a neutral boson, there are no nondiagonal interactions in the higgs sector and the Z and A sector of the SM, only non diagonal (CKM) in the W+/W- sector.

"how the existance of more than one generation of quark means that flavour change neutral currents do not occour at tree level?"

is not true, can you show where you find it?

3. May 16, 2010

### aw7879

It is my understanding that this argument of no flavour changing neutral currents at tree level was used to predict the existance of the charm quark. The rule basically states that there cannot be a change in flavour whilst the charge current is neutral.

http://en.wikipedia.org/wiki/Flavor_changing_neutral_current