Why Must a W Boson Always Decay into a Quark Anti Quark Pair of the Same Generation?

[Sturm]

TL;DR Summary

Why can't a W Boson decay into for ex. up and anti strange

Hi,
i was wondering why a W Boson must always decay into a quark anti quark pair of the same generation. Why can it not for example decay into an up ant anti strange pair?

 

[Orodruin]

The premise of the question is false. The W does indeed decay into quarks of different generations, but more rarely. The rates are controlled by the kinematics (heavier quarks are harder to produce) and the mixing between generations (which is small and given by the CKM matrix entries).