# Allowed Feynman diagrams for charged current weak interaction

I am reading about the charged current weak interaction. The notes I am using state that, with the charged current weak interaction, transitions across quark generations are possible, but suppressed. They then state that the Feynman diagrams (modulo crossing symmetry)

$$d \to W^- + u$$,

$$s \to W^- + u$$,

and $$b \to W^- + u$$

Firstly, I'm not too sure about in which sense these are "suppressed". Can anyone shed any light on this?

Secondly, I am not sure how to interpret the wording above. Are these three simply examples of vertices which are allowed, or is this a list of absolutely all of the vertices which are allowed?

Thanks!

$$d' = V_{ud} d + V_{us} s + V_{ub} b$$
where the coefficient $$|V_{ud}|^2$$ is much bigger than the other two. The CKM matrix is just the collection of these coefficients.
So even though only the $$d'$$ quark can go to the $$u'$$ quark, since $$d'$$ has a $$s$$ component this means that an $$s$$ quark can go to a $$u$$ quark (and similarly for the transition $$s'$$ to $$c'$$, $$c'$$ has a $$u$$ component), but it will be suppressed by factors of the CKM matrix coefficients