https://www.physicsforums.com/archive/index.php/t-187870.html
"If one looks at WW scattering, it is pretty easy to see that the process scales as energy squared (sometimes called s). This means that as energy increases, the probability for WW to scatter increases. At some point, this...
In the penguin diagrams you have a loop that allows the quark to change into another quark of the same charge such as b->s or s->d. I understand why the loop with the quark and W are needed to get you from b->s or s->d but why (in the example of the attached picture) do you need the gluon vertex...