- #1
guimauve
- 5
- 0
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 to make the additional particle/anti-particle pair? Why can't the diagram just be a "b" coming in, an intermediate loop with a W and a u,c,t quark, and then an "s" going out? Why does there need to be an additional vertex in the loop where a gluon, photon or Z branches off?