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Why are the legs needed in a penguin diagram?

  1. Sep 17, 2008 #1
    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?

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  3. Sep 17, 2008 #2
    If you only had b->s, or s->d, where would the extra energy go?
    For momentum and energy conservation you need another particle to be involved.
  4. Sep 17, 2008 #3


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    I have never formalised it, for me it's just intuitive. :wink:
  5. Sep 17, 2008 #4

    Vanadium 50

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    If you have a b->s transition with no additional particles coming out, that means you have the wrong basis and can re-diagonalize so this doesn't occur.
  6. Oct 7, 2008 #5


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    rephrasing JustinLevy's response in more brute-force terms: if you go ahead and write down that loop integral without the additional photon or gluon, it vanishes - go ahead and try it.
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