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Feynman Diagram term ordering

  1. Dec 10, 2012 #1
    I'm reading something which highlights the Feynman rules for QED, and it talks about what happens if you go along a fermionic line. For example this picture:

    According to these notes, there would be a term from the incoming electron on the bottom left (call it A) then a term from the vertex (call it B) then the propagator (C) then the next vertex (D) and finally the outgoing electron (E).

    So the notes would then say to do that part you would have:

    I get all that.. but what I don't understand is where to put the terms from the external photon lines.

    I would guess that it would be somewhere before or after the vertices.
    Can someone just tell me where they would go?

  2. jcsd
  3. Dec 10, 2012 #2
    The reason you have to be careful about ordering the terms along the fermion line is that these terms are matrices, so the order in which they are multiplied matters.

    The terms corresponding to photon lines are just numbers, not matrices, so it doesn't matter where you put them; you'll get the same answer.
  4. Dec 11, 2012 #3
    you will have to just write in the order of increasing time.you should just care that propagators should lie between free particle states overall.it is lorentz covariant.
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