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Physics
Quantum Physics
Feynman rule for closed fermion loop in QED
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[QUOTE="vanhees71, post: 5696753, member: 260864"] Yes, you always have to read the Feynman diagrams against the direction of the arrows. That's why it is most convenient to let run time from bottom to top of your page and then read the diagram from top to bottom. The additional sign in the closed loop comes from the fact that you have to reorder the fields to be contracted in Wick's theorem being on the ends of the expression to get a propagtor. Due to the fermion nature of the fields this reordering under the time-ordering symbol just gives an additional factor (-1) as mentioned in the fermion-loop Feynman rule. [/QUOTE]
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Physics
Quantum Physics
Feynman rule for closed fermion loop in QED
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