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I've been reading these notes and I can't figure out the why on P.120, it is said that

The fermionic statistics mean that the first diagram has an extra minus sign relative to the ψψ scattering of Figure 25.

Would someone mind explaining it to me? I see that the calculations below, illustrating this, but is there an intuitive way of seeing that the signs of the respective first diagrams in Figures 25, 27 should be opposite?

Thank you.

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    $\begingroup$ Isn't it explained on page 120? $\endgroup$
    – Trimok
    Commented Oct 1, 2013 at 9:43
  • $\begingroup$ Are you asking for an intuitive explanation for the fact that the anti-commutator (which is inherent to fermions) leads to a minus sign? $\endgroup$
    – Frederic Brünner
    Commented Oct 1, 2013 at 9:46
  • $\begingroup$ @Trimok: Yes, I see the calculations below. But I was wondering if there's a Feynman rule or intuition that would allow us to read off that minus sign by looking at the diagram alone, without doing calculations? $\endgroup$
    – Alex
    Commented Oct 1, 2013 at 9:47
  • $\begingroup$ @FredericBrünner: I was wondering if there's a Feynman rule or intuition that would allow us to read off that minus sign by looking at the diagram alone, without doing calculations? $\endgroup$
    – Alex
    Commented Oct 1, 2013 at 9:49
  • $\begingroup$ It could be also helpful Relative Minus signs from different Feynman Diagrams. $\endgroup$
    – Boris Valderrama
    Commented Jul 7, 2022 at 4:06

1 Answer 1

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every time you change two fermions you have to add a minus sign because of spin statistics. lets say for example you have fermion a and fermion b. lets say you have a diagram with two contributions one where fermions a and b interact in some manner, but for the other contribution you have to change the role of fermion a for that of fermion b then you have to add a minus sign.

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