I was suddenly confused by the calculation of symmetry factors of Feynmann diagrams.(adsbygoogle = window.adsbygoogle || []).push({});

For example, in Peskin's text book, as the attached pdf file,

Below eq(4.45), he calculated the symmetry factor in detail,

however, I was confused by the last 1/2 factor.

I'm trying to realize this factor as for the double-counting when we connect two of four legs of [tex]u[/tex]-vertex. But, I am not quite convinced by this above argument.

If we label the four legs of [tex]u[/tex]-vertex with 1,2,3,4.

Connecting legs 1,2 to form a loop, and then connecting legs 3,4 to [tex]w-[/tex]vertex is different from connecting legs 3,4 to form a loop and then connecting 1,2 to [tex]w-[/tex]vertex, right? In this way there is no double-counting!?

Where does that 1/2 factor come from?

Thanks.

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# Symmetry factor of diagrams

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