I'm probably missing something small but I haven't been able to figure this out. In the Feynman rules (for a scalar field that obeys the Klein-Gordon equation), you write a propagator for internal lines as(adsbygoogle = window.adsbygoogle || []).push({});

[tex]

\frac{i}{k^2 - m^2 + i \epsilon}.

[/tex]

The propagator integrand is originally

[tex]

\frac{e^{i k (x-y)}}{k^2 - m^2 + i \epsilon}.

[/tex]

Since we're dealing with an internal line, both exponentials, in [itex]x[/itex] and [itex]y[/itex], are integrated out to delta functions, leaving you with

[tex]

\frac{1}{k^2 - m^2 + i \epsilon}.

[/tex]

That I see, but where does the [itex]i[/itex] in the numerator of the first expression above come from?

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# Propagator in Feynman rules

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