I am reading a nice book (Quarks and Leptons, by Halzen and Martin) about particle physics. It states that the general form of the propagator of a virtual particle is:(adsbygoogle = window.adsbygoogle || []).push({});

[tex]

\dfrac{i\sum_{\text{spins}}}{p^2 - m^2}

[/tex]

I see that this is the case for the Dirac propagator:

[tex]

\dfrac{i(\displaystyle{\not}{p} + m)}{p^2 - m^2} = \dfrac{i\sum_{s}u_s(p)\bar{u}_s(p)}{p^2 - m^2}

[/tex]

but how can I prove that always holds?

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# Virtual particle propagators in QFT

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