I'm wondering about the Faddeev-Popov method described in Peskin Schroeder and also on page 7 in this link.

What gives them the right to simply add the Gaussian $\omega$ and thus introduce the $\xi$ parameter? It seems so arbitrary to me.

Are there any rigorous derivation of the Faddeev-Popov method?

  • $\begingroup$ Minor comment to the post (v4): Please consider to mention explicitly author, title, etc. of link, so it is possible to reconstruct link in case of link rot. $\endgroup$
    – Qmechanic
    Mar 1, 2014 at 17:02

1 Answer 1


You are always allowed to introduce a new integration variable as long as its not its already being summed over. This might be more clear in discrete form: \begin{align} \int d x \, f (x) & \rightarrow \Delta x\sum _i \,f ( x _i ) \\ & = \big( N \Delta y\sum _j g ( y _j ) \big) \Delta x \sum _i f ( x _i ) \\ \end{align} where $ N \Delta y\sum _j g ( y _j ) = 1 $ (note that it is very important this doesn't depend on $x$). Then, \begin{align} \int d x \, f (x)& \rightarrow N \Delta x \Delta y \sum _{i,j} f ( x _i ) g ( y _j ) \end{align}
The only difference with the Fadeev Poppov procedure is that now $ g ( y ) $ is also a function of a new unphysical parameter,$\xi$. In order to not change the value of the integral over $ f (x) $, the constant $ N $ needs to also change with $ \xi $.

  • 1
    $\begingroup$ You make sense, PS didn't. $\endgroup$ Mar 13, 2014 at 9:24

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