Hey, I've got this problem from Peskin & Schroeder (chapter 15). I'm not particularly confident with functional integration, as I'm pretty new to it, and working through such a book by myself is pretty tricky in places. Well here goes(adsbygoogle = window.adsbygoogle || []).push({});

The Wilson Loop for QED is defined as

[tex]U_p(z, z)=\exp \left[-ie\oint_pdx^{\mu}A_{\mu}\right][/tex]

With the Wilson line defined similarly (just change it so that there's not a closed contour integral and with the end points (z,z) changed to (z, y), or whatever you like).

Where A is the photon field, the gauge connection asociated with transformations in U(1).

Now it says: using functional integration, show that the expectation of the Wilson loop for the electromagnetic field free of fermions is

[tex]\langle U_p(z, z)\rangle =\exp \left[-e^2\oint_pdx^{\mu}\oint_pdy^{\nu}\frac{g_{\mu\nu}}{8\pi^2(x-y)^2}\right][/tex]

Where x and y are integrated over the closed loop P.

I think the Feynman propogator might be useful here, so to save anyone having to look it up,

[tex]D_F^{\mu\nu}(x-y)=\int\frac{d^4q}{(2\pi )^4}\frac{-ig^{\mu\nu}e^{-ip\cdot (x-y)}}{q^2+i\epsilon }[/tex]

(The imaginary term in the denominator of the integrand is the application of the Feynman boundary conditions, ensuring the convergence of the Gaussian integral involved in the derivation of the propogator.)

I have a vague idea of how to go about it, but I'm not particularly confident about it, it's finding the relevant starting point that's causing me problems, i.e. putting together and computing the functional integral for the expectation. I'm just going through this to gain some confidence in functional integration etc. so if anyone can give a few pointers as to going about this it'd be much appreciated. This isn't a homework question, if that perturbs anyone from helping me, I doubt my A level teacher would set something like this :).

Cheers

**Physics Forums | Science Articles, Homework Help, Discussion**

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Expectation of the Wilson Loop

**Physics Forums | Science Articles, Homework Help, Discussion**