Editing Feynman diagram (section)

==== Scattering of fermions ====
    {|align="right"
    |[[File:Feynman-diagram-ee-scattering.png|class=skin-invert-image|right|thumb|360px|The Feynman diagram of the term <math>N\bar\psi(x)ie\gamma^\mu\psi(x)\bar\psi(x')ie\gamma^\nu\psi(x')A_\mu(x)A_\nu(x')</math>]]
    |}
The [[Wick's theorem|Wick's expansion]] of the integrand gives (among others) the following term

:<math>N\bar\psi(x)\gamma^\mu\psi(x)\bar\psi(x')\gamma^\nu\psi(x')\underline{A_\mu(x)A_\nu(x')}\;,</math>

where

:<math>\underline{A_\mu(x)A_\nu(x')}=\int\frac{d^4k}{(2\pi)^4}\frac{-ig_{\mu\nu}}{k^2+i0}e^{-ik(x-x')}</math>

is the electromagnetic contraction (propagator) in the Feynman gauge. This term is represented by the Feynman diagram at the right. This diagram gives contributions to the following processes:
# e<sup>−</sup> e<sup>−</sup> scattering (initial state at the right, final state at the left of the diagram);
# e<sup>+</sup> e<sup>+</sup> scattering (initial state at the left, final state at the right of the diagram);
# e<sup>−</sup> e<sup>+</sup> scattering (initial state at the bottom/top, final state at the top/bottom of the diagram).