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I'm studying electron-electron scattering, starting with the Dirac equation it ends up calculating the invariant transition amplitude, defined as:

[tex]-iM=(ie{\overline{u}^f}_A}\gamma^\mu u^i}_A) \frac{-ig_{\mu\nu}}{q^2}(ie{\overline{u}^f}_B}\gamma^\nu u^i}_B)[/tex]

With [tex]u_A[/tex] and [tex]u_B[/tex] the electron spinors (initial and final)

After this it says that in the nonrelativistic limit a Dirac-delta spin-spin term arises in the corresponding potential. How is that?

Could anyone explain where does this dirac-delta come from? (And btw a better book to study QED)

Thanks

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# Dirac-delta term in a non-relativistic interaction

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