# Feynman Diagram in QED non-relativistic limit

I am following Peskin & Schroeder to read a Feynman diagram. But in this given image, they used the non-relativistic limit to write the incoming fermion gamma mu product. How did he derive the bottom formula? Any explanations in detail?

• What is your question explicitly? Mar 27, 2015 at 11:10
• That seems pretty clear to me. The OP is asking how to derive the nonrelativistic limit form of the quoted relation (or equality) below. Simply enough. I don't say I have an answer though, but I can spend some time thinking about it. Dec 22, 2019 at 5:48

Eq 3.55 of the pdf online version of Peskin and Schroder shows that: $$u^\dagger u=2E_p\xi^\dagger \xi$$ and since $$\gamma_0^2=1$$ we know that $$\bar u\gamma_0u=u^\dagger u$$ and, in the non-relativistic limit (i.e. $$p<) $$E_p\to m\;.$$ (N.b., $$c=1$$)