# What does this feynman diagram represent?

This feynman diagram represents the elastic proton scattering of electrons.

The $e^-$ has an initial momentum $k$ and a final $k^\prime$. The circle represents that the target(the proton that is) has a structure and $q$ is the momentum transfer between the electron and the proton, which is mediated by a photon.

What does the $p$ and $p^\prime$ lines mean?

• p roton and p' roton? Jun 30, 2013 at 8:27
• @DavidH: Thank you very much for your comment! So what does the circle mean? The proton is suppossed to be at rest before the scattering, but if $p$ is the proton before the impact, this diagram shows that it is a moving target. That's the point where I am confused! Jun 30, 2013 at 8:37
• There's a reason I ended my comment with a question mark. I know very little about QFT. Jun 30, 2013 at 9:02
• @DavidH: I see. I thought, it was too obvious for you! Sorry! Jun 30, 2013 at 9:14
• I think you missed my point. The four-velocity of a particle can never be zero - it always has the value $c$. No Feynmann diagram would ever show a stationary particle because no such thing exists. Jun 30, 2013 at 9:30