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?

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p roton and p' roton? –  David H Jun 30 '13 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! –  Thanos Jun 30 '13 at 8:37
There's a reason I ended my comment with a question mark. I know very little about QFT. –  David H Jun 30 '13 at 9:02
@DavidH: I see. I thought, it was too obvious for you! Sorry! –  Thanos Jun 30 '13 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. –  John Rennie Jun 30 '13 at 9:30