# Parallel lines in Feynman Diagram

I am trying to draw this feynman diagram

The part that's a bit tricky is the parallel lines, in the laeft and right side of the diagram. How to do that?

I've tried searching in feynmf's manual. I saw a similar diagram, which was using

\fmffreeze
\fmfi{plain}{vpath (__i1,__b) shifted (thick*(0,2))}
\fmfi{plain}{vpath (__i1,__b) shifted (thick*(1,-2))}


but in my case it only produces 2 lines on the left side...

My code is

\documentclass{article}
\usepackage{feynmp-auto}
\unitlength=1mm
\begin{document}
\begin{fmffile}{ElasticScattering}
\fmfframe(1,7)(1,7){
\begin{fmfgraph}(40,25)
\fmfleft{i1,i2}
\fmfright{o1,o2}
\fmfbottom{b}
\fmf{fermion}{i2,v1,o2}
\fmf{photon}{v1,b}
\fmfblob{.15w}{b}
\fmf{phantom}{i1,b,o1}
\fmffreeze
\fmfi{plain}{vpath (__i1,__b) shifted (thick*(0,2))}
\fmfi{plain}{vpath (__i1,__b) shifted (thick*(1,-2))}
\end{fmfgraph}}
\end{fmffile}
\end{document}


My result is

-

\documentclass{article}
\usepackage{feynmp-auto}
\unitlength=1mm
\begin{document}
\begin{fmffile}{ElasticScattering}
\begin{fmfgraph*}(40,25)
% External vertices on the left
\fmfleft{i1,i2}
% External vertices on the right
\fmfright{o1,o2}
% Labels on external vertices
\fmflabel{$$k$$}{i2}
\fmflabel{$$p$$}{i1}
\fmflabel{$$k'$$}{o2}
\fmflabel{$$p'$$}{o1}
% Add a fermion line between i2', new internal vertex v' and o2'
\fmf{fermion}{i2,v,o2}
% Add a single plain line between i1', new internal vertex b' and o1'
\fmf{plain}{i1,b,o1}
% Add photon line, with label, between v' and b'
\fmf{photon,label=$$q$$}{v,b}
% Insert a blob at b'
\fmfblob{.15w}{b}
\fmffreeze
% Add two plain lines between i1' and b', one shifted above and the
% other below the already present plain line
\fmfi{plain}{vpath (__i1,__b) shifted (thick*(-0.5,2))}
\fmfi{plain}{vpath (__i1,__b) shifted (thick*(0.5,-2))}
% Same as above, between b' and o1'
\fmfi{plain}{vpath (__b,__o1) shifted (thick*(0.5,2))}
\fmfi{plain}{vpath (__b,__o1) shifted (thick*(-0.5,-2))}
\end{fmfgraph*}
\end{fmffile}
\end{document}


If you want an empty circle at b vertex (as in your original example) instead of a shaded one, replace \fmfblob{.15w}{b} with \fmfv{decor.shape=circle,decor.filled=empty,decor.size=.15w}{b}.

-
Thank you very much once again for your help! I understand that \fmfi{plain}{vpath (__i1,__b) shifted (thick*(-0.5,2))} adds a line above/bellow. How do you define the slope? I suspect it must be the thick argument... –  Thanos Jul 4 '13 at 14:32
@Thanos that's probably metafont/metapost code, but... I don't know neither :-( That's why I'd prefer a package based on TikZ but with a feynmp-like syntax. I supposed (-0.5,2) stands for the shift value, I adjusted it to get a visually acceptable result and it seems that worked :-) –  giordano Jul 4 '13 at 15:24
I couldn't agree more about the tikz-feynmp` mixture! If only I could do such thing!!! –  Thanos Jul 4 '13 at 15:27