# How do I make a circle of fermions in feynmp? (a self-energy photon)

I'm trying to draw a photon turning into a circle formed by one fermions going one way and the other going the other way, but they both go up. Here is my MWE:

\documentclass[amsmath,amssymb,aps,12pt,eqsecnum]{revtex4}
\usepackage[a4paper]{geometry}
\usepackage{slashed}
\usepackage{simplewick}
\usepackage{feynmp-auto}
\usepackage[pdftex]{graphicx}
\usepackage{textcomp}
\usepackage{gensymb}

\begin{fmfgraph*}(80,120)
\fmftop{t1,t2}
\fmfbottom{b1,b2}
\fmf{fermion}{b1,v1,t1}
\fmf{fermion}{b2,v4,t2}
\fmffreeze
\fmf{photon}{v1,v2}
\fmf{phantom}{v2,v3}
\fmf{photon}{v3,v4}
\fmffreeze
\fmf{fermion,left,tension=0}{v2,v3}
\fmf{fermion,right,tension=0}{v3,v2}
\end{fmfgraph*}
\end{fmffile}


This is what I'm currently getting, any ideas?
(i.e. I would like one of the fermions in the middle to go down and form a circle)
Thanks for the edit egreg!

Change that last fermion to

\fmf{fermion,left,tension=0}{v3,v2}


Since it's going backwards from v3 to v2 it needs to bend left not right. Then you get:

Here's my complete, minimal compilable version of your picture:

\documentclass[border=1cm]{standalone}
\usepackage{feynmp-auto}
\begin{document}
\begin{fmfgraph*}(80,120)
\fmftop{t1,t2}
\fmfbottom{b1,b2}
\fmf{fermion}{b1,v1,t1}
\fmf{fermion}{b2,v4,t2}
\fmffreeze
\fmf{photon}{v1,v2}
\fmf{phantom}{v2,v3}
\fmf{photon}{v3,v4}
\fmffreeze
\fmf{fermion,left,tension=0}{v2,v3}
\fmf{fermion,left,tension=0}{v3,v2}
\end{fmfgraph*}
\end{fmffile}
\end{document}

• I would like to generate the same diagram, but then with a dashed line coming out of the loop in the middle. It should go to the top. Any ideas on how to do that? – Funzies Feb 26 '16 at 13:38
• I tried \begin{fmfgraph*}(75,50) \fmfleft{i1,o1} \fmfright{i2,o2} \fmftop{t1} \fmf{fermion}{i1,v1,o1} \fmf{fermion}{i2,v5,o2} \fmf{photon}{v1,v2} \fmf{phantom}{v2,v4} \fmf{photon}{v4,v5} \fmf{fermion,left,tension=0}{v2,v3} \fmf{fermion,left,tension=0}{v3,v4} \fmf{fermion,left,tension=0}{v4,v2} \fmf{dashes}{v3,t1} \end{fmfgraph*} But it looks horrible. – Funzies Feb 26 '16 at 13:49
• @Funzies - the usual thing to do here is to ask a new question as a new question not just as a comment. – Thruston Feb 26 '16 at 16:38
• @Funzies as I didn't see you posting a separate question about this, I'll suggest solution here. You should first put an \fmffreeze command after the diagram in this answer, and after that add your t1 and v3 vertices. And with \fmfforce you can set their position as you like. See for example the TeX code of this diagram with a gluon line drawn after \fmffreeze. – BartoNaz Jul 7 '16 at 11:04