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I want to use feynmp to draw the one-loop correction to the vertex in phi^3 theory. This is just a circle with three evenly-spaced incoming lines. The thing is I want the figure to be dashed, so using pre-defined polygons won't work.

The best I can do is this, but I want it to be a circle in the middle, not a tragically deformed hexagon:

\unitlength = 1mm




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Welcome to TeX.sx! A tip: If you indent lines by 4 spaces, they'll be marked as a code sample. You can also highlight the code and click the "code" button (with "{}" on it). – egreg Dec 9 '12 at 23:24
up vote 3 down vote accepted

I normally use TikZ for typesetting Feynman diagrams, but I don't typeset that many. I looked a little in the documentation feynMF and came up with this:

\fmfleft{i1,i2} % External lines (left) i1 & i2
\fmfright{o1} % External line (right) o1
\fmf{dashes}{i1,v1} % Dashed line from i1 to vertex v1
\fmf{dashes}{i2,v2} % Dashed line from i2 to vertex v2
\fmf{dashes}{o1,v3} % Dashed line from o1 to vertex v3
\fmf{dashes,left=1/2,tension=2/3}{v1,v2,v3,v1} % The loop: left curves the lines & tension change the size of the loop

Output: loop

Please change the values for left and tension and see what happens.

I don't like this package. Hmm... maybe I just don't understand it properly.

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This is exactly it!! Thank you!! I had been trying to do something with left, but I hadn't realized you could set left=1/2 as you did. – Alex Dec 11 '12 at 1:19

I'm not familiarized with feynmp package nor with phi^3 theory, so I'm not really sure of the output you want. Perhaps something like the following?


If tikz is an option...

\tikz[baseline=0, scale=0.4]{
\draw[dashed] circle(1cm);
\foreach \angle in {-60,60,180} {
 \draw (\angle:1cm) -- (\angle:2cm);

See also Feynman Diagrams with tikz and http://www.hep.wisc.edu/~grogg/files/2011/TikzDiagrams/

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Thanks for this, this is exactly what I wanted. I had been stubbornly trying to avoid learn another package, but you've convinced me to give tikz a try. – Alex Dec 10 '12 at 21:47

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