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I want to reproduce the attached Feynman diagram with the feynmp package. In particular the bottom left gluon line goes through the 3 vertices and from the middle vertex another gluon line starts off. I don't know how to make the gluon line not to stop and start again at the middle vertex but go like there is no vertex. How it can be achieved?

The basic diagram without the additional gluon radiation can be obtained with the following code, but adding an additional vertex and gluon line without disturbing anything is more tricky:

\begin{fmffile}{feyngraph}
\begin{fmfgraph*}(150,80)

\fmfleft{i1,i2}
\fmflabel{$g$}{i1}
\fmflabel{$g$}{i2}

\fmfright{o1,o2}
\fmflabel{$t$}{o1}
\fmflabel{$\bar{t}$}{o2}

\fmf{gluon}{i1,v1}
\fmf{gluon}{i2,v1}
\fmf{fermion}{v2,o1}
\fmf{fermion}{o2,v2}
\fmf{gluon, label=$g$, label.dist=0.09w}{v1,v2}

\fmfdot{v1}
\fmfdot{v2}

\end{fmfgraph*}
\end{fmffile}

Feynman diagram

  • 1
    Welcome to TeX.SX! (Although you have been a member for quite some time I have noticed this is your first question.) Could you please post what you have tried so far (as a minimal working example (MWE))? Otherwise this is a "do-it-for-me" kind of question and risks to be closed. – Pier Paolo Jan 5 '15 at 0:27
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They key to adding vertices to a diagram without disturbing it are the following two commands, which have a detailed description in the feynmf documentation :

  • \fmffreeze to not disturb what was drawn before
  • \fmfforce to force the position of a vertex

Thus, adding something like the following code will work:

\fmffreeze   % freezing the drawn elements

\fmfright{v3,o3}   % adding two more vertices
\fmfforce{(0.23w,0.23h)}{v3}   % setting position of the first vertex
\fmfforce{(0.4w,0h)}{o3}   % setting position of the second vertex
\fmfdot{v3}   % drawing the first vertex with a dot
\fmf{gluon, label=$g$, label.dist=0.03w}{v3,o3}   % drawing a gluon line

Finished code for a better-styled version of this diagram can be found on this page.

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