3

This diagram:

enter image description here

is generated from the following code:

\documentclass{article}
\usepackage{feynmp-auto}
\begin{document}

\begin{fmffile}{crazy}
  \begin{fmfgraph*}(140,85)
  \fmfstraight
  \fmfleft{i1,i2}
  \fmfright{o1,o2,o3}
  \fmflabel{$\overline{d}$}{i2}
  \fmflabel{$u$}{i1}
  \fmf{fermion}{i1,v1}
  \fmf{fermion,label=$d$,label.side=right}{v1,v2}
  \fmf{fermion}{v2,i2}
  \fmf{gluon,label=$g$}{v2,v3}
  \fmf{fermion}{v3,o3}
  \fmf{fermion}{v3,o2}
  \fmf{boson}{v1,o1}
  \fmflabel{$W^+$}{o1}
  \fmflabel{$t$}{o2}
  \fmflabel{$\overline{t}$}{o3}
  \end{fmfgraph*}
\end{fmffile}

\end{document}

Would someone be able to explain the proper tensioning/creation of dummy vertices for the result to be of the following form?

enter image description here

5
+50

It seems like it would be wiser to use \fmfforce instead of playing around with tensions.

\documentclass{article}
\usepackage{feynmp-auto}
\begin{document}

\begin{fmffile}{crazy}
  \begin{fmfgraph*}(140,85)
  \fmfstraight
  \fmfleft{i1,i2}
  \fmfright{o1,o2,o3}
  \fmflabel{$\overline{d}$}{i2}
  \fmflabel{$u$}{i1}
  \fmf{fermion}{i1,v1}
  \fmf{fermion,label=$d$,label.side=right}{v1,v2}
  \fmf{fermion}{v2,i2}
  \fmf{gluon,label=$g$,label.dist=10}{v3,v2}        % note: different from OP
  \fmf{fermion}{o3,v3}                              % note: different from OP
  \fmf{fermion}{v3,o2}
  \fmf{boson}{v1,o1}
  \fmflabel{$W^+$}{o1}
  \fmflabel{$t$}{o2}
  \fmflabel{$\overline{t}$}{o3}

  % this is the relevant code
  \fmfforce{(0.3w,0.25h)}{v1}
  \fmfforce{(0.3w,0.75h)}{v2}
  \fmfforce{(0.65w,0.75h)}{v3}
  \end{fmfgraph*}
\end{fmffile}
\end{document}

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