2

The code

\documentclass[class=minimal,border=20pt]{standalone}
\usepackage[utf8]{inputenc}
\usepackage{feynmf}
\begin{document}
\begin{fmffile}{diagram}
\begin{fmfgraph*}(250, 250)
    \fmfleft{i1,i2,i3}
    \fmfright{o1,o2,o3}
    \fmftop{t1,t2,t3,t4}
    \fmf{fermion,label=$q_{1}$}{i1,vp1,o1}
    \fmf{fermion,label=$q_{2}$}{i2,vp2,o2}
    \fmffreeze
    \fmf{gluon}{vp1,vp2}
    \fmf{gluon}{vp2,t3}
\end{fmfgraph*}
\end{fmffile}
\end{document}

produces the following Feynman diagram enter image description here

How can I keep the length of lines corresponding to q1 and q2 equal?

1

You need the \fmfstraight command. By default the galleries along the sides and top and bottom are curved as often this produces a nicer appearance but they can be switched to straight using the \fmfstraight command. This should be inserted in the fmfgraph block. Sorry I can't include a graph but I don't have access to a LaTeX install with feynmf at the second.

\documentclass[class=minimal,border=20pt]{standalone}
\usepackage[utf8]{inputenc}
\usepackage{feynmf}
\begin{document}
  \begin{fmffile}{diagram}
    \begin{fmfgraph*}(250, 250)
      \fmfstraight
      \fmfleft{i1,i2,i3}
      \fmfright{o1,o2,o3}
      \fmftop{t1,t2,t3,t4}
      \fmf{fermion,label=$q_{1}$}{i1,vp1,o1}
      \fmf{fermion,label=$q_{2}$}{i2,vp2,o2}
      \fmffreeze
      \fmf{gluon}{vp1,vp2}
      \fmf{gluon}{vp2,t3}
    \end{fmfgraph*}
  \end{fmffile}
\end{document}
1

Very simple:

    \documentclass[class=minimal,border=20pt]{standalone}
    \usepackage[utf8]{inputenc}
    \usepackage{feynmf}
    \begin{document}
    \begin{fmffile}{diagram}
    \begin{fmfgraph*}(250,250)
        \fmfleft{i0,i1,i2,i3}
        \fmfright{o0,o1,o2,o3}
        \fmf{fermion,label=$q_{1}$}{i1,vp1,o1}
        \fmf{fermion,label=$q_{2}$}{i2,vp2,o2}
        \fmffreeze
        \fmf{phantom}{i3,t3,o3}
        \fmf{gluon}{vp1,vp2}
        \fmf{gluon}{vp2,t3}
    \end{fmfgraph*}
    \end{fmffile}
    \end{document}

enter image description here

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