2

I would like to add colour flow lines to Feynman diagrams as in this example:

enter image description here Is there a convenient way to have such a functionality?

The code is the following:

\documentclass[class=minimal,border=20pt]{standalone}
\usepackage{feynmf}
\begin{document}
\begin{fmffile}{diagram}
\begin{fmfgraph*}(150, 150)                                                                                                                         
  \fmfleft{i}
  \fmfright{o1,o2}
  \fmf{fermion}{i,v,o1}
  \fmf{gluon}{v,o2}
\end{fmfgraph*}
\end{fmffile}
\end{document}
4
4

This is a code which produces something rather similar to what you want:

 \documentclass[class=minimal,border=20pt]{standalone}
 \usepackage{feynmp}

 \newcommand{\LFermionFlow}[7][X]
 % Usage: see \LFermionFlow[Deformation]{Node1}{Node2}{Length}{Shift x}{Shift y}{color}
 {  
 \fmffreeze
 \fmf{phantom}{#2,#2FFNode#3}
 \fmf{phantom,tension=#4}{#2FFNode#3,#3FFNode#2}
 \fmf{phantom}{#3FFNode#2,#3}
 \fmffreeze
 \fmfshift{#5,#6}{#2FFNode#3,#3FFNode#2}
 \fmffreeze
 \if X#1 {
 \fmf{fermion,width=0.5,fore=#7,left=0.3}{#2FFNode#3,#3FFNode#2}
 }
 \else
 {
 \fmf{fermion,width=0.5,fore=#7,left=#1}{#2FFNode#3,#3FFNode#2}
 }
 \fi
 }

 \newcommand{\RFermionFlow}[7][X]
 % Usage: see \RFermionFlow[Deformation]{Node1}{Node2}{Length}{Shift x}{Shift y}{color}
 {  
 \fmffreeze
 \fmf{phantom}{#2,#2FFNode#3}
 \fmf{phantom,tension=#4}{#2FFNode#3,#3FFNode#2}
 \fmf{phantom}{#3FFNode#2,#3}
 \fmffreeze
 \fmfshift{#5,#6}{#2FFNode#3,#3FFNode#2}
 \fmffreeze
 \if X#1 {
 \fmf{fermion,width=0.5,fore=#7,right=0.3}{#2FFNode#3,#3FFNode#2}
 }
 \else
 {
 \fmf{fermion,width=0.5,fore=#7,right=#1}{#2FFNode#3,#3FFNode#2}
 }
 \fi
 }
 \begin{document}
 \begin{fmffile}{diagram}
 \begin{fmfgraph*}(150, 150)                                                                                                                         
   \fmfleft{i}
   \fmfright{o1,o2}
   \fmf{fermion,label=$~$}{i,v,o1}
   \fmf{gluon,label=$~$}{v,o2}
   \fmffreeze
   \RFermionFlow{i}{o2}{0.5}{0.5}{0}{blue}
   \RFermionFlow{o2}{o1}{0.5}{0.5}{0}{red}
 \end{fmfgraph*}
 \end{fmffile}
 \end{document}

I removed the corners from the flow lines.

enter image description here

Of course, here this is not a fermion flow (for which the command originally was designed), but it can perhaps also serve as a `color flow'.

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