% !TEX program = pdflatexmk
% !TEX parameter = -shell-escape
% Author: Izaak Neutelings (September 24)
% Instructions: To compile via command line, run the following twice
%   pdflatex -shell-escape extract/LQ_decay$TAG.tex
\documentclass[11pt,border=4pt]{standalone}
\usepackage{feynmp-auto}
\usepackage{xcolor}
\definecolor{colvtx}{rgb}{0,.1,1} % LQ vertex (blue)
\begin{document}
\begin{fmffile}{feyngraph}
  \fmfframe(14,14)(2,14){ % padding (L,T)(R,B)
  \begin{fmfgraph*}(60,60) % dimensions (WH)
    % line style
    \fmfset{dash_len}{8} % dashes length
    % external vertices
    \fmfleft{i}
    \fmfright{q,l}
    \fmf{dashes,l.s=left,t=1.2}{i,v}
    % LQ -> ql
    \fmf{fermion}{v,q}
    \fmf{fermion}{v,l}
    % vertex
    \fmfv{d.sh=circle,d.f=full,d.si=4,f=(0,,.1,,1),
          l=\strut\color{colvtx}$\lambda$,l.d=6,l.a=-8}{v}
    % labels
    \fmfv{l=LQ}{i}
    \fmfv{l.a=-28,l=b}{q}
    \fmfv{l.a=28,l=$\tau$}{l}
  \end{fmfgraph*}
  } % close \fmfframe
\end{fmffile}
\end{document}