% !TEX program = pdflatexmk
% !TEX parameter = -shell-escape
% Author: Izaak Neutelings (March 25)
% Instructions: To compile via command line, run the following twice
%   pdflatex -shell-escape extract/LQ$TAG.tex
\documentclass[11pt,border=4pt]{standalone}
\usepackage{feynmp-auto}
\usepackage{xcolor}
\definecolor{colvtx}{rgb}{0,.1,1} % LQ vertex (blue)
\begin{document}
\begin{fmffile}{feyngraph}
  \fmfframe(6,12)(16,12){ % padding (L,T)(R,B)
  \begin{fmfgraph*}(100,90) % dimensions (WH)
    % line style
    \fmfset{dash_len}{10} % dashes length
    \fmfset{wiggly_len}{12} % boson wavelength
    \fmfset{wiggly_slope}{65} % boson slope of waves
    % external vertices
    \fmfstraight
    \fmfleft{i2,i1}
    \fmfright{b2,o2,o1,b1}
    % incoming
    \fmf{gluon}{v1,i1}
    \fmf{gluon}{i2,v2}
    % outgoing b quarks from gluon splitting
    \fmf{fermion}{b1,v1}
    \fmf{fermion}{v2,b2}
    \fmffreeze
    % t-channel
    \fmf{fermion}{v1,lq1} % t-channel
    \fmf{dashes,l.s=right,l.d=5,label=LQ}{lq1,lq2} % t-channel
    \fmf{fermion}{lq2,v2} % t-channel
    \fmffreeze
    % outgoing tau leptons
    \fmf{fermion}{lq1,o1} % tau out
    \fmf{fermion}{o2,lq2} % tau out
    % vertices
    \fmfv{d.sh=circle,d.f=full,d.si=4,f=(0,,.1,,1),
          l=\color{colvtx}$\lambda$,l.d=5,l.a=140}{lq1}
    \fmfv{d.sh=circle,d.f=full,d.si=4,f=(0,,.1,,1),
          l=\color{colvtx}\vspace{-2pt}$\lambda$,l.d=5,l.a=-160}{lq2}
    % labels
    \fmfv{l.d=6,l.a=170,l=$g$}{i1}
    \fmfv{l.d=6,l.a=-170,l=$g$}{i2}
    \fmfv{l.d=6,l.a=10,l=$\tau^-$}{o1}
    \fmfv{l.d=6,l.a=0,l=\strut$\tau^+$}{o2}
    \fmfv{l.d=6,l.a=10,l=$\overline{\mathrm{b}}$}{b1}
    \fmfv{l.d=6,l.a=-10,l=b}{b2}
  \end{fmfgraph*}
  } % close \fmfframe
\end{fmffile}
\end{document}