% !TEX program = pdflatexmk
% !TEX parameter = -shell-escape
% Author: Izaak Neutelings (December 24)
% Instructions: To compile via command line, run the following twice
%   pdflatex -shell-escape extract/singletop$TAG.tex
\documentclass[11pt,border=4pt]{standalone}
\usepackage{feynmp-auto}
\begin{document}
\begin{fmffile}{feyngraph}
  \fmfframe(-9,9)(18,9){ % padding (L,T)(R,B)
  \begin{fmfgraph*}(110,70) % dimensions (WH)
    % line style
    \fmfset{wiggly_len}{12} % boson wavelength
    \fmfset{wiggly_slope}{65} % boson slope of waves
    % external vertices
    \fmfleft{i2,i1}
    \fmfright{o2,l2,l1,o1}
    \fmfshift{8 right}{l1,l2}
    \fmfshift{6 right}{o2}
    % internal vertices (exact placement)
    \fmfforce{(.44w,.78h)}{v1} % exact placement
    \fmfforce{(.44w,.22h)}{v2} % exact placement
    \fmfforce{(.82w,.50h)}{z} % exact placement
    % main
    \fmf{fermion,t=1.5}{i1,v1}
    \fmf{fermion,t=1.5}{i2,v2}
    \fmf{boson,t=0.8,l.s=left,l.d=4,label=W}{v2,v1} % t-channel
    \fmf{fermion}{v1,o1}
    \fmf{phantom}{v2,o2} % balance v1-o1
    \fmffreeze
    % b -> tZ -> ll
    \fmf{fermion}{v2,vz}
    \fmf{fermion}{vz,o2} % top quark
    \fmf{boson,t=0,l.s=right,l.d=4,label=Z}{z,vz} % Z boson
    \fmf{fermion,t=0}{l2,z,l1} % decay
    %%%% coupling
    %%%\fmfv{d.sh=circle,d.f=full,d.si=4,f=(.15,,.75,,.15)}{vz}
    % labels
    \fmfv{l.a=158,l.d=5,l=$q$}{i1}
    \fmfv{l.a=-158,l.d=5,l=b}{i2}
    \fmfv{l.a=22,l.d=5,l=$q'$}{o1}
    \fmfv{l.a=22,l.d=5,l=$\ell^-$}{l1}
    \fmfv{l.a=-22,l.d=5,l=$\ell^+$}{l2}
    \fmfv{l.a=-22,l.d=5,l=t}{o2}
  \end{fmfgraph*}
  } % close \fmfframe
\end{fmffile}
\end{document}