\documentclass{amsart}
%
\usepackage{feynmf}
%
\begin{document}
%
\begin{fmffile}{graph}
\begin{fmfgraph*}(100,75)
\fmfpen{thick}
\fmfleft{i1,i2}
\fmflabel{$\bar{b}$}{i1}
\fmflabel{$d$}{i2}
\fmfright{o1,o2}
\fmflabel{$\bar{d}$}{o1}
\fmflabel{$b$}{o2}
\fmf{fermion}{i1,v1}
\fmf{fermion,tension=.5,label=$\bar{t},,\bar{c},,\bar{u}$,l.side=right}{v1,v3}
\fmf{fermion}{v3,o1}
\fmf{fermion}{o2,v4}
\fmf{fermion,tension=.5,label=$t,,c,,u$,l.side=right}{v4,v2}
\fmf{fermion}{v2,i2}
\fmf{photon,tension=.2,label=$W^+$,l.side=left}{v1,v2}
\fmf{photon,tension=.2,label=$W^-$,l.side=right}{v3,v4}
\fmfdotn{v}{4}
\end{fmfgraph*}
\end{fmffile}
%
\end{document}