\documentclass[a4paper]{article}
\usepackage{feynmp}

\begin{document}

\setlength{\unitlength}{1mm}

\begin{fmffile}{diagram}

\begin{center}
\begin{fmfgraph*}(41,17)
\fmfleftn{i}{2}
\fmfrightn{o}{2}
\fmflabel{$g_2$}{i1}
\fmflabel{$g_1$}{i2}
\fmflabel{$p_2$}{o1}
\fmflabel{$p_1$}{o2}
\fmf{quark}{i1,v1}
\fmf{quark}{i2,v1}
\fmfblob{.35w}{v1}
\fmf{quark}{v1,o1}
\fmf{quark}{v1,o2}
\end{fmfgraph*}
\end{center}

\end{fmffile}

\end{document}