TeX source:

\unitlength = 1mm \begin{fmffile}{tree} \begin{fmfgraph*}(100,60) \fmfleftn{i}{2} \fmfrightn{o}{4} \fmflabel{$e_-$}{i1}\fmflabel{$e_+$}{i2} \fmflabel{$\mu_+$}{o1} \fmflabel{$\nu_{\mu}$}{o2} \fmflabel{$s$}{o3} \fmflabel{$\noexpand\bar c$}{o4} \fmf{fermion}{i1,v1,i2} \fmf{boson,label=$\gamma,,Z$}{v1,v2} \fmf{boson}{v3,v2,v4} \fmf{fermion}{o1,v3,o2} \fmf{fermion}{o4,v4,o3} \fmfdot{v1,v3,v4}\fmfblob{.12w}{v2} \end{fmfgraph*} \end{fmffile}