documentclass{beamer}

usepackage{graphicx}
usepackage{grffile}
usepackage{gmp}
usepackage{feynmp-auto}

begin{document}
begin{frame}{Test}
    Test diagram
    begin{figure}
        centering
        begin{fmffile}{testdiagram}
            begin{fmfgraph*}(150,100)
                fmfpen{thin}
                fmfleft{a}
                fmfright{lept1,lept2,nu}
                fmf{fermion}{a,mixing}
                fmf{fermion,label.side=left,label.dist=3thick,tension=2}{mixing,decay}
                fmf{fermion,tension=0.8}{decay,nu}
                fmf{phantom}{a,mixing}
                fmf{wiggly,tension=2}{decay,Wdecay}
                fmf{fermion}{lept1,Wdecay,lept2}
                fmf{phantom}{lept1,Wdecay}
                fmfv{l=$gamma$,l.a=-90,decoration.shape=circle,decoration.size=4.2,decoration.filled=hatched}{mixing}
            end{fmfgraph*}
        end{fmffile}
    end{figure}
end{frame}
end{document}