\documentclass{article}
\usepackage[paperheight=60mm,paperwidth=95mm, margin=10mm]{geometry}
\usepackage{feynmp-auto}
\addtolength{\oddsidemargin}{-10mm}
\addtolength{\evensidemargin}{-10mm}
% commas in feynmp sometimes lead to strange errors, use \comma instead
\DeclareMathSymbol{\comma}{\mathpunct}{letters}{"3B}

\begin{document}\LARGE
    \centering
    \begin{fmffile}{bsll_DM_diagram} \centering 
        \begin{fmfgraph*}(280,110) 
            \fmfleft{l1,l2}
            \fmfright{r1,r2}            
                                  
            \fmf{phantom}{l1,w1,w2,r1}           
            \fmf{phantom}{l2,v1,v2,r2}
            
            \fmffreeze
            
            \fmf{fermion,label=$\l^+$,label.side=left}{r1,w2}
            \fmf{fermion,label=$\chi$,label.side=left}{w2,w1}
            \fmf{fermion,label=$\bar{s}$,label.side=left}{w1,l1}
            
            \fmf{fermion,label=$b$,label.side=left}{l2,v1}
            \fmf{fermion,label=$\chi$,label.side=left}{v1,v2}            
            \fmf{fermion,label=$\l^-$,label.side=left}{v2,r2}
            
            \fmf{dashes,label=$\phi_q$,label.side=left}{w1,v1}
            \fmf{dashes,label=$\phi_q$}{w2,v2}
            
        \end{fmfgraph*}
    \end{fmffile}
    \pagestyle{empty}
\end{document}