\documentclass{article}
\usepackage[paperheight=50mm,paperwidth=95mm, margin=5mm]{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}{bsmumu_eff_diagram} \centering 
        \begin{fmfgraph*}(300,100) 
            \fmfstraight   
            
            \fmfleft{l3,l4,l5}
            \fmfright{r3,r4,r5}     
              
            \fmf{phantom}{l5,q1,r5} 
            \fmf{phantom}{l4,a1,r4} 
            \fmf{phantom}{l3,m1,m2,m3,r3}    
            
            \fmfv{d.shape=square,d.filled=hatched,d.size=10}{q1}
            
            \fmffreeze
            
            \fmf{fermion,label=$b$,tension=10.0}{l5,q1}            
            \fmf{fermion,label=$s$,tension=10.0}{q1,r5}
                        
            \fmf{fermion,label=$\mu^+$, label.side=right}{m2,q1}
            \fmf{fermion,label=$\mu^-$}{q1,m3}
            
        \end{fmfgraph*}
    \end{fmffile}
    \pagestyle{empty}
\end{document}