\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}{bsll_Zprime_diagram} \centering 
        \begin{fmfgraph*}(280,100) 
            \fmfstraight               
            \fmfleft{l1,l2,l3}
            \fmfright{r1,r2,r3}     
                      
            \fmf{phantom}{l1,q1,r1}            
            \fmf{phantom}{l2,p1,p2,p3,p4,r2}
            
            \fmffreeze
            
            \fmf{fermion,label=$b$,tension=1.0}{l1,q1}       
            \fmf{fermion,label=$s$,tension=1.0}{q1,r1}
            
            \fmf{photon,label=$Z^{'}$, label.side=left,tension=1}{q1,p3}    
            
            \fmf{fermion,label=$\l^-$, label.side=left}{p3,r3}
            \fmf{fermion,label=$\l^+$, label.side=left}{r2,p3}
        
        \end{fmfgraph*}
    \end{fmffile}
    \pagestyle{empty}
\end{document}