\documentclass{article}
\usepackage[paperheight=60mm,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}{Kaon_box_diagram} \centering 
        \begin{fmfgraph*}(300,130)
            \fmfleft{i1,i2}
            \fmfright{o1,o2}         
            \fmf{fermion,label=$\bar{s}$}{w1,i1}       
            \fmf{fermion,label=$u\comma c$,tension=0.0}{w2,w1}
            \fmf{fermion,label=$d$}{i2,w2}
            \fmf{fermion,label=$\mu^+$}{o1,w3}
            \fmf{fermion,label=$\nu$,tension=0.0}{w3,w4}
            \fmf{fermion,label=$\mu^-$}{w4,o2}
            \fmf{photon,label=$W^+$,tension=0.7}{w1,w3}
            \fmf{photon,label=$W^-$,tension=0.7}{w2,w4}
        \end{fmfgraph*}
    \end{fmffile}
\pagestyle{empty}
\end{document}