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\begin{document}
  
    \begin{tabular}{c|cc}
    {} & \multicolumn{2}{c}{$|m_{K\kern-0.1em{K}}-m_\phi|$ ( $ {\mathrm{\,Me\kern -0.1em V\!/}c^2}$ \xspace)} \\
    $|\cos\theta_K|$ & $< 20$ & $(20,40)$ \\ 
      \hline
      $> 0.4$ &  A & B       \\ 
      $< 0.4$ & C & D   \\ 
    \end{tabular}
  
\end{document}
