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\usepackage{microtype}
\usepackage{lineno}
\usepackage{xspace}
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\usepackage{colortbl}
\usepackage{amsmath}
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\usepackage{hyperref}
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\begin{document}
\begin{tabular}{C{2cm}|C{3cm}|C{3cm}|C{3cm}|C{3cm}|}
\cline{2-5}
 & \multicolumn{2}{c|}{$\mathbf{\mu}$ \textbf{sample}} & \multicolumn{2}{c|}{$\mathrm{e}$ \textbf{sample}} \\ \hline
\multicolumn{1}{|c|}{\textbf{Uncertainty}} & \textbf{Signal} & \textbf{Background} & \textbf{Signal} & \textbf{Background} \\ \hline
        \multicolumn{1}{|c|}{Theory} &  3.2 \% & 32.4 \%  & 3.2  \% &  28.3  \% \\ \hline
        \multicolumn{1}{|c|}{b-tagging}  & 18.3 \% & 16.1 \%  & 17.8 \% & 14.7 \%  \\ \hline
        \multicolumn{1}{|c|}{JER} &  6.7 \% & 7.5 \% &  8.0 \% & 7.0  \%   \\ \hline
        \multicolumn{1}{|c|}{MC statistics} & 7.5 \% & 34.4 \%  &  10.8 \% & 50.0  \%   \\ \hline
\end{tabular}
\end{document}
