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\begin{document}
\begin{tabular}{lccc|ccc}
\toprule
                            & \multicolumn{3}{c|}{Forward}  &  \multicolumn{3}{c}{Backward} \\
Source                      & \Y1S & \Y2S & \Y3S & \Y1S & \Y2S & \Y3S \\
\hline 
Muon identification         &  1.3 &  1.3 &  1.3 &  1.3 &  1.3 &  1.3 \\
Tracking efficiency         &  1.5 &  1.5 &  1.5 &  1.5 &  1.5 &  1.5 \\
Mass fit model              & 1.1 (1.0) & 4.9 & 13 & 1.8 (1.7)&19 & 90 \\
Luminosity                  &  1.9 &  1.9 &  1.9 &  2.1 &  2.1 &  2.1 \\
Trigger                     &  2.1 &  2.1 &  2.1 &  5.0 &  5.0 &  5.0 \\
MC generation kinematics    & 3.9 (3.8) & 3.9 & 3.9 & 7.6 (6.3)&7.6 &7.6 \\ 
Reconstruction              &  1.5 &  1.5 &  1.5 &  1.5 &  1.5 &  1.5 \\
\hline 
Total                       & 5.5 (5.4) & 7.3 & 14 & 9.8 (8.8) & 21 & 91 \\
\bottomrule
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\end{document}
