
\documentclass[border=5pt,multi=true]{standalone}
\usepackage{standalone}
\usepackage{ifthen}
\usepackage{siunitx}
\usepackage{subfig}
\usepackage{xcolor}
\usepackage{booktabs}
\usepackage{collcell}
\usepackage{microtype}
\usepackage{lineno}
\usepackage{xspace}
\usepackage{caption}
\usepackage{graphicx}
\usepackage{color}
\usepackage{colortbl}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{amsfonts}
\usepackage{upgreek}
\usepackage{hyperxmp}
\usepackage{hyperref}
\usepackage{todonotes}
\usepackage{footmisc}
\usepackage{hypcap}
\usepackage{xspace}
\usepackage{upgreek}
\usepackage{cite}
\usepackage{mciteplus}

\begin{document}
  
    
    \centerline{\begin{tabular}{l 
>{\collectcell\num}r<{\endcollectcell} @{${}\pm{}$} >{\collectcell\num}l<{\endcollectcell}
>{\collectcell\num}r<{\endcollectcell} @{${}\pm{}$} >{\collectcell\num}l<{\endcollectcell}
}
\toprule
Background &  \multicolumn{2}{c}{Same sign} & \multicolumn{2}{c}{Opposite sign}\\
\midrule
   $ W\xspace$ \xspace $\rightarrow$ \xspace $\mu\xspace$ \xspace $\nu\xspace$ \xspace   & 1.8 & 1.3 &    2.7 &   1.6\\
 $ b\xspace\xspace\overline  b\xspace\xspace\xspace$ \xspace             & 1.7 & 1.7 &    1.7 &   1.7\\
 $ Z\xspace$ \xspace $\rightarrow$ \xspace $\mu\xspace$ \xspace $\mu\xspace$ \xspace    & 1.3 & 0.6 &   2251 &   161\\
light QCD          & 0.3 & 1.4 &    3.1 &   5.4\\
\bottomrule
\end{tabular}
}
\end{document}
