
\documentclass[border=5pt,multi=true]{standalone}
\usepackage{standalone}
\usepackage{ifthen}
\usepackage{microtype}
\usepackage{lineno}
\usepackage{xspace}
\usepackage{graphicx}
\usepackage{overpic}
\usepackage{color}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{amsfonts}
\usepackage{upgreek}
\usepackage{hyperref}
\usepackage{hypcap}
\usepackage{cite}
\usepackage{mciteplus}
\usepackage{longtable}

\begin{document}
\scriptsize

\begin{tabular}{lccccccc}
\hline\\[-2.5ex]
 & $R_{K/\pi}^{K3\pi}$  & $A_{K}^{K3\pi}$  & $A_{\pi}^{K3\pi}$  & $R_{K-}^{K3\pi}$  & $R_{K+}^{K3\pi}$  & $R_{\pi-}^{K3\pi}$  & $R_{\pi+}^{K3\pi}$ \\
\hline\\[-2.5ex]
$R_{K/\pi}^{K3\pi}$ &       1 &  0.003 &  0.001 & -0.060 & -0.024 &  0.017 &  0.021 \\[1.0ex]
    $A_{K}^{K3\pi}$ &         &      1 & -0.035 & -0.007 &  0.006 & -0.002 &  0.002 \\[1.0ex]
  $A_{\pi}^{K3\pi}$ &         &        &      1 & -0.006 &  0.008 & -0.002 &  0.005 \\[1.0ex]
  $R_{K-}^{K3\pi}$ &         &        &        &      1 &  0.043 &  0.006 &  0.029  \\[1.0ex]
  $R_{K+}^{K3\pi}$ &         &        &        &        &      1 &  0.022 &  0.025  \\[1.0ex]
$R_{\pi-}^{K3\pi}$ &         &        &        &        &        &      1 &  0.032  \\[1.0ex]
$R_{\pi+}^{K3\pi}$ &         &        &        &        &        &        &      1  \\[1.0ex]
\hline
\end{tabular}
\end{document}
