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\usepackage{ifthen}
\usepackage{microtype}
\usepackage{lineno}
\usepackage{xspace}
\usepackage{caption}
\usepackage{graphicx}
\usepackage{color}
\usepackage{colortbl}
\usepackage{amsmath}
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\usepackage{upgreek}
\usepackage{hyperref}
\usepackage{hypcap}
\usepackage{xspace}
\usepackage{upgreek}
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\usepackage{mciteplus}

\begin{document}
\begin{tabular}{c c@{\hskip 20pt} c}
Missed particle & $\lambda$ & PDF \\
\hline
$\pi\xspace\xspace^0\xspace$ & 0 & $\left(m - \frac{a_\pi\xspace\xspace^0\xspace+b_\pi\xspace\xspace^0\xspace}{2} \right)^2$ \\
$\pi\xspace\xspace^0\xspace$ & $-1$ or $+1$ & $- (m-a_\pi\xspace\xspace^0\xspace)(m-b_\pi\xspace\xspace^0\xspace)$ \\
$\gamma$ & 0 & $- (m-a_\gamma)(m-b_\gamma)$ \\
$\gamma$ & $-1$ or $+1$ & $\left(m - \frac{a_\gamma+b_\gamma}{2} \right)^2 + \left (\frac{a_\gamma-b_\gamma}{2} \right)^2$\\
\end{tabular}
\end{document}
