
\documentclass[border=5pt,multi=true]{standalone}
\usepackage{standalone}
\usepackage{ifthen}
\usepackage{color}
\usepackage{cancel}
\usepackage{microtype}
\usepackage{verbatim}
\usepackage{lineno}
\usepackage{xspace}
\usepackage{caption}
\usepackage{graphicx}
\usepackage{color}
\usepackage{colortbl}
\usepackage{rotating}
\usepackage{amsmath}
\usepackage{amssymb}
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\usepackage{upgreek}
\usepackage{multirow}
\usepackage{hyperref}
\usepackage{hypcap}
\usepackage{cite}
\usepackage{mciteplus}
\usepackage{relsize}
\usepackage{longtable}


\graphicspath{{./figs/}}

\begin{document}
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 \centering
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    \put(0,0){
      \includegraphics*[width=80mm,height=60mm%
      ]{Fig2a.pdf}
   }
    \put(80,0){
      \includegraphics*[width=80mm,height=60mm%
      ]{Fig2b.pdf}
   }
   \put(14.6,49){(a)}
   \put(94.6,49){(b)}
    \put(0,13){\begin{sideways}\small{ Candidates/(10\ensuremath{{\mathrm{\,Me\kern -0.1em V\!/}c^2}}\xspace) }\end{sideways}}
   \put(80,13){\begin{sideways}\small{ Candidates/(10\ensuremath{{\mathrm{\,Me\kern -0.1em V\!/}c^2}}\xspace) }\end{sideways}}
   \put(61,49){\footnotesize{\mbox{LHCb}\xspace}}
   \put(141,49){\footnotesize{\mbox{LHCb}\xspace}}
   \put(34 ,-1){\small{M$(\ensuremath{{\ensuremath{J}\xspace\mskip -3mu/\mskip -2mu\ensuremath{\psi}\xspace\mskip 2mu}}\xspace\ensuremath{\ensuremath{\eta}\xspace^{\prime}}\xspace)$}}
   \put(114,-1){\small{M$(\ensuremath{{\ensuremath{J}\xspace\mskip -3mu/\mskip -2mu\ensuremath{\psi}\xspace\mskip 2mu}}\xspace\ensuremath{\eta}\xspace)$}}
   \put(59 ,-1){\small{$\left[\ensuremath{{\mathrm{\,Ge\kern -0.1em V\!/}c^2}}\xspace\right]$}}
   \put(139,-1){\small{$\left[\ensuremath{{\mathrm{\,Ge\kern -0.1em V\!/}c^2}}\xspace\right]$}}
  \end{picture}
\end{document}
