documentclass{beamer}

usepackage{tikz}
begin{document}

  begin{frame}{my pic}
  begin{tikzpicture}
   node at (0,0) (pt) {$p$};
  node at (.5,0) (at) {$a$};
  node at (1,0) (zt) {$Z$};
  end{tikzpicture}
  end{frame}
end{document}