\documentclass{standalone}

\usepackage{tikz}
\usetikzlibrary{automata,positioning}

\begin{document}
\begin{tikzpicture}[shorten >=1pt,node distance=3cm,on grid,>=stealth,
  every state/.style={draw=none}]

  \node[state] (T)  {$T$};
  \node[state] (R1) [below left=of T] {$R_1$};
  \node[state] (R2) [below right=of T] {$R_2$};
  \node[state] (R) [below right=of R1] {$R$};

  \path [every node/.style={font=\footnotesize, below}]
  (T) edge[->>] node[near end] {$\beta$}  (R1)
      edge[->>] node[near end] {$\beta$}  (R2)
  (R1) edge[->>] node[near end] {$\beta$} (R)
  (R2) edge[->>] node[near end] {$\beta$} (R);    
\end{tikzpicture}
\end{document}