% TeX root = ../main.tex
\begin{tikzpicture}[program,initial above,font=\scriptsize]
    \node[state,initial]    (l0)                       {$\ell_0$};
    \node[state] (l1) [below=2cm of l0] {$\ell_1$};
    \node[state] (l2) [right=5cm of l1] {$\ell_2$};
    \node[state] (l3) [below=2cm of l1] {$\ell_3$};

    \path[->]   
        (l0) edge node[swap]{ $t_0$ } (l1)
        (l1) edge node[swap]{ $t_1: \tau = x > 0$ } (l2)
        (l1) edge node[swap]{ $t_4: \tau = x \leq 0$ } (l3)
        (l2) edge[color=rot-75,bend right=60] node { $t_{2a}:
        \begin{aligned}&p=0.5\\&\tau=y < z\end{aligned}$ } (l1)
        (l2) edge[color=rot-75,bend right=80] node[swap] { $t_{2b}:
        \begin{aligned}&p=0.5\\&\tau=y < z\\&\eta(y)=y+1\end{aligned}$ } (l1)
        (l2) edge[color=rwth-75,bend left=60] node[swap] {
        $t_{3a}:\begin{aligned}&p=0.5 \\&\tau=y \geq z\end{aligned}$ } (l1)
        (l2) edge[color=rwth-75,bend left=80] node {
        $t_{3b}:\begin{aligned}&p=0.5 \\&\tau=y \geq z\\&\eta(x) = x-1\end{aligned}$ } (l1)
                    ;
\end{tikzpicture}