\begin{tikzpicture}[
    roundnode/.style={circle, draw=black, fill=black, very thick, minimum size=1pt,},
    ignorednode/.style={circle, draw=black!20, fill=black!20, very thick, minimum size=1pt,},
    arrow/.style={|->, thick,},
    message/.style={->, blue!50, dashed, -{Circle[length=4pt,]}},
    ]

    \node[roundnode] (11) {};
    \node[left] at (11.west) {$p_0$};
    \node[above] at (11.north) {$w('he')$};
    \node[roundnode] (12) [right=40pt of 11] {};
    \node[above] at (12.north) {$r/\{'he'\}$};
    \node[roundnode] (13) [right=40pt of 12] {};
    \node[above] at (13.north) {$w('llo')$};
    \node[roundnode] (14) [right=40pt of 13] {};
    \node[above] at (14.north) {$r/\{'hello'\}^\omega$};

    \draw[arrow] (11) -- (12);
    \draw[arrow] (12) -- (13);
    \draw[arrow] (13) -- (14);
\end{tikzpicture}