\documentclass[tikz,border=10pt]{standalone}
\usepackage{tkz-euclide}

\begin{document}
  \begin{tikzpicture}
    \tkzDefPoint(0,0){O}
    \tkzDefPoint(1,4){S1}
    \tkzDefPoint(2,4){S2}

    \draw [line width=0.5pt] (O) -- (S1) -- (S2) -- cycle;

    \begin{scope}
      \clip (O) -- (S1) -- (S2) -- cycle;
      \draw [red] (O) circle (1);
     %\draw [red] (S1) circle (0.25);
      \draw [red] (S2) circle (0.25);
      \node[red] at (0.4,1.1) {$\alpha$};
      \node[red] at (1.6,3.7) {$\beta$};
    \end{scope}

    \node[below] at (O) {$O$};
    \node[above] at (S1) {$S'$};
    \node[above] at (S2) {$S$};
    \node[right,blue] at (1,2) {$c\tau$};
    \node[above,blue] at (1.5,4) {$u\tau$};
  \end{tikzpicture}
\end{document}
