\documentclass[11pt]{standalone}
% \usepackage{amssymb} %maths
% \usepackage{amsmath} %maths
% \usepackage[utf8] %useful to type directly diacritic characters
\usepackage{tikz-feynman} 
% \tikzfeynmanset{compat=1.0.0}

\begin{document}
\begin{tikzpicture}[]
  \begin{feynman}
    \vertex (a);
    \vertex [below=of a] (b);
    \vertex [below left=of b] (c);
    \vertex [below right=of b] (d);

    \diagram * {
      (a) -- [photon, edge label=\(\gamma\)] (b);
      (c) -- [fermion, edge label=\(\mu\)] (b);
      (b) -- [fermion, edge label=\(\mu\)] (d);
    };
  \end{feynman}
\end{tikzpicture}
\end{document}
