adding missing pieces

tikz-feynman/d.tex

@@ -0,0 +1,23 @@
+\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}