How to draw this Feynman diagram?

Question

I have the following Feynman diagram, which I obtained from an article. I want to draw a similar diagram, so I went through the tikz package but didn't understand how to draw this specific diagram. Any help or reference will be highly appreciated.

Answer 1

This does not necessarily look like a common Feynman diagram. So you may just draw it with plain TikZ.

\documentclass[tikz,border=3mm]{standalone}
\begin{document}
\begin{tikzpicture}
 \draw[thick,cyan,dash pattern=on 2pt off 2pt,line cap=round] (120:1) edge ++ (120:0.4)
 -- (0:1)  edge ++ (0:0.4)-- (-120:1)  edge ++ (-120:0.4) -- cycle;
 \draw[thick] (0,0) node[circle,fill,inner sep=3pt,draw](O) {}
 foreach \X [count=\Y]  in {180,60,300} { 
 (\X:1)node[circle,draw,inner sep=1.2pt,label={\X-90:{$a_\Y$}}]{}
 edge (O)  };
\end{tikzpicture}
\end{document}