Since I wasn't able to comment the answer to this question:
Propagator loop in Feynman diagram using tikz-feynman package
I need to ask separately. I was looking for the same thing except that I would like the propagator loop to be a proper circle. Does anybody know how this can be done?
%% luatex85 is only necesary to fix a bug in standalone
\RequirePackage{luatex85}
\documentclass{standalone}
\usepackage[compat=1.1.0]{tikz-feynman}
\begin{document}
\feynmandiagram [horizontal=a to b, layered layout] {
a -- b [dot] -- [out=135, in=45, loop, min distance=3cm] b -- c,
};
\end{document}