\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}

\feynmandiagram [vertical=a to b] {
  % a [crossed dot] -- [photon, thick, edge label=\(\gamma\)] b [dot],
  a -- [photon, thick, edge label=\(\gamma\)] b,
  i1 [particle=\(\mu\)] -- [fermion, thick] iv1 -- [fermion, thick] b 
  -- [fermion, thick] iv2 -- [fermion, thick] f1 [particle=\(\mu\)],
  {[same layer] iv1 -- [photon, thick] iv2},
};

% \feynmandiagram [horizontal=a to b] {
  % i1 [particle=\(e^{-}\)] -- [fermion] a -- [fermion] i2 [particle=\(e^{+}\)],
  % a -- [photon, edge label=\(\gamma\), momentum'=\(k\)] b,
  % f1 [particle=\(\mu^{+}\)] -- [fermion] b -- [fermion] f2 [particle=\(\mu^{-}\)],
% };
  
\end{document}
