\documentclass[11pt]{article}
% \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 [horizontal=a to b] {
  i1 -- [fermion] a -- [fermion] i2,
  a -- [photon] b,
  f1 -- [fermion] b -- [fermion] f2,
};

\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}