Tikzfeynman in beamer frame not where I want

Question

As you can see in the immage I attached, the code produces the diagram correctly, yet not where I'd like it to be, i.e. below the first line of text (the second line was just a test)

\documentclass[lualatex]{beamer}

\usepackage{graphicx}
\usepackage{tikz-feynman}

\begin{frame}

  \frametitle{Involved Feynman diagrams}

  At LO in $\alpha$ there is only one diagram: 
  
  \begin{figure}[!h]
    \begin{tikzpicture}
      \feynmandiagram[horizontal=a to b]{
        i1 [particle=\(e^{-}\)] -- [fermion] a -- [fermion] i2  [particle=\(e^{+}\)],
        a -- [photon] b,
        f1 [particle=\(\mu^{-}\)] -- [fermion] b -- [fermion] f2  [particle=\(\mu^{+}\)],      
      };
    \end{tikzpicture}
  \end{figure}

  prova
    
\end{frame}

Any help would be much appreciated. Thank you.

Edit: possibly useful information, compiling with lualatex

Answer 1

Remove the tikzpicture environment. The command feynmandiagram already includes tikzpicture. From the docs:

Feynman diagrams can be declared with the \feynmandiagram command. It is analogous to the \tikz command from TikZ and requires a final semi-colon (;) to finish the environment.

\documentclass{beamer}
\usepackage{tikz-feynman}
\begin{document}

\begin{frame}

  \frametitle{Involved Feynman diagrams}

  At LO in $\alpha$ there is only one diagram: 
  \begin{center}
      \feynmandiagram[horizontal=a to b]{
        i1 [particle=\(e^{-}\)] -- [fermion] a -- [fermion] i2  [particle=\(e^{+}\)],
        a -- [photon] b,
        f1 [particle=\(\mu^{-}\)] -- [fermion] b -- [fermion] f2  [particle=\(\mu^{+}\)],      
      };
  \end{center}
  prova
    
\end{frame}
\end{document}