Simple Feynman Diagram

\usepackage[compat=1.1.0]{tikz-feynman}

\begin{tikzpicture}
  \begin{feynman}
    \vertex (a);
    \vertex[right=1.5cm of a] (b);
    \vertex[above left=1cm of a] (i);
    \vertex[above right=1cm of b] (f);
    \vertex[below=0.8cm of a] (p);
    \diagram*{
      (i) -- [fermion] (a) -- [fermion] (b) -- [fermion] (f),
      (a) -- [photon] (p)
    };
  \end{feynman}
\end{tikzpicture}