s-Channel Diagram

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

\begin{tikzpicture}
  \begin{feynman}
    \vertex (a) {a};
    \vertex[right=2cm of a] (b) {b};
    \vertex[above left=1.5cm of a] (i1);
    \vertex[below left=1.5cm of a] (i2);
    \vertex[above right=1.5cm of b] (f1);
    \vertex[below right=1.5cm of b] (f2);
    \diagram*{
      (i1) -- [fermion] (a) -- [fermion] (i2),
      (a) -- [photon] (b),
      (f1) -- [fermion] (b) -- [fermion] (f2)
    };
  \end{feynman}
\end{tikzpicture}