Particle-particle bubble

Question

I want to draw a Feynman diagrams for the particle-particle bubble. I'm used to drawing Feynman-Diagrams with tikz-feynman in the following way:

\documentclass{article}
\usepackage{tikz-feynman,contour}
\begin{document}
\begin{tikzpicture}
  \begin{feynman}
    \vertex[blob,label={above:$P$}] (m) at ( 0, 0) {\contour{white}{$\leftarrow$}};
    \vertex (a) at (-2,-1) {$K' + P \\ \uparrow$};
    \vertex (b) at ( 2,-1) {$\uparrow \\ K + P$};
    \vertex (c) at (-2, 1) {$-K' \\ \downarrow$};
    \vertex (d) at ( 2, 1) {$\downarrow \\ -K$};
    \diagram* {
      (d) -- [fermion] (m) -- [fermion] (c),
      (b) -- [fermion] (m) -- [fermion] (a),
    };
  \end{feynman}
\end{tikzpicture}
\end{document}

In this case I drew a 4-point vertex with a blob in the middle. But this time, I need a diagram that looks like this

but has a similar "style" as the one I made above (except for the big blob, that one won't be needed here). The main problem I had, was making the fermonic lines in the middle, that have a curve, which I'm not used to making.

Answer 1

Found the answer with the help of JP-Ellis

\documentclass{article}
\usepackage{tikz-feynman,contour}
\begin{document}
\begin{tikzpicture}
\feynmandiagram [layered layout, horizontal=c to b] { a -- [photon] b
-- [fermion, half right, looseness=1.5] c
-- [fermion, half right, looseness=1.5] b, c -- [photon] d,
};
\end{tikzpicture}
\end{document}