# Particle-particle bubble

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.

• Have you had a look at the documentation for TikZ-Feynman? You can find it on CTAN and also on the project's website. In particular, there is an example of a self-energy diagram on page 25. – JP-Ellis Aug 30 '16 at 13:39
• @JP-Ellis I edited my initial post. – Zyrax Aug 30 '16 at 14:03
• I mention in the documentation how to add momentum arrows to any propagator. Hint: use the momentum=… key :) – JP-Ellis Aug 30 '16 at 14:07
• @JP-Ellis I tried that, but unfortunately, the momenta were squeezed on the inside of the bubble. It would be better, if they were on the outside. And I couldn't find a way to implement an arrow on the boson propagator. – Zyrax Aug 30 '16 at 14:21
• Again, the documentation mentions how you can adjust which side the momentum arrows go on (hint, change between momentum=… and momentum'=…). Also, have you had a look at the list of edge styles TikZ-Feynman offers? You'll see that charged boson and anti charged boson are bosons with arrows. Lastly, it is perfectly acceptable to answer your own question, and in fact, it is preferred to do that than to edit your answer into the original question because it is clearer for other readers. – JP-Ellis Aug 30 '16 at 14:37

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}