# Feynman Diagramm: 4-point vertex into 3-point vertex

I asked a question before about how to make a 4-point vertex before involving a blob. I got an answer in the form of

\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}


Now I want to make one that looks like this:

How do I have to modify my initial code, to get this one?

//Edit: Arrows pointing to the left in the bosonic propagators (curly lines) at the right side would be nice, too.

//Edit2: This is what I have:

\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}

\begin{feynman}
\vertex(m) at ( 0, 0);
\vertex (a) at (-2,-1) {$K + P \\ \uparrow$};

\vertex (c) at (-2, 1) {$-K \\ \downarrow$};

\vertex(n) at ( 2, 0) {$P$};
\diagram* {
(m) -- [fermion] (a),
(m) -- [fermion] (c),
(n) -- [boson] (m),
};
\end{feynman}

\begin{feynman}
\vertex(m) at ( 0, 0);
\vertex (a) at (2,-1) {$K + P \\ \uparrow$};

\vertex (c) at (2, 1) {$-K \\ \downarrow$};

\vertex(n) at ( -2, 0) {$P$};
\diagram* {
(m) -- [fermion] (a),
(m) -- [fermion] (c),
(n) -- [boson] (m),
};
\end{feynman}

\end{tikzpicture}
\end{document}


I got all 3 diagrams that I want. My problem is, that they are all beneath each other, but I need them next to each other (with an arrow and a plus sign). Any suggestions?

• Can you show us what you have tried so far? Also, can I recommend that you read the documentation for TikZ-Feynman? You can find it on CTAN and also on the project's website and there are examples of the diagrams you're looking for. Lastly, you should make sure accept previous answer, it's a nice way ot say "thank you" and people in the future will be more likely to help out :) – JP-Ellis Aug 30 '16 at 13:44
• @JP-Ellis I did, that before, but my "reputation" was not enough, so it could not be recorded. – Zyrax Aug 30 '16 at 13:48
• To accept an answer, you have to click on the tick (✓) next to it which doesn't require much/any reputation (and that's distinct to an upvote, which you can also go, but I think requires more reputation). Let me know if you have any trouble adapting an example from the documentation :) – JP-Ellis Aug 30 '16 at 14:00
• @JP-Ellis I edited my post. I got the 3 diagrams, but they are not next to each other. Any suggestions on that? – Zyrax Aug 30 '16 at 14:26

Good to see you have a go at your own question! I'll help you with the last little bit :)

To get them side-by-side, you'll can do one of two things:

1. Translate all the TikZ coordinates of the other diagrams, so that instead of using (0,0) for the origin, use (5,5) and have things relative to that.
2. Separate the TikZ environments so that LaTeX then places each graphic on their own.

Of the two methods, I prefer the second because it also allows you to use the diagrams in equations. The only problem is that LaTeX will want to line up the bottom of the diagram with the rest of the line, meaning that the equal and plus signs won't be centred. This can be fixed by using TikZ's baseline key which informs LaTeX of how high the diagram ought to be (in much the same way that the letter 'g' dips below the line). Finally, the argument to the baseline that I'm giving is a little trick I learnt from another question which ensures that everything lines up with the + and = signs.

Here' the code:

\RequirePackage{luatex85}
\documentclass{standalone}
\usepackage{amsmath}
\usepackage[compat=1.1.0]{tikz-feynman}
\usepackage{contour}
\begin{document}
\begin{equation*}
\begin{tikzpicture}[baseline=-\the\dimexpr\fontdimen22\textfont2\relax]
\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}
=
\begin{tikzpicture}[baseline=-\the\dimexpr\fontdimen22\textfont2\relax]
\begin{feynman}
\vertex (m) at ( 0, 0);
\vertex (a) at (-2,-1) {$K + P \\ \uparrow$};
\vertex (c) at (-2, 1) {$-K \\ \downarrow$};
\vertex (n) at ( 2, 0) {$P$};

\diagram* {
(m) -- [fermion] (a),
(m) -- [fermion] (c),
(n) -- [charged boson] (m),
};
\end{feynman}
\end{tikzpicture}
+
\begin{tikzpicture}[baseline=-\the\dimexpr\fontdimen22\textfont2\relax]
\begin{feynman}
\vertex (m) at (0, 0);
\vertex (a) at (2,-1) {$K + P \\ \uparrow$};
\vertex (c) at (2, 1) {$-K \\ \downarrow$};
\vertex (n) at ( -2, 0) {$P$};

\diagram* {
(m) -- [fermion] (a),
(m) -- [fermion] (c),
(n) -- [anti charged boson] (m),
};
\end{feynman}
\end{tikzpicture}
\end{equation*}
\end{document}


• Thanks, that helps. You have a small distance right and left of the +. How can I get such a distance? – Zyrax Aug 30 '16 at 15:09
• This is the usual spacing that comes within maths environments. In your own version, make sure you have the {equation} environment too (or whichever maths environment you prefer). – JP-Ellis Aug 30 '16 at 15:16