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I want to draw $\phi^4$ theory vacuum bubbles. The simplest one is the "figure eight", and the documentation does not refer to such diagrams at all. I can't find a way to manually set vertices and draw this. Help?

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Here is an example that does not require lualatex, i.e. that can be uploaded to arXiv.

\documentclass{article}
\usepackage{amsmath}
\usepackage{tikz-feynman}
\tikzfeynmanset{compat=1.1.0}
\begin{document}
\begin{align}
\Delta E&~=~
\begin{tikzpicture}[line width=.7pt,baseline=(v.south)]
\begin{feynman}
            \vertex (v);
            \vertex[above=2cm of v](t);
            \vertex[below=2cm of v](b);
            \diagram*{
            (v)  -- [scalar,out=135,in=180] (t) --[scalar,out=0,in=45] (v)
             -- [scalar,out=-135,in=180] (b) --[scalar,out=0,in=-45] (v)
            };
            \draw[fill=black] (v) circle (2pt);
\end{feynman}
\end{tikzpicture}\;.
\end{align}
If you really want the diagram to be centerd vertically, you could use
\verb|\vcenter|. 
\begin{align}
\Delta E&~=~\vcenter{\hbox{
\begin{tikzpicture}[line width=.7pt]
\begin{feynman}
            \vertex (v);
            \vertex[above=2cm of v](t);
            \vertex[below=2cm of v](b);
            \diagram*{
            (v)  -- [scalar,out=135,in=180] (t) --[scalar,out=0,in=45] (v)
             -- [scalar,out=-135,in=180] (b) --[scalar,out=0,in=-45] (v)
            };
            \draw[fill=black] (v) circle (2pt);
\end{feynman}
\end{tikzpicture}}}\;.
\end{align}
\end{document}

enter image description here

  • Could you also mention a way to scale such diagrams to fit in an eqnarray, if possible? – Razor Apr 12 '18 at 15:29
  • @Razor I updated my answer. But I was using align instead of eqnarray because otherwise egreg might want to grill me. ;-) – marmot Apr 12 '18 at 15:50

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