Gravitons with tikz-feynman

Question

Gravitons (in Feynman diagrams) are basically two wavy lines. I am interested in this type of diagram here:

However, the single wavy line should become a double wavy line. The code used to generate the above image is this:

\documentclass{article}
\usepackage{tikz}
\usepackage[compat=1.1.0]{tikz-feynman}
\usepackage[active,pdftex,tightpage]{preview}
\PreviewEnvironment[{[]}]{tikzpicture}

\begin{document}

\feynmandiagram[layered layout,inline=(a.base),horizontal=a to c] {
a -- [charged scalar,momentum'={[arrow shorten=0.2]\(p\)}] b -- [double,double distance=0.5ex, photon, out=135, in=45, loop, min distance=1.5cm,momentum={[arrow shorten=0.35]\(k\)}] b,
b -- [charged scalar,momentum'={[arrow shorten=0.2]\(p\)}] c,
};

\end{document}

I added a double,double distance=0.5ex to the photon propagator, but apparently the photon overrides that style. Does anybody know how to double the wavy line?

Answer 1

You can define a new key graviton using the snake decoration and double. Options amplitude, segment length, pre length and post length can be adjusted to your liking. The momentum arrow does not work well with repeating decorations like snake, so I made two loops, one with draw=none and the momentum arrow, and a second with graviton.

\documentclass{article}
\usepackage{tikz}
\usepackage[compat=1.1.0]{tikz-feynman}

\tikzset{graviton/.style={decorate, decoration={snake, amplitude=.4mm, segment length=1.5mm, pre length=.5mm, post length=.5mm}, double}}

\begin{document}

\feynmandiagram[layered layout,inline=(a.base),horizontal=a to c] {
a -- [charged scalar,momentum'={[arrow shorten=0.2]\(p\)}] b -- [draw=none, out=135, in=45, loop, min distance=1.5cm, text=black, momentum={[arrow shorten=0.35]\(k\)}] b
  -- [graviton, out=135, in=45, loop, min distance=1.5cm] b -- [charged scalar,momentum'={[arrow shorten=0.2]\(p\)}] c,
};

\end{document}

Answer 2

An alternative solution: draw the path twice, once thick and black and once thin and white:

\feynmandiagram[layered layout,inline=(a.base),horizontal=a to c] {
        a -- [charged scalar,momentum'={[arrow shorten=0.2]\(p\)}] b -- [double,double distance=0.5ex, photon, out=135, in=45, loop, min distance=1.5cm, line width=0.05cm] b,
        b -- [photon, out=135, in=45, loop, min distance=1.5cm, white, text=black, momentum={[arrow shorten=0.35]\(k\)}]
        b -- [charged scalar,momentum'={[arrow shorten=0.2]\(p\)}] c,
    };