Propagator style for an effective theory

[martin-ueding]

For hadron physics, I could use some pion propagators. Those are usually double lines. I think the rationale is that those are compound particles. These are a couple of examples I could find quickly:

Would it be possible to have some double-line (and then perhaps also triple-line) style for tikz-feynman?

[JP-Ellis]

It's a feature I've been intending to add, but haven't gotten around to doing that yet. There's already a double option from Ti_k_Z which I can tweak to make it compatible with Ti_k_Z-Feynman, but triple lines will probably take slightly more work.

[martin-ueding]

I would be happy with the double line. The triple line is just something that I thought to work out analogously. As it does not seem to be the case, I think it could wait.

[JP-Ellis]

Actually, it seems like it might be a little trickier than I naively expected, as it isn't immediately obvious how they lines should reach the vertices (this becomes very noticeable for large line separations as shown below, and is the same issue as with #21).

If you just want straight doubled lines, you can just use double from Ti_k_Z. Note that this actually draws two lines on top of each other, with the second one being filled white, so this doesn't work well if you have a background image for example.

\documentclass[tikz,border=5p]{standalone}

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

\begin{document}
\feynmandiagram {
  a -- [double] b -- [photon] {c, d},
};
\feynmandiagram {
  a -- [double distance=2pt] b -- [photon] {c, d},
};
\end{document}

I hope that helps in the mean time.

[Turgon-Aran-Gondolin]

But how can I draw a double fermion line? i.e. a double line with arrows?