# TikZ-Feynman: how to draw scattering Feynman diagram

I would like to produce the following Feynman diagram using the TikZ-Feynman package.

So far this is what I have:

\begin{tikzpicture}
\begin{feynman}
\vertex (li);
\vertex [below=2cm of li] (hi);
\vertex [right=of li] (a);
\vertex [above right=of a] (lf);
\vertex [below right=of a] (b);
\vertex [right=of b] (hf1);
\vertex [blob, right=of hi] (c) {};
\vertex [below right=of c] (hf2);

\diagram* {
(li) -- [fermion] (a) -- [fermion] (lf),
(hi) -- [fermion] (c) -- [fermion] (hf2),
(a) -- [photon] (b) -- [fermion] (hf1),
(c) -- [fermion] (b),
};
\end{feynman}
\end{tikzpicture}


which produces the following

What I'm having issues with is drawing three parallel lines that go into a single blob. If I generate three vertically-stacked vertices to the left of the blob and draw lines between them and the blob, clearly the lines are not gonna be parallel. If I "cheat" and draw three vertices within the blob itself, so that the three lines are parallel, the lines are drawn on top of the underlying blob. Clearly then neither of these methods works. Any help would be much appreciated.

• As for your just deleted question: it looks to me a bit like a \textpsi. \documentclass{article} \usepackage{textgreek} \newcommand{\varpsi}{\mathord{\mbox{\textpsi}}} \begin{document} \textpsi$\psi\varpsi$ \end{document} – user194703 May 16 '20 at 8:13

Something like this?

\documentclass[tikz,border=3mm]{standalone}
\usepackage{tikz-feynman}
\begin{document}
\begin{tikzpicture}
\begin{feynman}
\vertex (li);
\vertex [below=2cm of li] (hi);
\vertex [right=of li] (a);
\path (a) ++ (20:2) node[vertex] (lf);
\vertex [below right=of a] (b);
\vertex [right=of b] (hf1);
\vertex [blob, right=of hi] (c) {};
\path (c.-10) ++ (00:2) node[vertex] (hf2);
\path (c.-40-|hf2.center) node[vertex] (hf3);

\diagram* {
(li) -- [fermion] (a) -- [fermion] (lf),
(hi) -- [fermion] (c) -- [fermion] (b),
(a) -- [photon] (b) -- [fermion] (hf1),
(c.-10) -- [fermion] (hf2),
(c.-40) -- [fermion] (hf3)
};
\end{feynman}
\end{tikzpicture}
\end{document}


You may want to nudge the positions of the arrows a bit.

\documentclass[tikz,border=3mm]{standalone}
\usepackage{tikz-feynman}
\begin{document}
\begin{tikzpicture}
\begin{feynman}
\vertex (li);
\vertex [below=2cm of li] (hi);
\vertex [right=of li] (a);
\path (a) ++ (20:2) node[vertex] (lf);
\vertex [below right=of a] (b);
\vertex [right=of b] (hf1);
\vertex [blob, right=of hi] (c) {};
\path (c.-5) ++ (00:2) node[vertex] (hf2);
\path (c.-40-|hf2.center) node[vertex] (hf3);

\diagram* {
(li) -- [fermion] (a) -- [fermion] (lf),
(hi) -- [fermion] (c) -- [fermion] (b),
(a) -- [photon] (b) -- [fermion] (hf1),
(c.-5) -- [fermion] (hf2),
(c.-40) -- [with arrow=0.52] (hf3)
};
\end{feynman}
\end{tikzpicture}
\end{document}


Source of inspirations: tikzfeynman.keys.code.tex.

• That's lovely, thanks! – user-2147482634 Mar 29 '20 at 14:29
• Surely upvoted! Just two suggestions (if they are possible); but is it possible to have the line more tick and the tip of the arrow similar to the initial picture? Thank you very much. – Sebastiano Mar 29 '20 at 16:42
• @Sebastiano I actually like the arrows of feynmp most and am sometimes debating if I should write a short add-on that allows one to add similar arrows to tikz-feynman, along with arrows that indicate the fermion flow. Since you are living in Sicily you undoubtedly know why the latter is important. ;-) – user194703 Mar 29 '20 at 17:40
• @Schrödinger'scat Maybe I have understood... to stop the flow.... pandemic? :-) These days... I don't understand anything. I hope very much that we return to normal and find a solution. I wish you all the best. – Sebastiano Mar 29 '20 at 17:50
• @Sebastiano No, it has to do with Majorana fermions. – user194703 Mar 29 '20 at 19:21