# Change edges' arrow style with tikz-feynman

How can I change the edges' arrow style? For example with a fermion line:

\RequirePackage{luatex85}
\documentclass[tikz]{standalone}
\usepackage{tikz-feynman}
\begin{document}
\tikzfeynmanset{ fermion/.style = {???} }
\begin{tikzpicture}
\begin{feynman}
\vertex (a);
\vertex [right of a] (b);
\diagram*{ (a) -- [fermion](b) };
\end{feynman}
\end{tikzpicture}
\end{document}

• While code snippets are useful in explanations, it is always best to compose a fully compilable MWE that illustrates the problem including the \documentclass and the appropriate packages so that those trying to help don't have to recreate it. This is especially important for tikz as there are numerous libraries – Peter Grill May 5 '17 at 11:16

Internally, TikZ-Feynman (CTAN) does not use 'arrows' in the sense of the 'arrows' library from TikZ; instead, it decorates the path using a triangle:

/tikz/decoration={
markings,
mark=at position 0.5 with {
\node[
transform shape,
xshift=-0.5mm,
fill,
inner sep=〈some distance〉,
draw=none,
isosceles triangle
] { };
},
},
/tikz/decorate=true,


In particular, you can change the shape from isosceles triangle to whatever you want. Alternatively, it is also possible to use the \arrow command within the decoration (refer to the TikZ manual for the exact details).

I have illustrated two cases below.

\RequirePackage{luatex85}
\documentclass[tikz, border=10pt]{standalone}

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

\tikzfeynmanset{
fermion1/.style={
/tikz/postaction={
/tikz/decoration={
markings,
mark=at position 0.5 with {
\node[
transform shape,
xshift=-0.5mm,
fill,
dart tail angle=100,
inner sep=1.3pt,
draw=none,
dart
] { };
},
},
/tikz/decorate=true,
},
},
fermion2/.style={
/tikz/postaction={
/tikz/decoration={
markings,
mark=at position 0.5 with {
\arrow{>[length=6pt, width=5pt]};
},
},
/tikz/decorate=true,
},
},
}

\begin{document}
\feynmandiagram [horizontal=a to b] {
a -- [fermion1] b -- [fermion2] {c, d},
};
\end{document}


Since you probably want to change the fermion style completely, then I would recommend creating a new fermion style instead of overwriting the default one. Having said that, have a look at Torbjørn T.'s answer as he is going even more general than I am in this answer! He is modifying one of the underlying styles in TikZ-Feynman (the with arrow style) so that the arrows are changed for all particles.

• This is also a great starting point for further customizations! Thank you. – Luigi Pertoldi May 5 '17 at 14:34

The arrow tip in the middle of the lines are placed using a decoration from the decoration.markings library. I think you need to specify a new decoration altogether, for example:

\documentclass{article}
\usepackage{tikz-feynman}
\usetikzlibrary{positioning,arrows.meta}
\tikzfeynmanset{ fermion/.style = {
decoration={
markings,
mark=at position 0.5
with {\arrow[xshift=2mm]{Stealth[red,width=3mm,length=4mm]}}
},
postaction=decorate}
}

\begin{document}
\begin{tikzpicture}
\begin{feynman}
\vertex (a);
\vertex [right=of a] (b);
\diagram*{ (a) -- [fermion](b) };
\end{feynman}
\end{tikzpicture}
\end{document}


More generally, fermion and other styles with an arrow in the middle use a style called with arrow, that is defined by tikz-feynman. If you want the same arrow tip for all of these, you could modify that style instead. Not sure if JP-Ellis, the package author, intended for people to do that though.

\documentclass[review]{article}
\usepackage{tikz-feynman}
\usetikzlibrary{positioning,arrows.meta}
\tikzfeynmanset{ with arrow/.style = {
decoration={
markings,
mark=at position 0.5
with {\arrow[xshift=2mm]{Stealth[red,width=3mm,length=4mm]}}
},
postaction=decorate}
}

\begin{document}
\begin{tikzpicture}
\begin{feynman}
\vertex (a);
\vertex [right=of a] (b);
\diagram*{ (a) -- [fermion](b) };
\end{feynman}
\end{tikzpicture}
\begin{tikzpicture}
\begin{feynman}
\vertex (a);
\vertex [right=of a] (b);
\diagram*{ (a) -- [charged boson](b) };
\end{feynman}
\end{tikzpicture}
\end{document}

• Great answer! I didn't see it until I posted my answer :) My personal preference is to avoid overwriting the default style too much; however, I realize people might want to change things and it is up to them ultimately. Nice modification of with arrow! – JP-Ellis May 5 '17 at 12:17
• I finally chose the @JP-Ellis approach to create a new style instead of overwriting the existing, it's a matter of personal preference, I guess. Anyway thank you for your detailed answer! – Luigi Pertoldi May 5 '17 at 14:32