[Tex/LaTex] Change edges’ arrow style with tikz-feynman

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}

Best Answer

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}

output

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.

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