Tikz-Feynman: Multiple arrows on one fermion line and alignment (0vββ)

Question

I'm trying to align the quark arrows in my 0vββ diagram:

So I'd like to have 2 arrows on each of the 4 'unmodified' quark lines: the first arrow should be horizontally aligned with the arrow from the interacting down quark (such that it is directly above the d quark arrow). Same for the 2nd arrow, it should be directly above the arrow of the outgoing up quark.

Basically like in the left diagram here:

So my question is

1) How do I get 2 arrows on one fermion line

2) How can the arrows be aligned correctly?

( 3) And by the way, is it possible to move the Majorana neutrino label a little bit more to the left?)

Didn't find anything in the manual.

This is my code:

\documentclass[convert]{standalone}

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

\begin{document}

\begin{tikzpicture}
\begin{feynman}
\vertex (b);
\vertex [below=of b] (c);
\vertex [below left=1cm and 1.4cm of c] (d);
\vertex [above left=1cm and 1.4cm of b] (a);
\vertex [left=of a] (i1) {d};
\vertex [left=of d] (i2) {d};
\vertex[right = 2cm of b] (f2) { \(e^{-}\)};
\vertex[right = 2cm of c] (f3) { \(e^{-}\)};
\vertex[below = 2cm of f3] (f4) {u};

\vertex[above = 2cm of f2] (f1) {u};

\vertex[above=0.35cm of i1] (f6) {d}; % d quark outgoing
\vertex[above=0.35cm of f1] (i3) {d}; % d quark ingoing

\vertex[above=0.35cm of i3] (f7) {u}; % u quark outgoing
\vertex[above=0.35cm of f6] (i4) {u}; % u quark ingoing

% copy quarks for bottom

\vertex[below=0.35cm of i2] (f8) {d}; % d quark outgoing
\vertex[below=0.35cm of f4] (i5) {d}; % d quark ingoing

\vertex[below=0.35cm of i5] (f9) {u}; % u quark outgoing
\vertex[below=0.35cm of f8] (i6) {u}; % u quark ingoing


\diagram* {
    (a) -- [boson, edge label'=\(W^{-}\)] (b) -- [anti majorana, insertion=0.5, edge label' = \(\nu_{M}\) ] (c) -- [boson, edge label'=\(W^{-}\)] (d),
    (i1) -- [fermion] (a),
    (i2) -- [fermion] (d),
    (a) -- [fermion] (f1),
    (b) -- [fermion] (f2),
    (c) -- [fermion] (f3),
    (d) -- [fermion] (f4),
    (f6) -- [fermion, out=0, in=200] (i3),
    (i4) -- [fermion, out=0, in=200] (f7),
    (f8) -- [fermion, out=0, in=160] (i5),
    (i6) -- [fermion, out=0, in=160] (f9),
};

\draw [decoration = {brace} , decorate] (i1.south west) -- (i4.north west) node [pos = 0.5 , left = 0.125cm] {\huge n};

\draw [decoration = {brace} , decorate] (f7.north east) -- (f1.south east) node [pos = 0.5 , right = 0.125cm] {\huge p};

\draw [decoration = {brace} , decorate] (i6.south west) -- (i2.north west) node [pos = 0.5 , left = 0.125cm] {\huge n};

\draw [decoration = {brace} , decorate] (f4.north east) -- (f9.south east) node [pos = 0.5 , right = 0.125cm] {\huge p};

\end{feynman}
\end{tikzpicture}
\end{document}

Thank you very much! :)

Answer 1

The fermion, Majorana and charged boson styles actually all use an internal (and undocumented) with arrow style. I didn't make the with arrow style public because I didn't initially envisage it being needed though I think I will be making it public in the next version of TikZ-Feynman.

The with arrow and with reversed arrow styles place an arrow or reversed arrow along the path at the given position. The position can be specified in of the following:

• A number in between 0 and 1: This tells what how far down the path to place the arrow such that 0 corresponds to the start, 0.5 is the halfway point, and 1 is the end;
• A positive distance: the arrow is placed at the given distance down the path;
• A negative distance: the arrow is placed at the given distance from the end of the path.

Since in your case, we want the arrows to be aligned, I actually defined two temporary commands to hold the distance, \tmpda and \tmpdb, and use these as the arguments to with arrow. This avoids having to manually adjust each distance.

\documentclass[tikz]{standalone}

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

\begin{document}

\begin{tikzpicture}
  \begin{feynman}
    \vertex (b);
    \vertex [below=of b] (c);
    \vertex [below left=1cm and 1.4cm of c] (d);
    \vertex [above left=1cm and 1.4cm of b] (a);
    \vertex [left=of a] (i1) {d};
    \vertex [left=of d] (i2) {d};
    \vertex [right = 2cm of b] (f2) {\(e^{-}\)};
    \vertex [right = 2cm of c] (f3) {\(e^{-}\)};
    \vertex [below = 2cm of f3] (f4) {u};

    \vertex [above = 2cm of f2] (f1) {u};

    \vertex [above=0.35cm of i1] (f6) {d}; % d quark outgoing
    \vertex [above=0.35cm of f1] (i3) {d}; % d quark ingoing

    \vertex [above=0.35cm of i3] (f7) {u}; % u quark outgoing
    \vertex [above=0.35cm of f6] (i4) {u}; % u quark ingoing

    % copy quarks for bottom

    \vertex [below=0.35cm of i2] (f8) {d}; % d quark outgoing
    \vertex [below=0.35cm of f4] (i5) {d}; % d quark ingoing

    \vertex [below=0.35cm of i5] (f9) {u}; % u quark outgoing
    \vertex [below=0.35cm of f8] (i6) {u}; % u quark ingoing

    \newcommand\tmpda{0.7cm}
    \newcommand\tmpdb{-1.7cm}
    \diagram* {
      (a) -- [boson, edge label'=\(W^{-}\)] (b) -- [anti majorana, insertion=0.5, edge label' = \(\nu_{M}\) ] (c) -- [boson, edge label'=\(W^{-}\)] (d),
      (i1) -- [with arrow=\tmpda] (a),
      (i2) -- [with arrow=\tmpda] (d),
      (a) -- [with arrow=\tmpdb] (f1),
      (b) -- [fermion] (f2),
      (c) -- [fermion] (f3),
      (d) -- [with arrow=\tmpdb] (f4),
      (f6) -- [with arrow=\tmpda, with arrow=\tmpdb, out=0, in=200] (i3),
      (i4) -- [with arrow=\tmpda, with arrow=\tmpdb, out=0, in=200] (f7),
      (f8) -- [with arrow=\tmpda, with arrow=\tmpdb, out=0, in=160] (i5),
      (i6) -- [with arrow=\tmpda, with arrow=\tmpdb, out=0, in=160] (f9),
    };

    \draw [decoration = {brace} , decorate] (i1.south west) -- (i4.north west) node [pos = 0.5 , left = 0.125cm] {\huge n};

    \draw [decoration = {brace} , decorate] (f7.north east) -- (f1.south east) node [pos = 0.5 , right = 0.125cm] {\huge p};

    \draw [decoration = {brace} , decorate] (i6.south west) -- (i2.north west) node [pos = 0.5 , left = 0.125cm] {\huge n};

    \draw [decoration = {brace} , decorate] (f4.north east) -- (f9.south east) node [pos = 0.5 , right = 0.125cm] {\huge p};

  \end{feynman}
\end{tikzpicture}
\end{document}

I hope that helps! Also, very nice diagram! It's good seeing TikZ-Feynman being used :)