Formatting cascading decay Feynman diagram with TikZ-Feynman

Question

I am trying to draw a relatively simple diagram with TikZ-Feynman but I can't quite get it to format correctly:

Here is my code

\feynmandiagram [layered layout, horizontal=a to b] {
  i1 [particle=\(q\)] -- [fermion] a,
  i2 [particle=\(\overline{q}\)] -- [anti fermion] a,
  a -- [boson, edge label=\(Z^*\)] b,
  b -- [boson, edge label=\(Z\)] b1,
  b1 -- [plain] f1 [particle=\(l^{\pm}\)],
  b1 -- [plain] f2 [particle=\(l^{\pm}\)],
  b -- [scalar, edge label=\(H\)] d,
  d -- [scalar, edge label=\(a\)] e,
  d -- [scalar, edge label=\(a\)] f,
  e -- [fermion] f3 [particle=\(b\)],
  e -- [anti fermion] f4 [particle=\(\overline{b}\)],
  f -- [fermion] f5 [particle=\(b\)],
  f -- [anti fermion] f6 [particle=\(\overline{b}\)],
};

which produces

How can I get the final state particles to format nicely, with equally long lines and angles similar to the incoming particles?

Answer 1

Sometimes it may be easier to not use the layout algorithms.

\documentclass[tikz,border=3mm]{standalone}
\usepackage{tikz-feynman}
\begin{document}
\begin{tikzpicture}
\begin{feynman}
\path coordinate[label=right:\(\ell^\pm\)]  (f1)
    -- ++ (0,-1.2) coordinate[label=right:\(\ell^\mp\)]  (f2)
    -- ++ (0,-1) coordinate[label=right:\(b\)]  (f3)
    -- ++ (0,-1.2) coordinate[label=right:\(\overline{b}\)]  (f4)
    -- ++ (0,-1) coordinate[label=right:\(b\)]  (f5)
    -- ++ (0,-1.2) coordinate[label=right:\(\overline{b}\)]  (f6)
    (f1) -- coordinate[left=1cm] (v3) (f2)
    (f3) -- coordinate[left=1cm] (v5) (f4)
    (f5) -- coordinate[left=1cm] (v6) (f6)
    (v5) -- coordinate[left=1cm] (v4) (v6)
    (v3) -- coordinate[left=1cm] (v2) (v4)
    (v2) -- ++ (-1.5,0) coordinate (v1)
     ++ (120:1) coordinate[label=left:\(q\)]  (i1)
    (v1)  ++ (-120:1) coordinate[label=left:\(\overline{q}\)]  (i2);
\diagram*{
(i1) -- [fermion] (v1) -- [fermion] (i2),
(v1) -- [photon, edge label=\(Z^*\)] (v2) -- [photon, edge label=\(Z\)] (v3),
(v2) -- [scalar, edge label=\(H\),swap] (v4) -- [scalar, edge label=\(a\)] (v5),
(v4) -- [scalar, edge label=\(a\),swap] (v6),
(f2) -- [fermion] (v3) -- [fermion] (f1),
(f4) -- [fermion] (v5) -- [fermion] (f3),
(f6) -- [fermion] (v6) -- [fermion] (f5),
};
\end{feynman}
\end{tikzpicture}
\end{document}