Properly align vertex labels (decorated with brace) of different lengths in tikz-feynman

Question

In the following (incomplete) Feynman diagram,

\documentclass{minimal}

\usepackage{tikz-feynman}

\begin{document}

  \begin{tikzpicture}
    \begin{feynman}
      \vertex (i1) {$b$};
      \vertex [right=of i1] (m1) ;
      \vertex [right=of m1] (o1) {$c$};
      \vertex [below=2em of i1] (i2) {$\bar u, \bar d$};
      \vertex [right=of i2] (m2) ;
      \vertex [right=of m2] (o2) {$\bar u, \bar d$};

      \diagram* {
        (i1) -- [fermion] (m1),
        (m1) -- [fermion] (o1),
        (o2) -- [fermion] (m2),
        (m2) -- [fermion] (i2)
      };
      
      % annotations
      \draw [decoration={brace}, decorate] (i2.south west) -- (i1.north west)
        node [pos=0.5, left] {$B^-, \bar B^{0}$};
    \end{feynman}
  \end{tikzpicture}

\end{document}

the bracket is drawn at an angle

because the labels have different lengths. Is there an easy way to fix this?

Answer 1

By use of the orthogonal coordinate (i2.west |- i1.north) for top side of brace:

\documentclass[border=3.141592]{standalone}
\usepackage{tikz-feynman}

\begin{document}
    \begin{tikzpicture}
\begin{feynman}
  \vertex (i1) {$b$};
  \vertex [right=of i1] (m1) ;
  \vertex [right=of m1] (o1) {$c$};
  \vertex [below=2em of i1] (i2) {$\bar{u}, \bar d$};
  \vertex [right=of i2] (m2) ;
  \vertex [right=of m2] (o2) {$\bar{u}, \bar{d}$};

  \diagram* {
    (i1) -- [fermion] (m1),
    (m1) -- [fermion] (o1),
    (o2) -- [fermion] (m2),
    (m2) -- [fermion] (i2)
            };
% annotations
  \draw [decoration={brace}, decorate] 
    (i2.south west) -- node [left] {$B^-, \bar{B}^{0}$} (i2.west |- i1.north);
\end{feynman}
  \end{tikzpicture}
\end{document}