Scaling of tikzpicture inside tikz-feynman seems not working

Question

Consider the following code:

\documentclass{article}
\usepackage{tikz}
\usepackage{tikz-feynman}

\begin{document}
$$\begin{tikzpicture}[scale=0.5, baseline=(current  bounding  box.center)]
     \begin{feynman}
    \vertex (x);
    \vertex[right=of x] (y);
    \vertex[above left=of x] (a);
    \vertex[below left=of x] (b);
    \vertex[above right=of y] (c);
    \vertex[below right=of y] (d);
    \diagram*{
        (x) --[fermion, half left] (y),
        (x) --[anti fermion, half right] (y),
        (a) --[charged scalar] (x),
        (x) --[charged scalar] (b),
        (y) --[anti fermion] (c),
        (y) --[fermion] (d),
    };
    \end{feynman}
    \end{tikzpicture}$$
\end{document}

Although I put scale=0.5 option in the tikzpicture environment, it seems that the scaling does not work. How can I fix this problem?

Answer 1

The key scale will act only on coordinates, not the size of nodes. If you use scale=0.5, transform shape also the nodes will be scaled. If you want to apply it to every picture, you can use

\tikzset{every picture/.style={
    scale=0.5, transform shape,
    }}

Notice however that the arrows and linewidths are not scaled with the picture (they go with the linewidth); the solution to this is not simple.

If you want a seamless scaling, the best option is perhaps to use \scalebox.

\documentclass{article}
\usepackage{tikz}
\usepackage{tikz-feynman}

\newcommand{\diagram}{%
\begin{tikzpicture}[baseline=(current  bounding  box.center)]
     \begin{feynman}
    \vertex (x);
    \vertex[right=of x] (y);
    \vertex[above left=of x] (a);
    \vertex[below left=of x] (b);
    \vertex[above right=of y] (c);
    \vertex[below right=of y] (d);
    \diagram*{
        (x) --[fermion, half left] (y),
        (x) --[anti fermion, half right] (y),
        (a) --[charged scalar] (x),
        (x) --[charged scalar] (b),
        (y) --[anti fermion] (c),
        (y) --[fermion] (d),
    };
    \end{feynman}
    \end{tikzpicture}%
}

\begin{document}

Normal:

\[ \diagram \]

\begingroup %this will make the changes to every picture local
    \tikzset{every picture/.style={
        ,scale=0.5, transform shape
    }}
    Scale and transform shape:
    \[\diagram\]
\endgroup

Using \texttt{scalebox}:
\[\scalebox{0.5}{\diagram}\]

\end{document}

Please notice however the following warning:

Package tikz-feynman Warning: Consider loading TikZ-Feynman with \usepackage[co
mpat=1.1.0]{tikz-feynman} so that you can be warned if TikZ-Feynman changes. on
 input line 26.