将箭头从费米子线的中间移开

Question

我正在尝试使用该包绘制下图tikz-feynman。

\documentclass{standalone}
\usepackage{tikz-feynman}
\begin{document}
\begin{tikzpicture}
    \begin{feynman}
        \vertex [dot, label=180:$x$]   (x)  at (0,0) {};
        \vertex [dot, label=0:$x'$]    (y)  at (2,0) {};
        \vertex [dot, label=180:$x_1$] (x1) at (0,2) {};
        \vertex [dot, label=0:$x_2$]   (x2) at (2,2) {};

        \diagram*{
            (x1) -- [fermion, half left, out=45, in=135] (x2)
                 -- [fermion] (x)
                 -- [fermion] (y)
                 -- [fermion] (x1),
            (x1) -- [photon] (x2),
        };
    \end{feynman}
\end{tikzpicture}
\end{document}

如您所见，该图有两条费米子线，它们在中间相交，箭头就画在那里。我想沿着费米子线移动箭头，这样两条费米子线就不会重叠。我该怎么做？

Answer 1

我可能会引入一个虚拟顶点，也以表明线不相交。

\documentclass{standalone}
\usepackage{tikz-feynman}
\begin{document}
\begin{tikzpicture}
    \begin{feynman}
        \vertex [dot, label=180:$x$]   (x)  at (0,0) {};
        \vertex [dot, label=0:$x'$]    (y)  at (2,0) {};
        \vertex [dot, label=180:$x_1$] (x1) at (0,2) {};
        \vertex [dot, label=0:$x_2$]   (x2) at (2,2) {};
        \vertex [empty dot, minimum size=0mm] (dummy) at (1,1) {};
        \diagram*{
            (x1) -- [fermion, half left, out=45, in=135] (x2)
                 -- [fermion] (dummy)
                 -- [fermion] (x)
                 -- [fermion] (y)
                 -- [fermion] (dummy)
                 -- [fermion] (x1),
            (x1) -- [photon] (x2),
        };
        \draw[-,line width=2mm,white] (dummy)++(-0.2cm,-0.2cm) -- ++(0.4cm,0.4cm);
        \draw[-,black] (dummy)++(-0.2cm,-0.2cm) -- ++(0.4cm,0.4cm);
    \end{feynman}
\end{tikzpicture}
\end{document}