Move arrow away from the middle of the line in a fermion line

Question

I'm trying to draw the following diagram with the tikz-feynman package.

\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}

As you can see, the diagram has two fermion lines that intersect right in the middle, where their arrows are drawn. I would like to move the arrows along the fermion lines, so that the two ones don't overlap. How can I do this?

Answer 1

I would probably introduce a dummy vertex, also to indicate that the lines do not intersect.

\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}