How to reproduce this Feynman diagram using tikz-feynman?

Question

I want to reproduce the following Feynman diagram

and my attempt at it yields something like this

Notice that the loop doesn't look circular and it would be nice to have those bent arrows as well. Here's my code (I'm using tikz-feynman with LuaLaTeX)

\documentclass{article}
\usepackage[compat=1.1.0]{tikz-feynman}
\begin{document}
    \begin{tikzpicture}
        \begin{feynman}
            \vertex (a);
            \node[rectangle, draw, minimum size=0.075cm,label=-120:\(F_2\)] (b) at (1.5,0);
            \node[dot, label=below:\(P_{lin}\)] (c) at (2.5,1);
            \node[dot, label=above:\(P_{lin}\)] (d) at (2.5,-1);
            \node[rectangle, draw, minimum size=0.075cm,label=-60:\(F_2\)] (e) at (3.5,0);
            \vertex [right=1.5cm of e] (f);
            \diagram*{(b) -- [fermion] (a),
            (b) -- [fermion, quarter left] (c),
            (e) -- [fermion, quarter right] (c),
            (e) -- [fermion, quarter left] (d),
            (b) -- [fermion, quarter right] (d),
            (e) -- [fermion] (f)};
        \end{feynman}
    \end{tikzpicture}
\end{document}

Please suggest a way to reproduce the Feynman diagram using tikz-feynman (in combination with tikz, if required).

Answer 1

In this simple case, it's easy to cheat:

• Draw a circle.
• Place the nodes on the circle.
  Make the rectangles use fill = white.
• Add the bended arrows without drawing the path again (tips).

Unfortunately, the fermion arrows are actually isosceles triangle nodes, so it's going to need a bit investing if we can make the arrow tips (for which the bending library makes it very easy to get bended forms of it) the same size.

(Nodes can be transformed nonlinear which is the kind of transformation applied for the bending of arrows but it's not trivial to set up.)

I also don't want to mess around with the right positioning which is why I've used the ext.arrows library from my tikz-ext package for the Centered Triangle arrow tip.

LuaLaTeX which tikz-feynman is made for would allow us to use the necklace routing for the circular edges …

Code

\documentclass{article}
\usepackage[compat=1.1.0]{tikz-feynman}
\tikzfeynmanset{warn luatex=false}
\usetikzlibrary{bending, ext.arrows}
\begin{document}
\begin{tikzpicture}
\begin{feynman}[/tikz/rect/.style={rectangle, draw, minimum size=.075cm, fill=white}]
\vertex (a);
\draw (2.5,0) coordinate (center) circle[radius=1];
\foreach \sa in {90, -90}
  \foreach \da in {45, -45}
    \path[-{Centered Triangle[scale length=2, bend]}, tips]
      ([shift=(center)]\sa:1) arc[start angle=\sa, delta angle=\da, radius=1];
\node[rect, label= -120:\(F_2\)             ] (b) at (1.5, 0);
\node[dot,  label=below:\(P_{\mathrm{lin}}\)] (c) at (2.5, 1);
\node[dot,  label=above:\(P_{\mathrm{lin}}\)] (d) at (2.5,-1);
\node[rect, label=  -60:\(F_2\)             ] (e) at (3.5, 0);
\vertex [right=1.5cm of e] (f);
\diagram*{
  (b) -- [fermion] (a),
  (e) -- [fermion] (f)
};
\end{feynman}
\end{tikzpicture}
\end{document}

Output