Feynman loop diagram

Question

I am new to feynman-tikz and I am trying to reproduce the following image:

Here is what I have so far:

\feynmandiagram [layered layout, horizontal=a to b]  {
        {i1, i2} -- a[empty dot] -- [half left] b[empty dot] -- [half left] a,
        b -- {f1, f2},  
        };

I'm just not sure how to place that the upper and lower vertices connecting the vertical loop. Any help is appreciated.

Answer 1

Here's a way to do it. Because spring layout seemed a bit hard to tame, I used absolut vertices, which I connected later.

The solution below produces the left part, while the right part is the solution posted in the question.

\documentclass[10pt, border=10pt]{standalone}
\usepackage{tikz-feynman}

\begin{document}
  % placing absolut vertices first
  \tikz{
  \begin{feynman}
    % specifying the vertices
    \vertex (a) [empty dot]                         {};
    \vertex (b) [dot, above right=2cm of a]         {};
    \vertex (c) [empty dot, below right=2cm of a]   {};
    \vertex (d) [empty dot, right=2.8cm of a]       {};
    
    \vertex (i1) [above left=2.5cm of a] {};
    \vertex (i2) [below left=2.5cm of a] {};
    
    \vertex (f1) [above right=2.5cm of d] {};
    \vertex (f2) [below right=2.5cm of d] {};
    
    % drawing the connections
    \diagram*{% NO spring layout, though it dowsn't seem to matter
        % loops
        (a) -- [quarter left] (b),
        (b) -- [quarter left] (d),
        (a) -- [quarter right](c),
        (c) -- [quarter right](d),
        (b) -- [quarter right] (c),
        (b) -- [quarter left] (c),
        
        % initial
        (i1) -- (a),
        (i2) -- (a),
        
        % final
        (f1) -- (d),
        (f2) -- (d)
    };

  \end{feynman}
}
\hspace{1em} 

% initial solution
\feynmandiagram [layered layout, horizontal=a to b]  {
        {i1, i2} -- a[empty dot] -- [half left] b[empty dot] -- [half left] a,
        b -- {f1, f2},  
        };

\end{document}