How to draw a colored Feynman diagram with tikz-feynman?

Question

I am trying to draw the diagram below on latex. I am not sure where to begin. I searched a lot but couldn't find any resolution. Here's what I faced:

It was easy to draw the line from left to right. But the rest had no slution. First I tried:

\begin{figure}[h]
        \centering
        \begin{tikzpicture}[baseline=(current bounding box.center)] 
            \begin{feynman}
                \vertex (x);
                \vertex[right=5cm of x] (y);
                \vertex[right=2.5cm of x] (a);
                \diagram*{
                (x) --[fermion] (a),
                (a) --[fermion] (y),
                a --[fermion, out=135, in=45, loop, min distance=5 cm] a,
                };
            \end{feynman}
        \end{tikzpicture}
        \caption{Feynman diagrams for two vertex $\phi^4$ interaction in real scalar field theory.} \label{fig:FD1}
    \end{figure}

But the result was not same. The loop is not elliptical as shown in the figure. It looked like the one in question here.

The next problem was to put color inside the loop. I found some draw options. But then the colored region has circular/square/elliptic shapes, which do not coincide with the loop. I am at a complete loss. Is there any possible solution on this?

Answer 1

The answer to the question that you linked is actually a pretty good starting point for how to draw the ellipse you want. I adapted my answer based on that answer, but I changed circle to ellipse and added a fill color of ellipseFillColor, and I set the text color to backgroundTextColor based on your screenshot. Furthermore, the tikzfeynman/dot at node b got covered by the fill of the ellipse, so I tacked on some TikZ code of my own at the end to ensure it was visible above the filled ellipse. This was my end result:

\documentclass{standalone}
\usepackage[compat=1.1.0]{tikz-feynman}
\begin{document}
\definecolor{backgroundTextColor}{HTML}{8C6D0B}
\definecolor{ellipseFillColor}{HTML}{ADDCF0}
\begin{tikzpicture}
\color{backgroundTextColor}
\makeatletter
  \begin{feynman}
  \diagram [horizontal=a to b, layered layout] {
    a  [particle={$(p_n,\mathbf p)$}]
    -- [fermion]
    b

    -- [fermion]
    c  [particle={$(p_n,\mathbf p)$}]
  };
% Define a path to the center of the ellipse at point A1, and another point
  % to the top of the ellipse at point A2
  \path (b) --++ (90:0.6) coordinate (A1) --++ (0,0.65) coordinate (A2);
% Next, draw the ellipse at point A, and color the inside
  \draw[
    fill = ellipseFillColor,
    decoration={
      markings,
      % This following part is borrowed from the tikzfeynman.keys.code.tex “with 
      % reversed arrow” style, but I changed the fill of the arrow to match the
      % fill color of the line. Otherwise, the arrow would have been blue like
      % the fill of the ellipse.
      mark=at position 0.25 with {\node[
        transform shape,
        xshift=0.5mm,
        rotate=180,
        fill=backgroundTextColor,
        inner sep=\tikzfeynman@arrow@size,
        draw=none,
        isosceles triangle
      ] {}; }
    },
    postaction=decorate
  ] (A1) ellipse (0.4 and 0.6);
% Draw dot at node b
  \drawfill = backgroundTextColor circle (0.05) node[anchor=north] {$\lambda_1$};
% Place text above ellipse
  \draw(A2) node[anchor=south] {$(r_n, \mathbf r)$};
\end{feynman}
\end{tikzpicture}
\end{document}