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I want to draw the diagram I show below (with its MWE), but the loop doesn't look very "circular". Setting looseness=1.7 is the best I can get, but it still a bit oval-like. Is there a way to make it a proper circle? Maybe using some tikz functions would work, but I know nothing about tikz, much less how to mix it with feynman-tikz. Thanks in advance!

\documentclass[11pt]{standalone}
\usepackage{tikz}

\usepackage[compat=1.1.0]{tikz-feynman}

\begin{document}

\feynmandiagram[layered layout, horizontal=b to c]{
    a [particle=\(\mu\)] -- [photon, momentum=\(p\)] b
    -- [fermion, half left, looseness=1.7, edge label=\(k\)] c
    -- [fermion, half left, looseness=1.7, edge label=\(k-p\)] b, c -- [photon, momentum=\(p\)] d [particle=\(\nu\)],
};

\end{document}

enter image description here

  • Have you read those (1, 2, 3)? First two seem quite easy to implement. – mavzolej Jul 26 at 22:46
  • I read 1 and 2. In the first one, the circle is added to an underlying diagram (just a line in that case), whereas in my case avoiding the circle and adding it afterwards would imply breaking the diagram in half. In the second one, he gets the same as me (although the circles seem much nicer when he bends the arrows, but I don't need that). I hadn't seen the third one. It seems quite involved and doesn't use feynman-tikz package, but maybe I could give it a try. – Alan Rapoport Jul 26 at 22:56
  • Also, try four quarters, like on the page 21 here. – mavzolej Jul 27 at 6:41
  • I thought of that too, but I don't know how to put the arrows in the middle of two quarters. – Alan Rapoport Jul 27 at 13:32
  • \feynmandiagram [horizontal=a to b] { a -- [fermion, quarter left] b -- [quarter left] c -- [fermion, quarter left] d -- [quarter left] a, }; But now it's hard to attach lines on the sides... – mavzolej Jul 27 at 23:32

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