Loop

Edit and compile if you like:

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\documentclass{standalone}
\usepackage{tikz-feynman}
\begin{document}
$\displaystyle\partial_t \frac{\partial V}{\partial \chi}
  = \raisebox{0.5ex}{\feynmandiagram [inline=(a.base)] {
  i1 -- a [dot]
  -- [charged boson, half left, edge label=$q$] b [crossed dot]
  -- [charged boson, half left, edge label=$q$] a,
  };}$
\end{document}
 
 
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