Loop

loop.tex (13 lines)

\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}