If we want to apply Cutkosky rule, how can we draw a cutting line like the dashed line in the figure (quickly made with paint)?
\documentclass[12pt]{book}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[a4paper,left=2.5cm,right=2.5cm,top=2.8cm,bottom=2.8cm]{geometry}
\usepackage[frenchb]{babel}
\usepackage{libertine}
\usepackage{luatex85}
\def\pgfsysdriver{pgfsys-pdftex.def}
\usepackage{lmodern}
\usepackage{xcolor}
\usepackage{tikz}
\usepackage{pgfkeys}
\usepackage[compat=1.1.0]{tikz-feynman}
\usepackage{tikz-feynman, contour}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{graphicx}
\usepackage[absolute,overlay]{textpos}
\newcommand{\autofontsize}[1]{\fontsize{#1}{\dimexpr #1*12/10}}
\usepackage{setspace}
\usepackage{pgfplots}
\usepackage{xparse}
\usetikzlibrary{positioning,arrows,patterns}
\usetikzlibrary{decorations.markings}
\usetikzlibrary{calc}
\tikzset{
photon/.style={decorate, decoration={snake}, draw=black},
fermion/.style={draw=black, postaction={decorate},decoration={markings,mark=at position .55 with {\arrow{>}}}},
vertex/.style={draw,shape=circle,fill=black,minimum size=3pt,inner sep=0pt},
}
\NewDocumentCommand\semiloop{O{black}mmmO{}O{above}}
{%
\draw[#1] let \p1 = ($(#3)-(#2)$) in (#3) arc (#4:({#4+180}):({0.5*veclen(\x1,\y1)})node[midway, #6] {#5};)
}
%% Syntax
%\semiloop[fermion][<draw options>]{<first node>}{<second node>}{<angle>}[<label>][<below, default: above>];
\begin{document}
\begin{center}
\scalebox{2}{
\fontsize{0.35714em}{0.35714em}
\feynmandiagram [horizontal=a to b] {
i1 -- [fermion, arrow size=0.8pt] a [label=${\mu}$]-- [fermion, arrow size=0.8pt] i2 ,
a -- [photon] b
-- [fermion, half left, looseness=1.6, arrow size=0.8pt] c
-- [fermion, half left, looseness=1.6, arrow size=0.8pt] b,
c -- [photon] d,
f1 -- [fermion, arrow size=0.8pt] d -- [fermion, arrow size=0.8pt] f2,
};
}
\end{center}
\end{document}