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\section{Event Selection}\label{sec:Selection}
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Event selection proceeds in several steps :
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\begin{itemize}
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\item {\bf Trigger : } We first require that one of the relevant double-lepton triggers has fired. No attempt is made to match reconstructed leptons to trigger objects.
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\item {\bf Lepton Pre-selection : } $p_{T} > 5$ muon and $p_{T} > 7$ electron candidates with impact parameter significance $< 4$ and with a relative $Iso_{PF}/p_{T}$ satisfying the requirements are selected for the analysis.
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\item {\bf Z1 Selection : } Next, we attempt to reconstruct an on-shell Z boson (Z1) from a pair of same-flavor, oppositely-charged lepton candidates with $m_{\ell\ell}$ between $60-120~\rm GeV$. One of the Z1 legs must have $p_{T} > 20~\rm GeV$ while the other must have $p_{T} > 10~\rm GeV$. Muons must pass our standard set of quality cuts. Z1 electrons are required to pass the loose BDT electron ID working point. Of the possible combinations of identified leptons, the Z1 pair is chosen as those that provide an $m_{\ell\ell}$ closest to $m(Z)$.
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\item {\bf Z2 Selection : } We then attempt to reconstruct a second Z boson (Z2) from a distinct pair of same-flavor, oppositely-charged, identified lepton candidates with $m_{\ell\ell} < 120~\rm GeV$ and $m_{\ell\ell} > 12~\rm GeV$. No additional $p_{T}$ requirements are imposed beyond those of the predilection. Z2 muons must pass our standard muon identification requirements while electrons must pass the medium BDT working point. Of the possible lepton combinations satisfying these requirements, the pair with the largest individual $p_{T}$ are associated with Z2.
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\item {\bf $m(4\ell)$ Selection : } From the remaining events we select those with $m(4\ell) > 100\rm~GeV$. We do not impose mass windows on candidate events when determining significance and limits.
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\end{itemize}
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