101.2$ & $81.2-101.2$ & $81.2-101.2$ & $81.2-101.2$ \\ \hline \met\ &$ > 50$ & $> 75$ &$>75$ &$75-120$ &$75-120$ &$>120$ \\ \hline $M_T $ &$-$ &$-$ &$>110$ &$<110$ &$>110$ &$>110$ \\ \hline $p_{T(\ell_3)}$ &$>10$ &$>10$ &$>30$ &$>10$ &$>10$ &$>10$ \\ \hline SR veto & SRnoZc & SRnoZc & $-$ & $-$ & $-$ & $-$ \\ \hline \end{tabular}} ]]>

0.5$\,GeV, within a cone centered along the momentum vector of \muco\ and satisfying $\Delta R = 0.4$. Impose $I_{\rm sum} < 3 $\,GeV. \end{enumerate} The main backgrounds, containing two collinear muons along with a third lepton and \met, include $W(\to \ell^{\pm}v)\gamma^*$ and $Z(\to \ell^+\ell^-)\gamma^*$, wherein the \muco\ comes from the photon, and $Wb\bar{b}$, when one of the $b$-jets produces the \muco. The $Z\gamma^*$ background fakes the signal process when one of the two leptons escape undetected. Note that while the $t\bar{t}$ background mentioned above for the $3\ell$ search is also relevant for this signal process, it becomes negligible here owing to the requirement of the two final state muons being highly collinear. In order to isolate these backgrounds, we implement the following cuts. \begin{enumerate} \item Since the $A_1$ resulting from the higgsino decay is highly boosted, we expect the \muco\ from its subsequent decay to have a large $p_T$. We therefore require $p_{T(\muco)} > 50 $\,GeV. We also require $p_{T(\ell_3)} > 20 $\,GeV. \item In order to reduce the background a large \met\ is required, so we add the cut $\met > 50$\,GeV. \item In the $W\gamma^*$ background the $M_T$ distribution has an end point around the $W$ boson mass, which leads us to impose $M_T > 80$\,GeV. \item Our signal would appear as a narrow peak in the $m_{\muco}$ distribution. Hence, we impose a narrow cut width, $5\times \sigma_{A_1}$, where $\sigma_{A_1}=0.26+0.0013m_{A_1}$, around $m_{A_1}$. This parametrization of $\sigma_{A_1}$ follows the mass resolution study of the $J/\Psi$ resonance in~\cite{Chatrchyan-ml-2011hr}. We also remove the $J/\Psi$ resonance region ($3.0\gev < m_{\muco}< 3.2\gev$). \end{enumerate} To get a sufficient number of Monte Carlo events in the kinematic regime of our interest, we require $m_{\mu\bar{\mu}} > 1.5$\,GeV and $p_{T(\ell_3)} > 10$\,GeV at the parton level for the $W\gamma^*$ and $Z\gamma^*$ backgrounds. For the $Wb\bar{b}$ background, we additionally require the $p_T$ of the $b$-jet to be larger than 30\,GeV. ]]>