\section{\bf Generic Search for Super-symmetry using Missing Transverse Energy\label{CMS-susy}}
A substantial increase of the center of mass energy relative to the Tevatron 
will provide us with a great opportunity of finding physics beyond the Standard Model (SM)
at the {\sc lhc}. Super-symmetry ({\sc susy}), one of the most popular beyond-the-SM scenario, could
show up in early days of {\sc lhc} operation if the {\sc susy} mass scale is reasonably low.
A generic signature of the production of squarks and gluinos (super-partners of the
SM quarks and gluons) is a missing transverse energy in events containing
multi-jets. For low mass {\sc susy} point LM1 ($m_{\rm gluino} \sim 600$ GeV, $m_{squarak}\sim 520$ GeV,
$\tan\beta=10$), the $E_T^{miss}$ distribution is shown
in Fig.~\ref{fig:MET-msugra}. The measured $E_T^{miss}$, after corrections, agrees
with the true distribution. 
We are working on a generic {\sc susy} search
by studying the missing transverse energy in multi-jet events. 
{\sc susy} search based on this signature has been performed at CDF\cite{CDF-JetMET-SUSY}.
%----------------
\begin{figure}[htb]
 \begin{center}
 \includegraphics[width=9.0cm,clip=]{msugra_met_corr.eps}
   \caption{\label{fig:MET-msugra}
            Missing transverse energy distribution in low mass {\sc susy} point LM1 ($m_{\rm gluino} \sim 600$ GeV, 
      $m_{squarak}\sim 520$ GeV,$\tan\beta=10$). The measured distribution, after corrections agrees with the
true $E_T^{miss}$ arising from LSPs in the event.}
 \end{center}
\end{figure}
%-----------------------

At CMS, the analysis has been initiated by M. Spiropulu {\it et al.}~\cite{CMS-PTDR}\cite{CMS-EtMiss}. 
Major backgrounds to this signal include a) beam/cosmic ray induced background, b) QCD
multi-jet events, c) $W$+multi-jet production and d) $Z$+multi-jet production.
The beam halo and cosmic ray background can be removed by selecting on event properties,
particularly cutting on charged particle energy and electromagnetic energy fractions of 
the event.
The QCD multi-jet production rate is very large but can be
effectively removed by utilizing the correlation between the $E_T^{miss}$ and jet directions 
in azimuthal angle. 

%------------------------
\begin{figure}[htb]
 \begin{center}
   \includegraphics[width=5.7cm,clip=]{met_zpt0_ee_vs_mm_allcorr.eps}
   \includegraphics[width=5.7cm,clip=]{met_zpt230_ee_vs_mm_allcorr.eps}
   \caption{\label{fig:susy-Zeemm}The $E_T^{miss}$ distribution in $\Zee$+multi-jet events compared to the same
distribution in $\Zmm$+multi-jet after electron, muon and jet corrections for inclusive events (left) and
event with $P_T^{Z}>230$ GeV (right). Two distributions agree quite well.}
 \end{center}
\end{figure}
%----------------



The $Z$+multi-jet production where $Z$ boson decays
into $\nu\bar\nu$ and $W$+multi-jet production where $W\rightarrow \tau\nu$
and $\tau$ lepton decays into hadrons constitute irreducible SM backgrounds. 
Those irreducible backgrounds must be  estimated from the data
by measuring rates in similar channels and normalizing them to the signal
region. In particular, the $Z\rightarrow \mu\mu$+multi-jet or $\Zee$+multi-jet 
rates can be used to predict the $Z\rightarrow \nu\nu$+multi-jet background.
The  $\Znunu$+multi-jet rate was estimated from $\Zmm$+multi-jet events in~\cite{CMS-PTDR} but
$\Zee$+multi-jet sample was not studied.

During last year we worked on missing $E_T$ in QCD and $Z$+jet events.
As described in Section~\ref{sec-CMS-MET-calibration}, we worked to determine the $E_T^{miss}$ calibrations.
For {\sc susy} search, we measured the corrected $E_T^{miss}$ in $Z$+jet events where $Z$ boson decays into
either electron or muon pairs and showed that these samples can be used to predict $\Znunu$ background.

The $E_T^{miss}$ from $\Zee$ is compared with $\Zmm$ in Fig.~\ref{fig:susy-Zeemm} for inclusive sample 
on left and for $230<P_T^{Z}<300$ GeV sample on the right after all the corrections including electron, 
muon and jet corrections.  The two distributions agree quite well.


%------------------------
\begin{figure}[h]
 \begin{center}
   \includegraphics[width=5.7cm,clip=]{met_zpt0_ee_vs_nn_allcorr.eps}
   \includegraphics[width=5.7cm,clip=]{met_zpt230_ee_vs_nn_allcorr.eps}
   \caption{\label{fig:susy-Znunu}Estimate of irreducible  $E_T^{miss}$ background from 
$Z\rightarrow \nu\nu$ events. The estimated distributed agrees very well with the true distributed.} 
 \end{center}
\end{figure}

The missing $E_T$ distribution in $Z\rightarrow \nu\nu$ can predicted by replacing the
electrons (muons) by neutrinos in the $\Zee$($\Zmm$) events. The predicted distributions are shown in
Fig.~\ref{fig:susy-Znunu}. The derived distributions match quite well the expected distribution from
$Z\rightarrow \nu\nu$ events. Thus this data driven technique can be used to estimate the irreducible background
from invisible $Z$ decays to the {\sc susy} signal.



