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\def\Title#1{\begin{center} {\Large {\bf #1} } \end{center}}
\begin{document}
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\Title{Search for Supersymmetric particles using final states with one lepton, jets and missing transverse momentum with ATLAS detector}
\bigskip\bigskip
\begin{raggedright}  
{\it Yuichi Sasaki on behalf of the ATLAS collaboration\index{Y. Sasaki}\\
Department of Physics, Graduate School of Science, The University of Tokyo.\\
113-0033, Tokyo, Japan.
}\bigskip\bigskip
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%% Introduction %{{{
%\section{Introduction}
%\section{Analysis}

Supersymmetry~(SUSY) is one of the promising theories
which propose a solution to the hierarchy problem of the Standard Model~(SM) and a natural candidate for dark matter.
We search for a SUSY signature in the final state with one isolated lepton, multijets, and large missing transverse momentum~($E_{T}^{miss}$).
The full dataset~(4.7~fb$^{-1}$) recorded by the ATLAS~\cite{ATLAS} detector at $\sqrt{s}=$7~TeV in 2011 is used in the analysis.
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%% Analysis %{{{
%\section{Analysis and Results}

As a pre-selection, exactly one isolated electron~(muon) with $p_{T}>$25~(20)~GeV, transverse mass\footnote{$m_{T}=\sqrt{2\,p_{T}^{lep}\,E_{T}^{miss}\,(1-\cos\Delta\Phi(lep,E_{T}^{miss}))}$} $m_{T}>$100~GeV, and $E_{T}^{miss}>$250~GeV are required. Events with additional leptons are vetoed.
%Two signal regions~(SRs) are then defined to cover different production processes, e.g., squark pair production and 
%gluino pair production, under vanilla SUSY assumptions. 
%3jet-SR : $p_{T}$ of the leading jet is required to be larger than 100~GeV, and the 3rd leading jets with $p_{T}>$25~GeV should exist.
%In order to be exclusive to the 4jet-SR, if the event has more than 4jets, the $p_{T}$ of the 4th leading jets is required to be smaller than 80~GeV.
%In addition, $E_{T}^{miss}/m_{eff}>$0.3, and $m_{eff}>$1200~GeV are required,
%where $m_{eff}$ is defined as a scalar sum of the transverse momentums of lepton, $E_{T}^{miss}$, and jets.
%4jet-SR : After the pre-selection, at least 4jets with $p_{T}>$80~GeV,
%$E_{T}^{miss}/m_{eff}>$0.2, and $m_{eff}>$800~GeV are required.
To cover both squark-pair production and gluino-pair production, two signal regions (3-jet SR and 4-jet SR) are defined after the pre-selection.
The 4-jet SR has at least four jets with $p_T >$ 80~GeV and $E_T^{miss}/m_{eff}>$0.2 and $m_{eff}>$800~GeV, where $m_{eff}$ is the scalar sum of the
transverse momenta of the lepton, $E_T^{miss}$ and jets. The 3-jet SR is required to have a leading jet with $p_T>$100~GeV, at least two additional
jets with $p_T>$25~GeV, $E_T^{miss}/m_{eff}>$0.3 and $m_{eff} >$ 1200~GeV;
Events in which the second, third and fourth jets have $E_T >$ 80~GeV are removed from the 3-jet SR as these events are in the 4-jet sample.

Further improvement in sensitivity is achieved by splitting the SRs into bins in $m_{eff}$,
thereby the shape differences of signal and backgrounds are taken into account.

The dominant backgrounds in both of the SRs are W+jets and $t\bar{t}$, which are estimated by Monte Carlo simulation.
They are normalized in two dedicated Control Regions~(CRs), 
where the following kinematic selections are applied to enhance the components:
$m_{T}=40-80$~GeV, $E_{T}^{miss}=30-120$~GeV, $m_{eff}>400$~GeV,
and at least three jets with $p_{T}>$25~GeV are required, where the leading jet should have $p_{T}>80$~GeV.
W+jets and $t\bar{t}$ components are then separated by the existence of $b$-jet in the leading 3jets.
If there is at least 1 $b$-tagged jet, the event is classified into $t\bar{t}$ CR, and if there is no $b$-tagged jet,
then the event goes to W+jets CR.

%W+jets component is enhanced by 
%$m_{T}$ selection~($m_{T}$=40-80~GeV) and $b$-jet veto,
%then extrapolated to the SRs based on the Monte Carlo shape.
%The number of events in each SR is then estimated based on Monte Carlo  by extrapolating the shape 
The dominant uncertainty in the SR prediction comes from the shape variation caused by the energy scale uncertainty of jets.
Multijets background is estimated in a data-driven way.
%Other minor backgrounds~(single $t$, Dibosons) are estimated based on the Monte Carlo simulations, where extra 20\% uncertainties are added to their cross-sections.
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%% Result and Interpretations %{{{
%\section{Result and Interpretation}

As shown in Figure~\ref{fig:meff}, three events are observed in 3jet-SR, where the SM predicts 5.7$\pm$4.0 events,
while six events are found in 4jet-SR, which is consistent with our expectation of 8.3$\pm$3.1.
Since no significant excess is observed in both of the SRs, 95\% CL exclusion limit is calculated
in the MSUGRA/CMSSM model~(Figure~\ref{fig:limit}).
Squark mass has been excluded up to 1.2~TeV.
%%
%\vspace{-5pt}
\begin{figure}[htb]
\begin{center}
\epsfig{file=3jets.eps, width=0.40\textwidth}
\epsfig{file=4jets.eps, width=0.40\textwidth}
\caption{m$_{eff}$ distributions in 3jet-SR(left) and 4jet-SR(right) \cite{CONF}.}
\label{fig:meff}
\end{center}
\end{figure}
\vspace{-20pt}
\begin{figure}[htb]
\begin{center}
\epsfig{file=limit.eps,width=0.55\textwidth}
\caption{Expected and observed 95\% CL exclusion limits \cite{CONF}.}
\label{fig:limit}
\end{center}
\end{figure}
\vspace{-20pt}
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%% Biblography %{{{
\begin{thebibliography}{99}
%\bibitem{LHC}
%L. Evans and P. Bryant, 2008 {\it JINST} {\bf 3} S08004.
\bibitem{ATLAS}
ATLAS Collaboration, 2008, {\it JINST} {\bf 3} S08003.
\bibitem{CONF}
ATLAS Collaboration, ATLAS-CONF-2012-041. %, http://cdsweb.cern.ch/record/1435195.
This result has been published in combination with 2-lepton and soft-lepton results : arXiv:1208.4688v1
\end{thebibliography}
\end{document}
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