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\Title{Searches for Exotics physics states in jets and boosted objects final states at ATLAS}

\bigskip\bigskip

%+\addtocontents{toc}{{\it D. Reggiano}}
%+\label{ReggianoStart}

\begin{raggedright}  

{\it Ludovica Aperio Bella\index{Aperio Bella, L.}, on behalf of the ATLAS Collaboration. \\
LAPP (Laboratoire d'Annecy-le-Vieux de Physique des Particules)\\
%Centre National de la Recherche Scientifique\\
9 Chemin de Bellevue - BP 110, 74941 Annecy-le-Vieux, FRANCE}
\bigskip\bigskip
\end{raggedright}

% 
% \begin{abstract}
% We present searches for new particles decaying to jets and
% boosted objects final states using data collected in 7
% TeV LHC proton-proton collisions with the ATLAS detector.
% We present studies of states like diljets, photons and jets,
% monojet, $t\bar{t}$ resonances and 4th generation particles decaying hadronically.
% These studies demonstrate the usage of novel techniques for reconstruction of 
% boosted objects and test wide set of predictions for Exotics extensions to Standard Model.
% \end{abstract}


\section{Introduction}
Due to the outstanding performance of the Large Hadron Collider~\cite{lhc} (LHC) that in 2011 delivered more
than $5~\mbox{fb}^{-1}$ of proton-proton collision data at center-of-mass energy of 7 TeV, the ATLAS experiment~\cite{atlas} has been
able to explore a wide range of exotic models in order to address the questions unanswered by the Standard Model (SM)
of particle physics.
%In particular jet signatures allow to probe the highest energies directly accessible in proton-proton collisions.
% At high energies, jet production is expected to be the dominant process at LHC,
% allowing us to search for evidence of new physics
% with cross sections potentially higher even than the SM jet production.
%
%ATLAS searches for new particles decaying to jets and
%boosted object final states using data collected in $\sqrt{s}=7$ TeV LHC proton-proton collisions %with the ATLAS detector
%%decaying to jets, monojet, new heavy quarks and resonant production of hight mass top quark pairs 
%are presented in this paper.
%
ATLAS searches for new particles decaying to jets, photons and jets,
 monojet, $t\bar{t}$ resonances and 4th generation particles decaying hadronically are presented in this paper.
% These studies demonstrate the usage of novel techniques for reconstruction of 
% boosted objects and test wide set of predictions for Exotics extensions to Standard Model.
%
No evidence for physics beyond the SM is found and model-independent limits and limits on the parameters of particular
models are set.

\section{New Physics searches with jets in the final state}
\subsubsection*{Searches in Dijet Mass and Angular Distribution}
 The production of events with two energetic jets of particles (dijet events)
is well understood within the Standard Model. An enhanced production of dijet final states
is expected in several scenarios of new physics.  
A variety of models of new physics, including models with excited quarks and axigluons,
predict the resonant production of states decaying predominantly to two jets. 
Other models, such as quark contact interactions, predict an excess of events 
with two jet of central rapidity and forming a high invariant mass.
By studying the dijet invariant mass ($m_{jj}$)
and the dijet angular distributions, sensitive searches for both resonant and non-resonant
deviations from the Standard Model are performed.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\begin{figure}[htb]
\begin{center}
\epsfig{file=fig_dijet1.eps,height=2.15in}
\epsfig{file=fig_dijet2.eps,height=2.15in}
\caption{Left: The $\chi$ distributions for different dijet mass bins. Right: $F_{\chi}(m_{jj})$ distribution versus $m_{jj}$. 
The QCD predictions are shown with theoretical and total systematic uncertainties (bands) 
as well as the data points with statistical uncertainties~\cite{dijet}. } 
% The dashed line is the prediction for a QBH signal for MD = 4.0 TeV and n = 6 in the highest mass bin. 
% The distributions have been offset by the amount shown in the legend to aid in visually comparing the shapes in each mass bin.}
\label{fig:dijet}
\end{center}
\end{figure}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
In the dijet resonance search, the $m_{jj}$ of the two leading jets is studied using $4.8~\mbox{fb}^{-1}$
of data collected in 2011. In the analysis the BumpHunter~\cite{bumphunter} algorithm is used to evaluate
the presence of an excess of events in the $m_{jj}$ spectrum on top of a smooth background estimate by fitting the
data with the formula 
$f(x) = p_1(1 - x)^{p2} x^{p3 +p4 \ln x}$ (where the $p_i$ are the fit parameters and $x \equiv m_{jj}/\sqrt{s}$). % and $p_i$ the fit parameters.
No resonant structure is founded and masses of excited quark less then 3.35 TeV are excluded at 95\% confidence level (C.L.).
%Details of the dijet analysis can be found on Reference~\cite{dijet}.\\
In the angular analysis convenient variables that emphasizes the dijet central scattering region are employed. 
The $\chi$ variable is defined as the exponential of the rapidity difference of the two jets 
with the highest transverse momenta ($p_T$). 
In Figure~\ref{fig:dijet} (left) the $\chi$ distribution for different $m_{jj}$ interval is shown in data
along with background predictions from NLO QCD.
% $\exp{|y_{1} - y_{2}|}$ for leading (second-leading) transverse momenta jet rapidities $y_1$ ($y_2$).
The $F_{\chi}(m_{jj})$ variable is defined as the
ratio  $F_{\chi}(m_{jj}) = N_{central}/N_{total}$ where $N_{central}$ is the number of dijet events
in a defined central region, and $N_{total}$ is the number of dijet events in the full distribution
extending to $\chi < 30$.
%fraction of events, as a function of $m_{jj}$, where the dijets have proximate rapidities. 
This variable has excellent sensitivity to resonant as well as non-resonant excesses of central jets.
The resulting distribution for different $m_{jj}$ values
is shown in Figure~\ref{fig:dijet} (right) for the data and the NLO QCD background.
Data and QCD prediction agrees in each $m_{jj}$ bin. Limits at 95\% C.L. are set on models of contact interactions and quantum black holes.
The full analysis details, including limits on additional models, are given in Reference~\cite{dijet}.




\subsubsection*{Searches in Photon-Jet Mass Distribution}
The photon-jet invariant mass distribution ($m_{\gamma j}$) 
is shown to be a useful tool for searching for resonances indicative of new
physics. The most recent ATLAS results~\cite{gammajet} are obtained from the analysis of $2.11~\mbox{fb}^{-1}$
of 2011 proton-proton collision data. Data events with $m_{\gamma j}$ masses up to 2 TeV
are compared with a smooth background estimate, and no evidence of resonant production is found. Limits are set on generic 
Gaussian-shape signals (excluded below 2 TeV) and on a benchmark excited-quark model excluding masses below 2.46 TeV. 


\subsubsection*{Searches in Monojet plus Missing Transverse Momentum Final States}

The search for new physics in events with a jet of high transverse
energy and large missing transverse energy constitute
one the simplest and most striking signatures that can be
observed at a hadron collider. Different theoretical models
for physics Beyond the Standard Model predict the
presence of \emph{monojet} signatures in the final state like, for
example, Large Extra Dimension scenarios.
In the analysis presented in Reference~\cite{monojet} $1~\mbox{fb}^{-1}$ of data are used to select events with one high $p_T$ 
jet and a large amount of missing transverse energy. A lepton veto is applied to reduce the electroweak
background. The shape of the dominant electroweak background is taken from Monte
Carlo normalized using data in control regions, while the multi-jet contribution and the non-collision background 
are estimated with a data-driven technique.
%The background and data distributions are shown in Figure 4. 
%This translates to model-independent limits on cross section times acceptance at 0.11 pb,
%and limits on MD for ADD extra dimensions of, for example, 3.16 TeV for two extra dimensions
%and 1.99 TeV for six extra dimensions. Details of the monojet search can be found elsewhere [8
Good agreement is observed between the data and the Standard Model predictions.
Model-independent upper limits at 95\% C.L. 
are set on the fiducial cross section for the non-Standard Model production
of different signal $p_T$-region %final states,
 varying between 2.02 pb and 0.045 pb. Additionally, an
interpretation is made in terms of the Large Extra Dimensions model. 
Values of the fundamental Planck scale between 3.2 and
2.0 TeV are excluded for a number of extra dimensions corresponding to 2 and 6, respectively.



\section{Searches of High-Scale physics scenarios with Boosted Objects in the Final State}
Hadronic decays of heavy particles such as W bosons, top quarks and
potential hitherto unobserved particles may be collimated into a single heavy jet characterized by a distinct substructure and
large mass. In this section exotic analyses using boosted or mostly resolved reconstruction topology are presented.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%
%%   use this format to include an .eps figure into your paper
%%
\begin{figure}[htb]
\begin{center}
\epsfig{file=fig_boosted3.eps,height=2.1in}
%\epsfig{file=fig_boosted2.eps,height=2in}
\epsfig{file=fig_Semiboosted.eps,height=2.1in}
\caption{Left: 
%Comparison of the data and the Standard Model prediction for the jet mass of the fat R = 1 jets 
%selected as the hadronically decaying top quark candidate.
%The shaded band indicates the normalization uncertainty on the Standard Model prediction. %but does not include the shape uncertainty or the impact of uncertainties on reconstructed objects. 
Reconstructed invariant mass distribution of the  $t\bar{t}$
candidates after the lepton plus jets signal selection. The shaded band indicates the uncertainty in the 
normalization of the Standard Model prediction~\cite{boost}. % The variable bin size is chosen to match the mass resolution for a resonant signal~\cite{boost}.
%Right: Expected (dashed line) and observed (solid line) upper limits on the production cross section times the $t\bar{t}$
%branching fraction of Z'. The smooth solid (red) lines correspond to the predicted production cross section times branching
% fraction for the Z' model. The band around the signal cross section curve represents the effect of the PDF uncertainty on the prediction. From Reference~\cite{boost}. } 
Right: Distribution of the numbers of events observed in the data and expected from SM processes
for $N_{jets} = 6, 7, \geq 8$ with $N_W = 0, 1, \geq 2$. The expected $b'$ signals for two masses are also shown, 
stacked on top of the backgrounds~\cite{Wsemiboosted}.}
\label{fig:boost}
\end{center}
\end{figure}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\subsubsection*{Search for $t\bar{t}$ resonances in lepton plus jets events with highly
boosted top quarks}
A search for resonant production of high-mass top-quark pairs is performed on $2.05~\mbox{fb}^{-1}$ of ATLAS 7 TeV data~\cite{boost}. 
The signal is assumed to originate from the resonance of a leptophobic Z' or a Kaluza-Klein gluon.
The analysis focuses on the lepton plus jets final 
state obtained when one W boson decays to a charged lepton and a neutrino, and the other
decays to a quark and an anti-quark pair. The selection and reconstruction are specifically
designed for the collimated topology that arises from the decay of boosted top quarks. The
hadronically decaying top quark candidate is identified as a single \emph{fat} jet with radius parameter
R = 1.
%This analysis of the lepton plus jets final state is specifically designed for the particular topology
%that arises from the harmonically decay of highly boosted top quarks, that are reconstructed as a \emph{fat} jet.
%In the events the lepton and jet form the leptonically decaying top quark and the hadronically decaying top quark are
%expected to form a single fat jet.
%We search for the distinct shape of a resonant signal in the reconstructed $t\bar{t}$ invariant mass distribution.
% The main irreducible background from the SM is the tt production. The background consisting
%  of the W boson production in association with jets is modeled using simulation and
% data-driven corrections. The multi-jet background, which contains fake leptons, is estimated
% with data-driven templates. 
% The reconstructed tt invariant mass is used to distinguish between
% signal and background, and its shape is taken into account to derive upper limits on the signal
% production cross section times branching ratio. The limits are compared to the theory.
% Leptophobic Z with masses between 500 and 860 GeV are excluded, as illustrated in Fig-
% ure 2. In the case of standard RS couplings between the KK-gluon and quarks, KK-gluon with
% masses between 500 and 1025 GeV are discarded.
% 
% The observed $t\bar{t}$ invariant mass spectrum is
% found to be compatible with the Standard Model prediction and 95\% confidence level upper limits are
% derived on the $t\bar{t}$ production rate through new massive states. An upper limit of 0.7 pb is set on the
% production cross section times branching fraction of a narrow 1 TeV resonance. A Kaluza–Klein gluon
% with a mass smaller than 1:5 TeV is excluded.
% 
% 
In Figure~\ref{fig:boost} (left) the reconstructed $t\bar{t}$ mass spectrum is compared with a template for the Standard
Model prediction constructed using a combination of Monte Carlo simulations and data-driven measurements
using control samples. The data are found to be compatible with the SM within
uncertainties. Upper limits at 95\% CL on the production cross section times the branching
ratio of narrow Z' resonances and broad colored resonances are derived.
Leptophobic Z' with masses between 600 GeV and 1.2 TeV is excluded and 
 %range from approximately 8 pb for a mass of 600 GeV
%to 220 fb for a mass of 1.6 TeV are founded in good agreement with the expected limits. 
%Slightly weaker limits are derived on a broad resonance. 
Kaluza-Klein gluon with a mass smaller than 1.5 TeV is also excluded. 
The sensitivity of this search obtained with highly boosted top quarks is
significantly enhanced in the 1-2 TeV region with respect to a previously published search
using the same data set~\cite{ttbar}. %The limit on the KK gluon is the most stringent limit on this model to date.


\subsubsection*{Search for exotic heavy quarks}

A search for pair production of a fourth generation down-type quark $b'$ in events with one
lepton is performed with $1.04~\mbox{fb}^{-1}$ of 7 TeV data~\cite{Wsemiboosted}.
In the model where $b'$ is a chiral quark with mass larger than $m_t + M_W$,
the predominant decay mode is $b' \rightarrow Wt\rightarrow WW b$, which leads to four W bosons and two $b$ quarks
in the $b'\bar{b'}$ production events.
Due to the high expected $b'$ mass, the W bosons coming from the $b' \rightarrow Wt$ decay are expected
to have a high momentum and the decay products of such W bosons should be closer together
than jets for background processes, but still resolvable as separate jets (mostly resolved topology). %with spatial separation ∆R ≈ 2mW /pW .
Considering this, the quantity suitable for distinguishing $b'$ signal from background is
the number of jet pairs with small opening angle and an invariant mass close to the W boson mass.
% %
% The dominant background in this analysis is the t t production with additional jets. The second
% largest contribution is due to W +jets production. Prediction of these backgrounds are signiﬁcantly
% affected by the theoretical uncertainties on the level of gluon radiation and experimental uncertain-
% ties on jet energy scale and resolution. The other backgrounds are due to single top production,
% diboson production (WW ,W Z, ZZ), Z+jets events in which a lepton is not detected and multi-jet
% production with one jet being misidentiﬁed as a lepton.
% miss
% The ﬁnal state signature of the decay contains exactly one lepton (e or µ), ET and at least
% six jets passing the object selection. To reduce the multi-jet background additional selection is
% miss
% miss
% required: ET > 35 GeV and mW > 25 GeV in the events with an electron, and ET > 20 GeV
% T
% miss
% and ET + mW > 60 GeV in the events with a muon.
% T
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% The quantity of interest is the number of reconstructed W bosons ($N_W$) defined as the number
% of the pairs of jets separated by $\Delta R < 1.0$ with an invariant mass in the range 70 − 100 GeV.
The
final discriminant consists of nine exclusive bins as a function of the multiplicity of hadronic W
decays ($N_W = 0, 1, \geq 2$)
and jet multiplicity ($N_{jet} = 6, 7, \geq 8$), as shown in Figure~\ref{fig:boost} (right).
% The most likely value of the b b cross section in the nine bins of (NW , N jet ) multiplicity
% is extracted with a binned maximum likelihood ﬁt using a proﬁle likelihood ratio, varying each
% background rate within its uncertainty, and allowing shape and rate variation due to the systematic
% uncertainties. The signal and background rates are ﬁtted simultaneously.
% Good agreement between data and expectation. Since no evidence of $b'$ quark production is observed
% 95\% C.L. exclusion limit is set on the $b'$ pair production cross section as a function of the $b'$ quark mass employing th CLS method. This
% translates into an observed (expected) lower bound on the b quark mass of 480 (470) GeV, as
% illustrated in Fig. 8.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%5
% Nine exclusive bins are examined, split by the multiplicity of hadronic W ($N_W = 0, 1, \geq 2$)
% and jet multiplicity ($N_{jet} = 6, 7, \geq 8$). 
A binned maximum likelihood fit is performed to derive
the most likely cross-section of $b'\bar{b'}$ pairs. No evidence of $b'$ production is observed with $1.04~\mbox{fb}^{-1}$
data. The $b'$ masses below 480 GeV are excluded at 95\% confidence level.




\begin{thebibliography}{99}

%%
%%  bibliographic items can be constructed using the LaTeX format in SPIRES:
%%    see    http://www.slac.stanford.edu/spires/hep/latex.html
%%  SPIRES will also supply the CITATION line information; please include it.
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\bibitem{lhc}
L. Evans and P. Bryant (editors) JINST (2008) 3 S08001.\\[-20pt]

\bibitem{atlas}
ATLAS Collaboration, JINST (2008) 3 S08003.\\[-20pt]

\bibitem{dijet}
ATLAS Collaboration, ATLAS-CONF-2012-038, {http://cdsweb.cern.ch/record/1432206/}.\\[-20pt]

\bibitem{bumphunter}
Choudalakis, G, arXiv.1101.0390, {On hypothesis testing, trials factor, hypertests and the BumpHunter}.\\[-20pt]
 
\bibitem{gammajet}
ATLAS Collaboration, PRL {\bf 108} (2012) 211802.\\[-20pt]

\bibitem{monojet}
ATLAS Collaboration, ATLAS-CONF-2011-096, {http://cdsweb.cern.ch/record/1369187/}.\\[-20pt]

\bibitem{boost}
ATLAS Collaboration, Submitted to JHEP, arXiv:1207.2409 (2012) [hep-ex].\\[-20pt]

\bibitem{ttbar}
ATLAS Collaboration, ATLAS-CONF-2012-029, {https://cdsweb.cern.ch/record/1430738/}.\\[-20pt]

\bibitem{Wsemiboosted}
ATLAS Collaboration, PRL {\bf 109} (2012) 032001.\\[-20pt]


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     \bigskip\bigskip      {\Large {\bf Discussion}} \bigskip}
\def\speaker#1{{\bf #1:}\ }
\def\endDiscussion{}

 
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