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\begin{document}

\Title{Search for Contact Interaction in dilepton events from pp collisions at $\sqrt{s}$ = 7 TeV with the ATLAS detector}

\bigskip\bigskip

\begin{raggedright}  

{\it Vikas Bansal, On behalf of the ATLAS Collaboration.\\
Department of Physics and Astronomy, University of Victoria\\
3800 Finnerty Road, Elliott Bldg. Victoria BC V8P5C2 Canada}
\bigskip\bigskip
\end{raggedright}



\section{Introduction}

A wide range of new physics phenomena such as 
quark/lepton compositeness, extra dimensions and new gauge bosons
 can produce
modifications to the dilepton mass spectra predicted by the standard model (SM).
The expected form of these deviations is often either a resonance 
or an excess in the number of events in the spectra at high mass. 
ATLAS~\cite{detector} has searched for such an excess in dilepton 
events produced in $\sqrt{s}$ = 7 TeV proton-proton collisions at the LHC,
interpreting the data in the context of contact interactions (CI)
assuming left-left isoscalar model (LLIM)~\cite{ELP},\cite{PDG},\cite{EHLQ}.
Under LLIM scheme,
the differential cross section
for the process $q \bar{q} \to \ell^+\ell^-$ can be written as
\begin{equation}\label{cross_section}
\frac{d\sigma}{dm_{\ell\ell}} = \frac{d\sigma_{DY}}{dm_{\ell\ell}}
 - \eta_{LL}\frac{F_I(m_{\ell\ell})}{\Lambda^2} + \frac{F_C(m_{\ell\ell})}{\Lambda^4}~,
\end{equation}
where $m_{\ell\ell}$ is the final-state dilepton mass and $\eta_{LL}$ is LLIM parameter defining interference type (destructive or constructive). 
$\Lambda$ is the contact interaction scale.
The expression above includes a SM Drell-Yan (DY) term, as well as DY-CI interference ($F_I$) 
and pure contact interaction ($F_C$) terms calculated with a coupling $g^2 = 4\pi$
(see Ref.~\cite{EHLQ} for the full form of this expression). The analysis uses early 2011 run data amounting to
$\integLumiElec$ and $\integLumiMuon$ fb$^{-1}$ of $pp$ collisions recorded by the 
ATLAS detector for final states with electron and muon pairs, respectively.

\section{Results}

Events were selected by requiring that they pass the single electron (muon) 
trigger with a transverse momentum $p_T$ 
threshold of 20 (22) \GeV{}. This analysis follows the same event selection 
as the search for new heavy resonances described in Ref.~\cite{Zprime}.

% dilepton mass yields
Figure~\ref{fig:mass} displays the dielectron and
dimuon mass spectra for all selected events with 
invariant mass greater than 70~\GeV{} along with the SM (including mass dependent NNLO k-factors)
and expected CI contributions for four values of $\Lambda$ for both constructive ($\Lambda^-$) and destructive ($\Lambda^+$) interference.
%A slight excess of events observed at high dimuon mass is consistent 
%with a statistical fluctuation.
%
\begin{figure}
  \includegraphics[width=0.48\textwidth]{./mass_elec.eps}
  \includegraphics[width=0.475\textwidth]{./mass_muon.eps}
  \caption{\label{fig:mass} Dielectron (left) and dimuon (right) invariant mass distributions
  for data (points) and Monte Carlo simulation (histograms)~\cite{dilepton_ATLAS_1ifb}. The open histograms correspond to
  the distributions expected in the presence of CI with different values
  of $\Lambda$ for both constructive (solid) and destructive (dashed)
  interference.}
\end{figure}


The observed limits~\cite{dilepton_ATLAS_1ifb} (at 95\% CL) with a flat prior of $1/\Lambda^2$ are
$\Lambda^- > \obsLimitMinusElec$~\TeV{} ($\Lambda^+ > \obsLimitPlusElec$~\TeV{})
in the electron channel and
$\Lambda^- > \obsLimitMinusMuon$~\TeV{} ($\Lambda^+ > \obsLimitPlusMuon$~\TeV{})
in the muon channel
for constructive (destructive) interference and are obtained using Bayesian Analysis Toolkit (BAT)~\cite{BAT}.
The resulting combined limits are
$\Lambda^- > \obsLimitMinusComb$~\TeV{} and $\Lambda^+ > \obsLimitPlusComb$~\TeV{}
for the same prior.



\begin{thebibliography}{99}

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\bibitem{detector} 
%G. Aad {\sl et al.} (ATLAS Collaboration), 
ATLAS Collaboration, 
%``The ATLAS Experiment at the CERN Large Hadron Collider'', 
JINST {\bf 3}, S08003 (2008).


\bibitem{ELP} 
E. Eichten, K. Lane, and M. Peskin,
% ``New Tests for Quark and Lepton Substructure'', 
Phys. Rev. Lett. {\bf 50}, 811 (1983).

\bibitem{PDG}
K. Nakamura {\sl et al.} (Particle Data Group), 
J. Phys. G {\bf 37}, 075021 (2010).

\bibitem{EHLQ} 
E. Eichten, I. Hinchliffe, K. Lane, and C. Quigg,
% ``Supercollider Physics'', 
Rev. Mod. Phys. {\bf 56}, 579 (1984);
Addendum-ibid. {\bf 58}, 1065 (1986).

\bibitem{Zprime}
%G.~Aad {\sl et al.} (ATLAS Collaboration),
ATLAS Collaboration,
%arXiv:1108.1582 [hep-ex],
Phys. Rev. Lett. {\bf 107}, 272002 (2011) 


\bibitem{dilepton_ATLAS_1ifb}
%G.~Aad {\sl et al.} (ATLAS Collaboration),
ATLAS Collaboration,
%arXiv:1104.4398 [hep-ex],
Phys.~Lett.~B {\bf 712}, 40 (2012).

\bibitem{BAT} A. Caldwell, D. Kollar, and K. Kr\"oninger,
%``BAT - The Bayesian Analysis Toolkit,''
Computer Phys.Comm. {\bf 180}, 2197 (2009).


\end{thebibliography}

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