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\Title{Searches for long-lived, Exotic particles at ATLAS }

\bigskip\bigskip

%+\addtocontents{toc}{{\it C.P. Marino}}
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\begin{raggedright}  

{\it Christopher Phillip Marino\index{Marino, C.P.}, On behalf of the ATLAS Collaboration\\
University of Victoria, 3800 Finnerty Road  Victoria, BC V8N 1M5, Canada\\}
\bigskip\medskip
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\section{Introduction}
Three recent searches for massive, exotic, long-lived particles are presented for 2011 LHC data using the ATLAS detector \cite{Aad:2008zzm}.  No new physics has been found but limits have been set, and tools for future searches have been developed.  Improvements are under way using timing, ionization, and displaced vertices in several ATLAS sub-detectors for searches with full 2011 data and 2012 data.

\section{Disappearing-track signature}
In models of anomaly-mediated supersymmetry breaking (AMSB), the lightest chargino ($\widetilde\chi^\pm_{1}$) is predicted to have a long lifetime due to a small mass difference between the $\widetilde\chi^\pm_{1}$ and the neutralino $\widetilde\chi^0_{1}$ to which it decays. The $\widetilde\chi^0_{1}$ leaves the detector allowing a trigger on a high-$p_T$ jet and missing $E_T$.  Candidates are identified as high-$p_T$ tracks with few hits in the outer part of tracker, as the low-$p_T$ $\pi$ from the $\widetilde\chi^\pm_{1} \rightarrow \widetilde\chi^0_{1} \pi^\pm$ decay sequence is not reconstructed. 
Backgrounds come primarily from hadronic interactions and mismeasured tracks. They are studied using control samples of non-disappearing tracks and low missing $E_T$, respectively.
The $p_T$ distributions are modeled for the control samples as well as the AMSB signal samples. A maximum likelihood fit of $p_T$ distributions to data shows no signal contribution in 4.7 $fb^{-1}$ of $pp$ collisions. Model-independent limits for a new physics process with an isolated, disappearing track and limits on the signal cross-section as a function of $\widetilde\chi^\pm_{1}$ lifetime are extracted \cite{ATLAS:2012ab}.  

\section{Hidden Valley}
One Standard Model (SM) extension is a hidden sector ($v$-sector) with a light Higgs communicator. The Higgs' high mass and weak couplings create a barrier which hides the $v$-sector and makes production of $v$-particles rare at low energies. Some $v$-particles may  decay to SM particles, and their production may be observable through displaced vertices.
A range of $\pi_v$ lifetimes gives decay signatures in all parts of the ATLAS detector.
Decays to two $b$-jets near the muon system (from 4.5-11 m) have been used by searching for two vertices of $>$ 3 muon segments not pointing back to the interaction point. 
A special trigger has been designed to improve efficiency by an order of magnitude near the muon system.
No events meeting the analysis selection are observed  in 1.94 $fb^{-1}$ of $pp$ collisions \cite{ATLAS:2012av}. Exclusion limits are shown in Fig. \ref{fig:hv}.

\section{R-hadrons}
%R-hadrons are charged, long-lived heavy SUSY particles. 
R-hadrons are hadronic states containing a heavy exotic long-lived parton.
They have $\beta < 1$, resulting in high ionization, but  can become neutral through hadronic interactions inside ATLAS. The Pixel detector provides ionization information close to the interaction point using minimum bias events to calibrate a d$E$/d$x$ measurement.  This d$E$/d$x$ is used to determine a $\beta$ via Bethe-Bloch and then a mass using momentum. The conversion is calibrated using protons and extrapolating to R-hadrons. Candidates are isolated, high $p_T$ tracks, and 333 are observed in 2.1 $fb^{-1}$ of $pp$ collisions  as is consistent with data-driven background estimation \cite{pixel}. The result can be interpreted as excluding gluino R-hadrons with $m < 810$ GeV  as shown in Fig. \ref{fig:pixel}.

\begin{figure}[htb]
\begin{center}
\subfigure{\epsfig{file=hv_fig_03.eps,height=1.95in}\label{fig:hv}} 
%\subfigure[]{\epsfig{file=amsb.eps,height=1.48in}}
\subfigure{\epsfig{file=pixel_fig_08.eps,height=1.95in}\label{fig:pixel}}
\caption{(a) Observed 95$\%$ CL (multiples of the SM cross-section for Higgs production with 100$\%$ branching ratio to $\pi_v$'s) for \mbox{$h^0\rightarrow\pi_v\pi_v$} vs. the $\pi_v$ proper decay length \cite{ATLAS:2012av}.  (b) Cross section as a function of mass for gluino R-hadrons \cite{pixel}. }
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\end{figure}


\begin{thebibliography}{99}

%\cite{Aad:2008zzm}
\bibitem{Aad:2008zzm} 
  ATLAS Collaboration,
  %``The ATLAS Experiment at the CERN Large Hadron Collider,''
  JINST {\bf 3}, S08003 (2008).
  %%CITATION = JINST,3,S08003;%%

%\cite{ATLAS:2012ab}
\bibitem{ATLAS:2012ab} 
  ATLAS Collaboration,
  %``Search for anomaly-mediated supersymmetry breaking with the ATLAS detector based on a disappearing-track signature in pp collisions at sqrt(s) = 7 TeV,''
  Eur.\ Phys.\ J.\ C {\bf 72}, 1993 (2012).
  %[arXiv:1202.4847 [hep-ex]].
  %%CITATION = ARXIV:1202.4847;%%

%\cite{ATLAS:2012av}
\bibitem{ATLAS:2012av} 
  ATLAS Collaboration,
  %``Search for a light Higgs boson decaying to long-lived weakly-interacting particles in proton-proton collisions at sqrt(s) = 7 TeV with the ATLAS detector,''
  Phys.\ Rev.\ Lett.\  {\bf 108}, 251801 (2012).
  %[arXiv:1203.1303 [hep-ex]].
  %%CITATION = ARXIV:1203.1303;%%

%\cite{pixel}
\bibitem{pixel} 
  ATLAS Collaboration,
  %"Search for charged long-lived heavy particles with the ATLAS Experiment at the LHC",
  ATLAS-CONF-2012-022 (2012). 
  %https://cdsweb.cern.ch/record/1430731/files/ATLAS-CONF-2012-022.pdf
  %[arXiv:1202.4847 [hep-ex]].
  %%CITATION = ARXIV:1202.4847;%%

\end{thebibliography}


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%Patient &  Initial level($\mu$g/cc) &  w. Magnet &  
%w. Magnet and Sound \\ \hline
% Guglielmo B.  &   0.12     &     0.10      &     0.001  \\
% Ferrando di N. &  0.15     &     0.11      &  $< 0.0005$ \\ \hline
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%\caption{Blood cyanide levels for the two patients.}
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%\caption{The constraint on the chargino mass and lifetime and on the mass splitting and the chargino mass for tan(beta) = 5 and mu > 0. The black dashed line shows the expected limits at 95\% CL, with the yellow bands indicating the 1sigma excursions due to experimental uncertainties. Observed limits are indicated by red curves, where the solid contour represents the nominal limit, and the dotted lines are obtained by varying the cross section by the theoretical scale and PDF uncertainties. The previous result and the combined LEP2 exclusion at 95\% CL are also shown by the blue dotted line and the region filled in cyan, respectively. }
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%\subfigure[]{\epsfig{file=hv_fig_03.eps,height=1.48in}}
%\subfigure[]{\epsfig{file=amsb.eps,height=1.48in}}
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%\caption{{\footnotesize (a) Observed 95$\%$ upper limits on the process \mbox{$h^0\rightarrow\pi_v\pi_v$}, vs. the $\pi_v$ proper decay length, expressed as a multiple of the SM cross-section for Higgs production (assumes 100$\%$ branching ratio for the Higgs decaying to $\pi_v$'s). (b) The constraint on the chargino mass and lifetime for $tan(\beta)$ = 5 and $\mu > 0$. The black dashed line shows the expected limits at 95\% CL, with the yellow bands indicating the 1$\sigma$ excursions due to experimental uncertainties. Observed limits are indicated by red curves. (c) Cross section as a function of mass for gluino R-hadrons.}}
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