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\Title{Search for Long-Lived Massive Particles at CMS}

\bigskip\bigskip

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

\begin{raggedright}  

{\it Seth I. Cooper on behalf of the CMS Collaboration\index{Cooper, S.I.}\\
School of Physics and Astronomy\\
116 Church St. S.E.\\
University of Minnesota\\
Minneapolis MN 55455\\
USA
}
\bigskip\bigskip
\end{raggedright}



Long-lived massive particles are predicted by several theories of beyond the Standard Model physics,
such as the minimal supersymmetric standard model, split supersymmetry, and gauge-mediated supersymmetry breaking.
%and pair-produced stau.
The first two models predict hadron-like particles, stops and gluinos, respectively, and the last predicts
a lepton-like particle, the stau.
%In each case, these particles are the next-to-lightest supersymmetric particles which are quasi-stable
%in their decay to the lightest supersymmetric particle.
The particles considered here carry electric charge in the case of the lepton-like particles,
and the hadron-like particles will hadronize to form a color-singlet ``R-hadron'' state.
%The R-hadron can interact strongly with detector materials and change the sign of its electric charge or even become neutral
%in a process called ``charge flipping.''
In both cases, due to their large masses, these particles travel at low values of $\beta$, resulting in increased
$dE/dx$ over a minimum ionizing particle and prolonged time of flight.
Here we consider the case in which their lifetimes are long enough to traverse the entire CMS detector (7.4~m)
before decaying.
%Their slow speed gives rise to a higher specific ionization energy and a longer time of flight than that expected
%from a light stable particle such as a muon.
%Given their relatively high production cross sections, especially those produced via the strong interaction,
%they could be one of the first signals of new physics.
A search for these particles was conducted with the CMS detector using data from the Large Hadron Collider
collected in pp collisions at $\sqrt{s} = 7$~TeV in 2011 \cite{Chatrchyan:2012sp}.

A complete description of the CMS detector can be found elsewhere \cite{Chatrchyan:2008aa}.
Here we note the silicon inner tracking system, used to measure the $dE/dx$,
and the muon system consisting of drift tubes and resistive plate chambers in the barrel region, used to measure the time of flight.
%Increased $dE/dx$ compared to a minimum ionizing particle results from the slow speed of these massive particles, which can be measured in the
%inner tracker, while their prolonged time of flight can be measured in the muon system.
Combining either of these measurements with the particle's momentum, measured in the tracker, yields a mass measurement.

Two different triggers are used: a single muon trigger, and a missing transverse energy trigger to add efficiency for R-hadrons becoming neutral
due to nuclear interactions in the detector.
Selection variables used include the transverse momentum ${P_T}$, the $dE/dx$ compatibility with
minimum ionizing particles $I_{as}$, and $1/\beta$ calculated from the time of flight.
The distributions of these variables for background and several models is
shown in Figure \ref{fig:selectionVars}.
\begin{figure}[h]
  \centering
  \begin{minipage}{.25\textwidth}
    \includegraphics[trim=92mm 12mm 38mm 10mm,clip,width=\textwidth]{Distrib_Pt_tktof}
  \end{minipage}
  \begin{minipage}{.25\textwidth}
    \includegraphics[trim=92mm 12mm 38mm 10mm,clip,width=\textwidth]{Distrib_I_tktof}
  \end{minipage}
  \begin{minipage}{.25\textwidth}
    \includegraphics[trim=92mm 12mm 38mm 10mm,clip,width=\textwidth]{Distrib_TOF_tktof}
  \end{minipage}
  \caption{Selection variables for several signal models, data, and Monte Carlo background.}
  \label{fig:selectionVars}
\end{figure}
Two different analyses were conducted: one using the tracker only and the other adding the time-of-flight information from the muon system.

The analysis was conducted as follows.
To predict the background, an ABCD or sideband method was used, taking advantage of the uncorrelated $P_T$, $I_{as}$, and $1/\beta$ measurements.
This estimates the amount of background in the signal region D.
%The background mass distribution is predicted using pseudoexperiments from the distributions in the A, B, and C regions.
%Figure \ref{fig:backgroundPredictions} shows the predicted mass spectra for the tracker-only and tracker+time-of-flight approaches.
%Next, selection cuts are applied which maximize the discovery reach for each model.
A mass cut is applied to the data at the nominal model reconstructed mass minus twice the expected mass resolution, and its affect
on the background estimate in the D region is accounted for.
Then the statistical analysis is performed using the estimated background and the observed number of events.
%\begin{figure}[h!]
%  \centering
%  \begin{minipage}{.25\textwidth}
%    \includegraphics[trim=92mm 12mm 38mm 10mm,clip,width=\textwidth]{Mass_tkonly}
%  \end{minipage}
%  \begin{minipage}{.25\textwidth}
%    \includegraphics[trim=92mm 12mm 38mm 10mm,clip,width=\textwidth]{Mass_tktof}
%  \end{minipage}
%  \caption{Predicted mass spectrum for background from ABCD method and Monte Carlo simulation along with data and a simulated signal model
%           for tracker-only (left) and tracker+time-of-flight (right).}
%  \label{fig:backgroundPredictions}
%\end{figure}

As there is no evidence of signal, 95\% confidence level upper limits are set using CLs.
The results are shown in Figure \ref{fig:results}.
Using theoretical calculations of HSCP cross sections, the calculated mass lower limits are: 314~GeV and 223~GeV for the GMSB stau and pair produced stau;
1098~GeV and 928~GeV for the gluino, using the cloud and charge suppression nuclear interaction models;
737~GeV and 626~GeV for the stop, using the cloud and charge suppression nuclear interaction models.
\begin{figure}[h!]
  \centering
  \begin{minipage}{.3\textwidth}
    \includegraphics[trim=85mm 12mm 38mm 10mm,clip,width=\textwidth]{ExclusionPlot_tkonly}
  \end{minipage}
  \begin{minipage}{.3\textwidth}
    \includegraphics[trim=85mm 12mm 38mm 10mm,clip,width=\textwidth]{ExclusionPlot_tktof}
  \end{minipage}
  \caption{
           Upper limits on the cross section for various models. Left: Tracker-only analysis. Right: Tracker+time-of-flight analysis.
           }
  \label{fig:results}
\end{figure}




\begin{thebibliography}{99}


\bibitem{Chatrchyan:2012sp} 
  S.~Chatrchyan {\it et al.}  [CMS Collaboration],
  %``Search for heavy long-lived charged particles in pp collisions at sqrt(s)=7 TeV,''
  Phys.\ Lett.\ B {\bf 713}, 408 (2012)
  [arXiv:1205.0272 [hep-ex]].
  %%CITATION = ARXIV:1205.0272;%%


\bibitem{Chatrchyan:2008aa} 
  S.~Chatrchyan {\it et al.}  [CMS Collaboration],
  %``The CMS experiment at the CERN LHC,''
  JINST {\bf 3}, S08004 (2008).
  %%CITATION = JINST,3,S08004;%%


\end{thebibliography}
 

\end{document}

