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%is availabe in your LaTeX system.
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\newcommand{\lagr}{\cal{L}}
\newcommand{\mueg}{$\mu^{+} \rightarrow e^{+}\gamma$}
\newcommand{\meee}{$\mu \rightarrow eee$}
\newcommand{\muenn}{$\mu \rightarrow e \nu \overline{\nu}$}
\newcommand{\muenng}{$\mu \rightarrow e \nu \overline{\nu} \gamma$}
\newcommand{\muec}{$\mu^{-} N \rightarrow e^{-} N$}
\newcommand{\muecaz}{$\mu^{-} + N(A,Z) \rightarrow e^{-} + N(A,Z)$}
\newcommand{\sindrumlimit}
{$\mathcal{B} (\mu^- + Au \rightarrow e^- +Au) < 7\times 10^{-13}$}

\title{Status of the COMET Experiment at J-PARC}

\author{Nam Hoai \textsc{Tran}$^{1}$, for the COMET Collaboration}

\inst{$^{1}$Department of Physics, Osaka
University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan}

\email{nam@kuno-g.phys.sci.osaka-u.ac.jp}

\recdate{July 15, 2013}

\abst{
	The COMET experiment at J-PARC with a staging approach to search for
	a charged lepton flavor violation in a muonic atom is described. Highlights
	of the ongoing R\&D activivities in preparation for physics runs of the first
	stage, COMET Phase--I, are presented.
}

\kword{charged lepton flavor violation, mu-e conversion, COMET}

\begin{document}
\maketitle

\section{Physics motivation}
Charged lepton flavor violation (CLFV) belongs to the class of flavor-changing
neutral currents, which are suppressed at tree level in the Standard Model
(SM) where they are mediated by $\gamma$ and $Z^0$ bosons, but arise at loop
level via weak charged currents mediated by the $W^{\pm}$ boson. Because flavor
violation requires mixing between generations, CLFV exactly vanishes in the SM
with massless neutrinos. Even in the framework of the SM with massive neutrinos
and their mixing, branching ratio of CLFV is still very small - for example, in
case of \mueg~\cite{marciano}:
\begin{equation}
	\mathcal{B}(\mu^{+} \rightarrow e^{+}\gamma) \simeq 
	10^{-54} \left( \frac{sin^{2}2\theta_{13}}{0.15}\right)
\end{equation}
This is an unobservably tiny
branching ratio so that any experimental evidence of CLFV would be a clear
sign of new physics beyond the SM.

One of the most prominent CLFV processes is
a process of coherent muon-to-electron conversion ($\mu
- e$ conversion) in the field of a nucleus: \muecaz. When muons are stopped in
a target, they are quickly
captured by atoms ($~10^{-10}$ s) and cascade down to the 1S orbitals. There,
they can undergo:
(a) ordinary decay, (b) weak capture, $\mu^- p \rightarrow \nu_\mu n$, or (c)
$\mu - e$ conversion, \muec. The last of these reactions is a CLFV process
where lepton flavor numbers, $L_\mu$ and $L_e$, are violated by one unit.

The $\mu - e $ conversion is attractive both from theoretical and experimental
points of view. Many extensions of the SM predict that it would has sizeable
branching ratio~\cite{altman}. One possible supersymmetric contribution to the
$\mu - e$ conversion is shown in Fig.~\ref{fig:susy_contr}. Experimentally, the
simplicity and distinctive signal, a mono-energetic electron of energy $E_{e}$:
\[	
	E_{e} = m_{\mu} - B_{\mu}(Z, A) - R(A) \simeq \textrm{105 MeV},
\]
where $m_\mu$ is the muon mass, $B_\mu(Z, A)$ is the muonic atom binding
energy, and $R(A)$ is the nuclear recoil energy, allow experimental searches
without accidentals and thus in extremely high rates. As a result, one of the
best upper limits of CLFV searches comes from a search for $\mu - e$ conversion
in muonic gold done by the SINDRUM--II collaboration:
\sindrumlimit~\cite{sindrumii}.

\begin{figure}[tbh]
	\centering
	\includegraphics[width=\textwidth]{figs/susy_contr}
	\caption{Possible SUSY contributions to the CLFV processes \mueg
	(left) and \muec (right).}
	\label{fig:susy_contr}
\end{figure}

%%%%%%%%%%%%%%%%%%%%
\section{The COMET experiment to search for \muec~at a sensitivity of
$10^{-17}$}
\subsection{The COMET experiment}
At the Japan Proton Accelerator Research Complex (J-PARC), an experiment to
search for \muec~conversion, which is called COMET (COherent Muon to Electron
Transition), has been proposed~\cite{comet07}. The experiment received Stage--1
approval in
2009. Utilising a proton beam of 56 kW (8 GeV $\times$ 7 $\mu$A) from the
J-PARC main ring, the COMET aims for a single event sensitivity of $3 \times
10^{-17}$, which is 10000 times better than the current best
limit at SINDRUM--II. As of April 2013, the COMET collaboration has 117
members in 27 institutes from 12 countries.

The COMET experiment is designed to be carried out at the Hadron
Experimental Facility using a bunched proton beam that is
slowly-extracted from the J-PARC main ring. The experimental set-up consists of
a dedicated proton beam line, a muon beam transport section, and a detector
section. The muon beam section is composed of superconducting magnets: pion
capture solenoid and a pion/muon transport solenoid. The
detector section has a multi-layered muon stopping target, an electron
transport beam line for $\mu - e$ conversion signals,
followed by detector systems. 

\subsection{Staging approach at the COMET}
The COMET collaboration has adopted a staging approach with two
phases~\cite{comet12}. COMET Phase--I  is scheduled to
have an engineering run in 2016, followed by a physics run in 2017. Phase--I
should achieve a sensitivity
of $3 \times 10^{-15}$, 100 times better than that of SINDRUM--II; while
Phase--II will reach a sensitivity of $2.6 \times 10^{-17}$, which is
competitive with the Mu2e project at Fermilab~\cite{mu2e08}.
A schematic layout of the COMET experiment with its two phases is
shown in Fig.~\ref{fig:comet_phase1}, and a schedule for two phases is shown in
Fig.~\ref{fig:sched}.
\begin{figure}[tbh]
	\centering
\includegraphics[width=\textwidth]{figs/comet_phase1}
\caption{Schematic layout of the COMET experiment with two phases: Phase--I
(left) and Phase--II (right).}
\label{fig:comet_phase1}
\end{figure}

\begin{figure}[tbh]
	\centering
\includegraphics[width=0.8\textwidth]{figs/sched}
\caption{The anticipated schedule of the COMET experiment.}
\label{fig:sched}
\end{figure}

COMET Phase--I has two major goals:
\begin{itemize}
	\item Background study for the COMET Phase--II by using the actual COMET beam
		line constructed at Phase--I, 
	\item Search for $\mu-e$ conversion with a single event sensitivity of $3
		\times 10^{-15}$.
\end{itemize}

In order to realize the goals, COMET Phase--I proposes to have two systems of
detector. A straw tube detector and an electromagnetic calorimeter will be used
for the background study. For the $\mu-e$ conversion search, a cylindrical
drift chamber (CDC) will be built.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Status of the COMET experiment}
The COMET collaboration is working very hard toward the
physics run in 2017. Some R\&D activities are hightlighted as follows.

\subsection{COMET beam line and experimental hall}
Budget for construction of the COMET beam line and experimental hall has been
approved. Construction has been started in 2013 and is expected to be completed
by spring 2015. Production of coils for superconducting magnets will begin in
July, 2013. Design of the radiation shielding and the pion production target
are in progress.

\subsection{R\&D for detectors}
\subsubsection{Cylindrical drift chamber}
The CDC is a detector dedicated for COMET Phase--I to maximize the experimental
sensitivity for the $\mu-e$ conversion search. A prototype CDC will be ready
for testing at Osaka University in September, 2013. Designing for the real
detector is ongoing based on simulation studies and experiments with a test
chamber and test readout boards.

One designing issue with the CDC is that its single hit rate would be dominated
by protons emitted after muons being captured in the target. The lack of
experimental data on protons from Al target makes it difficult to optimize
the inner wall of the CDC to achieve best momentum resolution. In order to
provide input for this optimization, a dedicated
measurement of protons (and other charged particles) after muon capture 
will be carried out at PSI, Switzerland in December, 2013. The measurement is
a joint effort between COMET and Mu2e.

\subsubsection{Straw tube tracker}
The straw tube tracker for background study in COMET Phase--I is being
developed with expertise from JINR, Russia group in NA62 experiment. A KEK-JINR
collaboration has been formed to produce a prototype tracker. R\&D for this
prototype will be completed in 2013. Design works and construction of the real
detector is ongoing.

\subsubsection{Electromagnetic calorimeter}
An electromagnetic calorimeter will be used in the COMET Phase--II as a trigger
detector that triggers on 105 MeV electrons, as well as in the Phase--I for the
background study. Currently, two types of crystal
are considered for this calorimeter: GSO and LYSO. Beam tests of calorimeter
prototypes with APD readout are underway. The choice of crystal will be made
in a few months. A prototype electronics system for the detector has been
developed by BINP, Novosibirsk, Russia group, and is being evaluated in the
beam tests at J-PARC.

\subsection{Software and analysis}
%Software used to simulate and analyze data is crucial to the success of the
%COMET experiment. The COMET software group has set up a new software framework
%called ICEDUST (Integrated COMET Experiment Data User Software Toolkit). The
%framework includes an improved simulation for the experiment. Two major improvements
%are: the ability to run the simulation on a computing grid for large simulation
%data production, and an optimized
%beam line with a realistic magnetic field map from the manufacturer of the
%superconducting magnets. The ICEDUST also
%has modules to perform calibration, reconstruction and analysis of both
%Monte-Carlo outputs and experimental data in a unified way. Documentations and
%implementations of the ICEDUST are underway.

The software used to simulate and analyze data is crucial to the success of the
COMET experiment. The COMET software group has set up a new software framework
called ICEDUST (Integrated COMET Experiment Data User Software Toolkit), which
provides the ability to simulate, reconstruct and analyze data. The ability to
reconstruct and analyze data, treating simulated and real data in the same way,
is an essential feature that will allow COMET to achieve its sensitivity.
ICEDUST uses accurate descriptions of the experiment geometry and magnetic
field maps from the manufacturer of the superconducting magnets.  The framework
also calibrates the data (through the use of constants stored in a database)
and distributes the data and processing using the Grid.  Implementation of
COMET Phase-I in ICEDUST is currently underway.

\section{Summary}
The COMET (J-PARC E21) experiment is going to push the limit of experimental
searches for CLFV $\mu-e$ conversion process, a very promising probe for new
physics beyond the SM. 
The first stage, COMET Phase--I, will improve the upper limit by two orders of
magnitude to a single event sensitivity of $3\times10^{-15}$.
Heavy R\&D activities are ongoing to realize the physics
run of the COMET Phase--I in the year of 2017.
%\appendix
%\section{}
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	Ann. Rev. Nucl. Part. Sci. \textbf{58}, 315 (2008).
\bibitem{altman} W. Altmannshofer, A. J. Buras, S. Gori, P. Paradisi and D. M.
	Straub: Nucl. Phys. B \textbf{830} (2010) 17.
\bibitem{sindrumii} W.~Bertl (The SINDRUM-II collaboration): 
	Eur. Phys. J. C \textbf{47} (2006).
337-346.
\bibitem{comet07} D.~Bryman (The COMET collaboration):  ``An experimental
	search for lepton flavor violating $\mu^- - e^-$ conversion at sensitivity of
	$10^{-16}$ with a slow-extracted bunched proton beam'', J-PARC Proposal,
	2007.
\bibitem{comet12} R. Akhmetshin (The COMET collaboration): ``Experimental
	proposal for Phase--I of the COMET experiment at J-PARC'', KEK/J-PARC-PAC
	2012-10.
\bibitem{mu2e08} R.M.~Carey (The Mu2e collaboration): ``Proposal to search for
	\muec~ with a single event sensitivity below $10^{-16}$'', a research
	proposal to Fermilab, 2008.
%\bibitem{jpsj} The abbreviation for JPSJ must be ``J. Phys. Soc. Jpn." in the
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%\bibitem{format} F. Author, S. Author, and T. Author: Abbreviated journal title
%\textbf{volume in bold face} (year of publication) initial page or article ID.
\end{thebibliography}

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