\documentclass[letterpaper]{jpsj-suppl}
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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.

Among CLFV processes, one of the most prominent one is
a process of coherent muon-to-electron conversion ($\mu
- e$ conversion) in the field of a nucleus: \muecaz. $\mu - e$ conversion would
occur in the following manner. Stopped muons 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 distintive signal, a mono-energetic electron of energy:
\[	
	E_{mec} = m_{\mu} - B_{\mu}(Z, A) - R(A) \simeq \textrm{105 MeV},
\]
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 extremely high rates. As a result, one of the best
limit 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 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.

The COMET experiment is designed to be carried out at the Nuclear and Particle
Experimental Hall (NP Hall) 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 a superconducting pion capture
solenoid with high magnetic field. The detector section has muon stopping
targets, an electron transport beam line for $\mu - e$ conversion signals,
followed by detector systems. 

\subsection{Staging approach at the COMET}
The COMET collaboration, which has 117 collaborators in 27 institutes from
12 countries as of April 2013, 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 $3 \times 10^{-17}$, which is
comparable to 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=\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 realized the goals, COMET Phase--I proposes to have two systems of
detector. A straw tube detector and 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 to realize the schedule with the
physics run in 2017. Some R\&D activities are hightlighted as follow.

\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 radiation shielding and 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 would be ready
for testing at Osaka University in September, 2013. Designing for the real
detector is ongoing based on simulation study and experiments with a test
chamber and test readout boards.

One designing issue with the CDC is its single hit rate would be dominated by
protons emitted after muons are 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 would 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. 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 would 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 analyse 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. It 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 ICEDUST are 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. Heavy R\&D activities are ongoing to realize the physics
run of its first stage, COMET Phase--I, in the year of 2017 with an immediate
goal: two orders of magnitude improvement in the single event sensitivity
compares the the current best limit. 
%\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).
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\bibitem{comet07} D.~Bryman (The COMET collaboration):  ``An experimental
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	$10^{-16}$ 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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%\textbf{volume in bold face} (year of publication) initial page or article ID.
\end{thebibliography}

\end{document}

