267 lines
13 KiB
TeX
267 lines
13 KiB
TeX
\documentclass[letterpaper]{jpsj-suppl}
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\usepackage{txfonts} %Please comment out this line unless the txfonts package
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%is availabe in your LaTeX system.
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\usepackage{mathtools}
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\newcommand{\lagr}{\cal{L}}
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\newcommand{\mueg}{$\mu^{+} \rightarrow e^{+}\gamma$}
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\newcommand{\meee}{$\mu \rightarrow eee$}
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\newcommand{\muenn}{$\mu \rightarrow e \nu \overline{\nu}$}
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\newcommand{\muenng}{$\mu \rightarrow e \nu \overline{\nu} \gamma$}
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\newcommand{\muec}{$\mu^{-} N \rightarrow e^{-} N$}
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\newcommand{\muecaz}{$\mu^{-} + N(A,Z) \rightarrow e^{-} + N(A,Z)$}
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\newcommand{\sindrumlimit}
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{$\mathcal{B} (\mu^- + Au \rightarrow e^- +Au) < 7\times 10^{-13}$}
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\title{Status of the COMET Experiment at J-PARC}
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\author{Nam Hoai \textsc{Tran}$^{1}$, for the COMET Collaboration}
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\inst{$^{1}$Department of Physics, Osaka
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University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan}
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\email{nam@kuno-g.phys.sci.osaka-u.ac.jp}
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\recdate{July 15, 2013}
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\abst{
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The COMET experiment at J-PARC with a staging approach to search for
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a charged lepton flavor violation in a muonic atom is described. Highlights
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of the ongoing R\&D activivities in preparation for physics runs of the first
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stage, COMET Phase--I, are presented.
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}
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\kword{charged lepton flavor violation, mu-e conversion, COMET}
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\begin{document}
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\maketitle
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\section{Physics motivation}
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Charged lepton flavor violation (CLFV) belongs to the class of flavor-changing
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neutral currents, which are suppressed at tree level in the Standard Model
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(SM) where they are mediated by $\gamma$ and $Z^0$ bosons, but arise at loop
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level via weak charged currents mediated by the $W^{\pm}$ boson. Because flavor
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violation requires mixing between generations, CLFV exactly vanishes in the SM
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with massless neutrinos. Even in the framework of the SM with massive neutrinos
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and their mixing, branching ratio of CLFV is still very small - for example, in
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case of \mueg~\cite{marciano}:
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\begin{equation}
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\mathcal{B}(\mu^{+} \rightarrow e^{+}\gamma) \simeq
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10^{-54} \left( \frac{sin^{2}2\theta_{13}}{0.15}\right)
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\end{equation}
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This is an unobservably tiny
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branching ratio so that any experimental evidence of CLFV would be a clear
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sign of new physics beyond the SM.
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One of the most prominent CLFV processes is
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a process of coherent muon-to-electron conversion ($\mu
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- e$ conversion) in the field of a nucleus: \muecaz. When muons are stopped in
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a target, they are quickly
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captured by atoms ($~10^{-10}$ s) and cascade down to the 1S orbitals. There,
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they can undergo:
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(a) ordinary decay, (b) weak capture, $\mu^- p \rightarrow \nu_\mu n$, or (c)
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$\mu - e$ conversion, \muec. The last of these reactions is a CLFV process
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where lepton flavor numbers, $L_\mu$ and $L_e$, are violated by one unit.
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The $\mu - e $ conversion is attractive both from theoretical and experimental
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points of view. Many extensions of the SM predict that it would has sizeable
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branching ratio~\cite{altman}. One possible supersymmetric contribution to the
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$\mu - e$ conversion is shown in Fig.~\ref{fig:susy_contr}. Experimentally, the
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simplicity and distinctive signal, a mono-energetic electron of energy $E_{e}$:
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\[
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E_{e} = m_{\mu} - B_{\mu}(Z, A) - R(A) \simeq \textrm{105 MeV},
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\]
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where $m_\mu$ is the muon mass, $B_\mu(Z, A)$ is the muonic atom binding
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energy, and $R(A)$ is the nuclear recoil energy, allow experimental searches
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without accidentals and thus in extremely high rates. As a result, one of the
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best upper limits of CLFV searches comes from a search for $\mu - e$ conversion
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in muonic gold done by the SINDRUM--II collaboration:
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\sindrumlimit~\cite{sindrumii}.
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\begin{figure}[tbh]
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\centering
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\includegraphics[width=\textwidth]{figs/susy_contr}
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\caption{Possible SUSY contributions to the CLFV processes \mueg
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(left) and \muec (right).}
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\label{fig:susy_contr}
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\end{figure}
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%%%%%%%%%%%%%%%%%%%%
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\section{The COMET experiment to search for \muec~at a sensitivity of
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$10^{-17}$}
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\subsection{The COMET experiment}
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At the Japan Proton Accelerator Research Complex (J-PARC), an experiment to
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search for \muec~conversion, which is called COMET (COherent Muon to Electron
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Transition), has been proposed~\cite{comet07}. The experiment received Stage--1
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approval in
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2009. Utilising a proton beam of 56 kW (8 GeV $\times$ 7 $\mu$A) from the
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J-PARC main ring, the COMET aims for a single event sensitivity of $3 \times
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10^{-17}$, which is 10000 times better than the current best
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limit at SINDRUM--II. As of April 2013, the COMET collaboration has 117
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members in 27 institutes from 12 countries.
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The COMET experiment is designed to be carried out at the Hadron
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Experimental Facility using a bunched proton beam that is
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slowly-extracted from the J-PARC main ring. The experimental set-up consists of
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a dedicated proton beam line, a muon beam transport section, and a detector
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section. The muon beam section is composed of superconducting magnets: pion
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capture solenoid and a pion/muon transport solenoid. The
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detector section has a multi-layered muon stopping target, an electron
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transport beam line for $\mu - e$ conversion signals,
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followed by detector systems.
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\subsection{Staging approach at the COMET}
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The COMET collaboration has adopted a staging approach with two
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phases~\cite{comet12}. COMET Phase--I is scheduled to
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have an engineering run in 2016, followed by a physics run in 2017. Phase--I
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should achieve a sensitivity
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of $3 \times 10^{-15}$, 100 times better than that of SINDRUM--II; while
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Phase--II will reach a sensitivity of $2.6 \times 10^{-17}$, which is
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competitive with the Mu2e project at Fermilab~\cite{mu2e08}.
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A schematic layout of the COMET experiment with its two phases is
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shown in Fig.~\ref{fig:comet_phase1}, and a schedule for two phases is shown in
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Fig.~\ref{fig:sched}.
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\begin{figure}[tbh]
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\centering
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\includegraphics[width=\textwidth]{figs/comet_phase1}
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\caption{Schematic layout of the COMET experiment with two phases: Phase--I
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(left) and Phase--II (right).}
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\label{fig:comet_phase1}
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\end{figure}
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\begin{figure}[tbh]
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\centering
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\includegraphics[width=0.8\textwidth]{figs/sched}
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\caption{The anticipated schedule of the COMET experiment.}
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\label{fig:sched}
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\end{figure}
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COMET Phase--I has two major goals:
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\begin{itemize}
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\item Background study for the COMET Phase--II by using the actual COMET beam
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line constructed at Phase--I,
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\item Search for $\mu-e$ conversion with a single event sensitivity of $3
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\times 10^{-15}$.
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\end{itemize}
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In order to realize the goals, COMET Phase--I proposes to have two systems of
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detector. A straw tube detector and an electromagnetic calorimeter will be used
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for the background study. For the $\mu-e$ conversion search, a cylindrical
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drift chamber (CDC) will be built.
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\section{Status of the COMET experiment}
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The COMET collaboration is working very hard toward the
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physics run in 2017. Some R\&D activities are hightlighted as follows.
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\subsection{COMET beam line and experimental hall}
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Budget for construction of the COMET beam line and experimental hall has been
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approved. Construction has been started in 2013 and is expected to be completed
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by spring 2015. Production of coils for superconducting magnets will begin in
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July, 2013. Design of the radiation shielding and the pion production target
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are in progress.
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\subsection{R\&D for detectors}
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\subsubsection{Cylindrical drift chamber}
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The CDC is a detector dedicated for COMET Phase--I to maximize the experimental
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sensitivity for the $\mu-e$ conversion search. A prototype CDC will be ready
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for testing at Osaka University in September, 2013. Designing for the real
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detector is ongoing based on simulation studies and experiments with a test
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chamber and test readout boards.
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One designing issue with the CDC is that its single hit rate would be dominated
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by protons emitted after muons being captured in the target. The lack of
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experimental data on protons from Al target makes it difficult to optimize
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the inner wall of the CDC to achieve best momentum resolution. In order to
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provide input for this optimization, a dedicated
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measurement of protons (and other charged particles) after muon capture
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will be carried out at PSI, Switzerland in December, 2013. The measurement is
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a joint effort between COMET and Mu2e.
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\subsubsection{Straw tube tracker}
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The straw tube tracker for background study in COMET Phase--I is being
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developed with expertise from JINR, Russia group in NA62 experiment. A KEK-JINR
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collaboration has been formed to produce a prototype tracker. R\&D for this
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prototype will be completed in 2013. Design works and construction of the real
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detector is ongoing.
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\subsubsection{Electromagnetic calorimeter}
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An electromagnetic calorimeter will be used in the COMET Phase--II as a trigger
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detector that triggers on 105 MeV electrons, as well as in the Phase--I for the
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background study. Currently, two types of crystal
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are considered for this calorimeter: GSO and LYSO. Beam tests of calorimeter
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prototypes with APD readout are underway. The choice of crystal will be made
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in a few months. A prototype electronics system for the detector has been
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developed by BINP, Novosibirsk, Russia group, and is being evaluated in the
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beam tests at J-PARC.
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\subsection{Software and analysis}
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%Software used to simulate and analyze data is crucial to the success of the
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%COMET experiment. The COMET software group has set up a new software framework
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%called ICEDUST (Integrated COMET Experiment Data User Software Toolkit). The
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%framework includes an improved simulation for the experiment. Two major improvements
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%are: the ability to run the simulation on a computing grid for large simulation
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%data production, and an optimized
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%beam line with a realistic magnetic field map from the manufacturer of the
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%superconducting magnets. The ICEDUST also
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%has modules to perform calibration, reconstruction and analysis of both
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%Monte-Carlo outputs and experimental data in a unified way. Documentations and
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%implementations of the ICEDUST are underway.
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The software used to simulate and analyze data is crucial to the success of the
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COMET experiment. The COMET software group has set up a new software framework
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called ICEDUST (Integrated COMET Experiment Data User Software Toolkit), which
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provides the ability to simulate, reconstruct and analyze data. The ability to
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reconstruct and analyze data, treating simulated and real data in the same way,
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is an essential feature that will allow COMET to achieve its sensitivity.
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ICEDUST uses accurate descriptions of the experiment geometry and magnetic
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field maps from the manufacturer of the superconducting magnets. The framework
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also calibrates the data (through the use of constants stored in a database)
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and distributes the data and processing using the Grid. Implementation of
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COMET Phase-I in ICEDUST is currently underway.
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\section{Summary}
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The COMET (J-PARC E21) experiment is going to push the limit of experimental
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searches for CLFV $\mu-e$ conversion process, a very promising probe for new
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physics beyond the SM.
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The first stage, COMET Phase--I, will improve the upper limit by two orders of
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magnitude to a single event sensitivity of $3\times10^{-15}$.
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Heavy R\&D activities are ongoing to realize the physics
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run of the COMET Phase--I in the year of 2017.
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%\appendix
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%\section{}
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%
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%Use the \verb|\appendix| command if you need an appendix(es). The
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%\verb|\section| command should follow even though there is no title for the
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%appendix (see above in the source of this file).
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\begin{thebibliography}{9}
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\bibitem{marciano} W. J. Marciano, T. Mori and J.M. Roney:
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Ann. Rev. Nucl. Part. Sci. \textbf{58}, 315 (2008).
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\bibitem{altman} W. Altmannshofer, A. J. Buras, S. Gori, P. Paradisi and D. M.
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Straub: Nucl. Phys. B \textbf{830} (2010) 17.
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\bibitem{sindrumii} W.~Bertl (The SINDRUM-II collaboration):
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Eur. Phys. J. C \textbf{47} (2006).
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337-346.
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\bibitem{comet07} D.~Bryman (The COMET collaboration): ``An experimental
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search for lepton flavor violating $\mu^- - e^-$ conversion at sensitivity of
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$10^{-16}$ with a slow-extracted bunched proton beam'', J-PARC Proposal,
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2007.
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\bibitem{comet12} R. Akhmetshin (The COMET collaboration): ``Experimental
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proposal for Phase--I of the COMET experiment at J-PARC'', KEK/J-PARC-PAC
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2012-10.
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\bibitem{mu2e08} R.M.~Carey (The Mu2e collaboration): ``Proposal to search for
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\muec~ with a single event sensitivity below $10^{-16}$'', a research
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proposal to Fermilab, 2008.
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%\bibitem{jpsj} The abbreviation for JPSJ must be ``J. Phys. Soc. Jpn." in the
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%reference list.
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%\bibitem{instructions} More abbreviations of journal titles
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%are listed in ``Instructions for Preparation of Manuscript", which is
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%available at our Web site (http://jpsj.ipap.jp).
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%\bibitem{format} F. Author, S. Author, and T. Author: Abbreviated journal title
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%\textbf{volume in bold face} (year of publication) initial page or article ID.
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\end{thebibliography}
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\end{document}
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