Update APPC2013 proceeding, according to reviewer's request
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APPC2013/Answer to the reviewer.pdf
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APPC2013/Answer to the reviewer.pdf
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APPC2013/Ref_cEbu61VYUP_CYDxeKrRf1.pdf
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APPC2013/Ref_cEbu61VYUP_CYDxeKrRf1.pdf
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@@ -124,7 +124,7 @@ Using the J-PARC proton beam and the pion capture by
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a solenoidal field, COMET will have a single event sensitivity 10,000 times
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better than the current limit. The COMET collaboration has taken a phased
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approach in which the first phase, COMET Phase-I [3], starts in 2013 and
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initial data taking in around 2016. This paper provides some highlights among
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initial data taking in around 2017. This paper provides some highlights among
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ongoing R\&D activities.
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%Charged lepton flavor violation (CLFV) has attracted much attention from
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@@ -157,18 +157,18 @@ ongoing R\&D activities.
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~\\
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~[1] Y.~Kuno {\it et al.} (COMET collaboration), ``A Experimental
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Search for Lepton Flavor Violating \muec Conversion at Sensitivity of
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$10^{-16}$ with A Slow-Extracted Bunched Proton Beam'', J-PARC Proposal, 2007
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and J-PARC Conceptual Design Report, 2009.\\
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~[1] 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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~[2] W.~Bertl {\it et al.} (SINDRUM-II collaboration), ``A search for $\mu
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- e$ conversion in muonic gold'', The European Physical Journal C 47 (2006)
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~[2] W.~Bertl (SINDRUM-II collaboration): Eur. Phys. J. C \textbf{47} (2006)
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337-346.\\
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~[3] Y.~Kuno {\it et al.} (COMET collaboration), ``Letter of
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Intent of Phase-I for the COMET Experiment at J-PARC'', unpublished, March
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2012.
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~[3] 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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%%%
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@@ -53,29 +53,30 @@ 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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Among CLFV processes, one of the most prominent one is
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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. $\mu - e$ conversion would
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occur in the following manner. Stopped muons are quickly captured by atoms
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($~10^{-10}$ s) and cascade down to the 1S orbitals. There, they can undergo:
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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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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 distintive signal, a mono-energetic electron of energy:
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simplicity and distintive signal, a mono-energetic electron of energy $E_{e}$:
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\[
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E_{mec} = m_{\mu} - B_{\mu}(Z, A) - R(A) \simeq \textrm{105 MeV},
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E_{e} = m_{\mu} - B_{\mu}(Z, A) - R(A) \simeq \textrm{105 MeV},
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\]
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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 extremely high rates. As a result, one of the best
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limit of CLFV searches comes from a search for $\mu - e$ conversion in muonic
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gold done by
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the SINDRUM--II collaboration: \sindrumlimit~\cite{sindrumii}.
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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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@@ -91,31 +92,32 @@ $10^{-17}$}
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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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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 from the
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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.
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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 Nuclear and Particle
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Experimental Hall (NP Hall) using a bunched proton beam that is
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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 a superconducting pion capture
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solenoid with high magnetic field. The detector section has muon stopping
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targets, an electron transport beam line for $\mu - e$ conversion signals,
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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, which has 117 collaborators in 27 institutes from
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12 countries as of April 2013, has adopted a staging approach with two
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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 $3 \times 10^{-17}$, which is
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comparable to the Mu2e project at Fermilab~\cite{mu2e08}.
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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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@@ -129,7 +131,7 @@ Fig.~\ref{fig:sched}.
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\begin{figure}[tbh]
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\centering
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\includegraphics[width=\textwidth]{figs/sched}
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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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@@ -142,32 +144,32 @@ COMET Phase--I has two major goals:
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\times 10^{-15}$.
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\end{itemize}
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In order to realized the goals, COMET Phase--I proposes to have two systems of
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detector. A straw tube detector and electromagnetic calorimeter will be used
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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 to realize the schedule with the
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physics run in 2017. Some R\&D activities are hightlighted as follow.
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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 radiation shielding and pion production target are in
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progress.
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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 would be ready
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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 study and experiments with a test
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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 its single hit rate would be dominated by
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protons emitted after muons are captured in the target. The lack of
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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 are 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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@@ -179,14 +181,15 @@ a joint effort between COMET and Mu2e.
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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 would be completed in 2013. Design works and construction of the real
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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. Currently, two types of crystal
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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 would be made
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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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@@ -195,18 +198,23 @@ beam tests at J-PARC.
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Software used to simulate and analyse 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. It also
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framework includes an improved simulation for the experiment. Some 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 ICEDUST are underway.
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implementations of the ICEDUST are 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. Heavy R\&D activities are ongoing to realize the physics
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run of its first stage, COMET Phase--I, in the year of 2017 with an immediate
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goal: two orders of magnitude improvement in the single event sensitivity
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compares the the current best limit.
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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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