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Update MLF Proposal, submitted version on July 8th, 2013

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AlCapTRIUMF/AlCap_DetailedStatement.tex
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@@ -229,7 +229,11 @@ has the potential to improve sensitivity by using a high muon rate
without suffering from accidental background events, which would be serious
for other processes, such as $\mu\rightarrow e\gamma$ and $\mu\rightarrow eee$ decays.
The previous search for \muec conversion was performed by the SINDRUM II collaboration at PSI. The SINDRUM II spectrometer consisted of a set of concentric cylindrical drift chambers inside a superconducting solenoid magnet of 1.2 Tesla. They set an upper limit of \muec in Au of $B(\mu^{-} + Au \rightarrow e^{-} + Au) < 7 \times 10^{-13}$.
The previous search for \muec conversion was performed by the SINDRUM II
collaboration at PSI. The SINDRUM II spectrometer consisted of a set of
concentric cylindrical drift chambers inside a superconducting solenoid magnet
of 1.2 Tesla. They set an upper limit of \muec in Au of
$B(\mu^{-} + Au \rightarrow e^{-} + Au) < 7 \times 10^{-13}$.
\begin{figure}[b!]
\vspace{-40mm}

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MLFProposal/proposal.tex
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@@ -45,49 +45,52 @@ linkbordercolor={1 0 0}
\section{Scientific background and aim of the experiment}
\label{sec:motivation}
\subsection{Scientific background}
The recent observation that neutrinos oscillate and change flavour and so have
mass requires an extension to the SM and demonstrates that lepton flavour is
not an absolutely conserved quantity. However, even in this minimal extension
to the SM, accommodating neutrino masses, the rate of charged lepton flavour
violating (CLFV) interactions is predicted to be $O(10^{-54})$, and is far too
small to be observed. As such, any experimental observation of CLFV
ould be a clear evidence of new physics beyond the SM.
Two new projects have ben established to search for a CLFV process,
$\mu^-N\rightarrow e^-N$ conversion. They are Mu2e experiment~\cite{mu2e08}
at FNAL and the COMET experiment~\cite{come07} at J-PARC. The two experiments
will both utilise multi-kW pulsed 8$-$9 GeV proton beams to achieve a branching
Charged lepton flavor violation (CLFV) has yet to be observed and is known to
be sensitive to new physics beyond the Standard Model (SM). The J-PARC E12
experiment, COMET~\cite{come07}, is a new experiment to search for a CLFV process of
neutrinoless muon-to-electron conversion in presence of a nucleus (\muec).
Utilising multi-kW pulsed 8$-$9 GeV proton beams, COMET can achieve a branching
ratio sensitivities lower than 10$^{-16}$, that is 10,000 better than current
best limit established by SINDRUM II. Both COMET Phase--I and Mu2e will be
subject to significant backgrounds from the products of the nuclear capture
process. Among them, the background for protons is a particularly acute one.
%, and its detailed
%investigation is the subject of this proposal, which is a joint proposal on
%behalf of both the Mu2e and COMET collaborations.
best limit established by SINDRUM II~\cite{sindrumii}.
The tracking chambers of COMET Phase--I~\cite{phaseI12} and Mu2e are designed
to be measure charged particles of their momenta greater than 70 MeV/$c$ and 53
MeV/$c$ respectively. In that momentum ranges, it turns out that single hit
rates of the tracking chambers would be dominated by protons after nuclear muon
capture. In order to limit the single hit rate of the tracking chamber to an
acceptable level, both experiments are considering to place proton absorbers in
front of the tracking chambers to reduce proton hit rates. However, the proton
Recently, COMET collaboration has adopted a staged approach, in which the COMET
Phase--I~\cite{phaseI12}, with a partial muon transport solenoid, will
have physics runs in 2016. The tracking chamber for COMET Phase--I are designed
to measure charged particles of their momenta in the range from 70 MeV/$c$ to
105 MeV/$c$.
In that momentum range, single hit
rate of the tracking chamber would be dominated by protons after nuclear muon
capture. In order to limit the single hit rate to an
acceptable level, a proton absorber would be installed in
front of the tracking chambers to reduce proton hit rate. However, the proton
absorber would deteriorate the reconstructed momentum resolution of electrons
at birth. And similarly the rate of proton emission is important to determine
thickness of the muon stopping target made of aluminum. Therefore it is
important to know the rate so that the detector system can be optimized in
terms of both hit rate and momentum resolution.
%Mu2e will be subject to significant backgrounds from the products of the
%nuclear capture
%process. Among them, the background for protons is a particularly acute one.
%, and its detailed
%investigation is the subject of this proposal, which is a joint proposal on
%behalf of both the Mu2e and COMET collaborations.
%The tracking chambers of COMET Phase--I~\cite{phaseI12} and Mu2e are designed
%to be measure charged particles of their momenta greater than 70 MeV/$c$ and 53
%MeV/$c$ respectively.
\subsection{Goal of the experiment}
The goal of the experiment is to measure the rate and energy spectra of the
charged particles emitted after a muon is captured on aluminum, silicon and
titanium targets. A precision of 5\% down to an energy of 2.5 MeV is required
for both the rate and the energy spectra.
protons emitted after a muon is captured on aluminum and silicon targets.
A precision of 5\% in the range from 3 to 6 MeV (momentum from 75 MeV/$c$ to
105 MeV/$c$) is required for both the rate and the energy spectra.
\subsection{Urgency}
The Mu2e experiment is now under the DOE Critical Decision Review process. The
COMET Phase--I construction, at least the beam line, might start next year in
2013. The COMET collaboration needs to complete the detector design as soon as
The construction of
COMET experimental hall has started in 2013, a prototype of the tracking
chamber is being designed, and chamber contruction should be finished by the
end of 2015.
The COMET collaboration needs to complete the final detector design as soon as
possible. Therefore, measurements of proton emission rates and spectrum that
can be done as early as possible become one of the critical path for the both
experiments.
@@ -96,8 +99,8 @@ experiments.
%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Present status of the research}
At present, the yield, energy spectrum and composition of the charged
particles emitted in muon capture on Al and Ti have not been measured in the
relevant energy range for COMET Phase--I and Mu2e.
particles emitted in muon capture on aluminum have not been measured in the
relevant energy range for COMET Phase--I.
Figure~\ref{fg:silicon-proton} shows the spectrum of charged particle emission
from muons being stopped and captured in a silicon detector \cite{sobo68}. The
peak below 1.4 MeV is from the recoiling heavy ions, mainly $^{27}$Al, when no
@@ -112,7 +115,7 @@ spectrum in Fig.~\ref{fg:silicon-proton} with an empirical function given by
\centering
\includegraphics[width=0.48\textwidth]{figs/si-proton.pdf}
\caption{Charged particle spectrum from muon capture on an active silicon
target ~\cite{sobo68}.}
target~\cite{sobo68}.}
\label{fg:silicon-proton}
\vspace{-10pt}
\end{wrapfigure}
@@ -154,12 +157,20 @@ Measday~\cite{measday}.
%\end{table}
The limited information available makes it difficult to draw
quantitative conclusive detector design. At this moment, for both COMET
Phase--I and Mu2e, the above analytical spectrum has been used to estimate proton
emission. And also the $p, d, \alpha$ composition is not known.
quantitative conclusive detector design. At this moment
the above analytical spectrum has been used to estimate proton
emission in COMET Phase--I detector designs.
A test experiment has been conducted at PSI in 2009 and we have had a better
understanding of the intrumentations and possible backgrounds involved.
For this measurement, a DC muon beam, such as at TRIUMF or PSI, is the best
choice. A test experiment has been conducted at PSI in 2009 by Mu2e, but
without a conclusive result. We have discussed with them to have a better
understanding of the instrumentations and possible backgrounds involved. An
experiment similar to the one in 2009 is scheduled at PSI in the end of
2013. This is a joint effort of the two collaborations, COMET and Mu2e.
The experiment in this proposal could serve two purposes: (a) cross check
for the PSI experiment, and (b) back up plan in case that the PSI experiment
could not be carried out.
\section{Experimental method} % (fold) \label{sec:expdescpription}
\begin{wrapfigure}{r}{0.4\textwidth}
@@ -170,26 +181,38 @@ understanding of the intrumentations and possible backgrounds involved.
\vspace{-10pt}
\end{wrapfigure}
A schematic layout of the experimental setup is shown in Fig.~\ref{fg:setup}.
It will be an improved version of a test experiment performed by part of this
collaboration at PSI in 2009.
Low energy negative muons (less than 30 MeV/c) will be detected by external beam
counters and then enter a vacuum vessel though a thin mylar
window. They will be stopped in passive Al and Ti foils of 25 to 200 $\mu m$
thickness, positioned under 45 degrees to the beam direction. As a cross check
they will also be stopped in active Si detectors used as target.
Low energy negative muons (28 MeV/$c$) will enter a vacuum vessel
though a thin mylar window, and will be stopped in passive Al foils of 25
-- 200 $\mu m$
thickness, positioned under 45 degrees to the beam direction.
As a cross check, an active silicon target of 140 $\mu m$ will also be used.
A long duct is prepared to avoid potential background from muons that are not
stopped in the target.
Two packages
of charged particle detectors are positioned on opposite sides, parallel
to the target surface. The thin Si detectors (65 $\mu m$) will provide dE/dx
information. The thick Si detectors (1500 $\mu m$) stop protons up to about 12
MeV. Plastic scintillators positioned
behind these Si detector observe potential higher energy protons and veto
through--going electrons. The symmetry
MeV. According to a simple Geant4 simulation, we can use dE/dx method to do
PID (Fig.~\ref{fg:dedx}).
%Plastic scintillators positioned
%behind these Si detector observe potential higher energy protons and veto
%through--going electrons.
The symmetry
between the left and right Si stations allows for a powerful monitor of
systematic effects. Differences between the detectors would indicate background
due to different stopping material, non--uniform stopping distribution or
differences due to muon scattering.
differences due to muon scattering.
Muon bunch signal will be used as the trigger for the DAQ system. The DAQ
system collects data in a fixed period of time after this trigger.
Timing and energy information from silicon detectors will be read out by
flash ADCs (FADC).
Particle identification can also be done using TOF method. In this case,
extended pipes need to be connected to the chamber, and distance between dE and
E detectors would be increased to about 15 cm.
Careful shielding of direct or scattered
muons is required, as the stopping fraction is small and proton emission is a
@@ -201,6 +224,13 @@ In order to normalize a number of muons stopping in the aluminum target, a
germanium detector to measure muonic X-rays from muons stopping in the aluminum
target is installed.
\begin{wrapfigure}{r}{0.4\textwidth}
\centering
\includegraphics[width=0.38\textwidth]{figs/dEdx-mlf}
\caption{PID using two silicon detectors: 65 $\mu m$ and 1500 $\mu m$ thick}
\label{fg:dedx}
\vspace{-10pt}
\end{wrapfigure}
The main systematic issues are as follows.
\begin{itemize}
@@ -208,19 +238,18 @@ The main systematic issues are as follows.
\setlength{\parskip}{0pt}
\setlength{\parsep}{0pt}
\item Deconvolute the orginal proton spectrum $f(T_i)$: firstly, an optimal
cloud muon beam is requested for the experiment. Second, the use of an active
\item Deconvolute the orginal proton spectrum $f(T_i)$: the use of an active
Si target allow the experimental calibration of the response function,
because both initial energy $T_i$ and final energy $T_f$ of protons are accessible with an active target.
\item Absolute calibration: the number of muon stops will be determined with
because both initial energy $T_i$ and final energy $T_f$ of protons are
accessible.
\item Absolute calibration: the number of
muon stops will be determined with
the Ge detector. Again, the use of an active Si target allows for a cross
calibration. The proton detection efficiency will be simulated by Geant4 and
calibrated with the active Si target.
\item The PID of emitted charged particles will be determined by dE/dx.
\item Background: electron background will be determined with $\mu^+$, neutron
recoils by absorbing the proton component before the Si detectors. A
dangerous background are muons stops in walls and scattered into the Si
detector.
\item Background: a dangerous background comes from muons stop in walls and
scatter into the Si detectors.
\end{itemize}
%A realistic Geant4 simulation is being developed. It will serve as an important
@@ -274,21 +303,20 @@ The main systematic issues are as follows.
%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{Beam time estimation}
We are requesting a 10--day beamtime for the test measurement. This is based on
the estimation as follows.
We are requesting a 6--day beam time (two consecutive blocks). This is based on
the estimation as follows:
\begin{itemize}
\setlength{\itemsep}{1pt}
\setlength{\parskip}{0pt}
\setlength{\parsep}{0pt}
\item 2 days setup time including the installation of the equipment,
\setlength{\itemsep}{1pt}
\setlength{\parskip}{0pt}
\setlength{\parsep}{0pt}
\item 1 days of beam tuning and adjustment of electronics,
\item 1 week for data taking with 2 different thick targets of 3.5 day data
taking for each.
Data taking of 7 days is based on the estimated rate of protons using
Geant4 simulation: the proton yield is $5\times 10^{-4}$ per stopped
muons, about 50\% of muons will be stopped in the 200 $\mu m$ target, and
we will use the double pulse to have 50 bunches of muons in one second.
We want to accumulate 3000 events for each sample in this test experiment.
\item 5 days of data taking: the proton hit rate on the detectors estimated
by the Geant4 simulation is $2 \times 10^{-3}$ per muon bunch. We want to
accumulate 5000 events in 20 bins from 3 MeV to 6 MeV, that means 10000
proton events in total. So, we will need $5 \times 10^{6}$ bunches, or $2
\times 10^5$ seconds = 2.3 days ( at 25 Hz beam frequency) for one sample. We
intend to have two measurements with two targets: active silicon, and
aluminum 200 $\mu m$.
\end{itemize}
\begin{thebibliography}{9}
@@ -300,14 +328,19 @@ the estimation as follows.
%\bibitem{masi06} L.~Calibbi, A.~Faccia, A.~Masierro, and S.K. Vempati, Phys.
%Rev. {\bf D74} 116002 (2006).
%
\bibitem{mu2e08} R.M.~Carry {\it et al.} (Mu2e collaboration), ``Proposal to
Search for \muec with a Single Event Sensitivity Below $10^{-16}$, FNAL
proposal, 2008.
%\bibitem{mu2e08} R.M.~Carry {\it et al.} (Mu2e collaboration), ``Proposal to
%Search for \muec with a Single Event Sensitivity Below $10^{-16}$, FNAL
%proposal, 2008.
%
\bibitem{come07} Y.~Kuno {\it et al.} (COMET collaboration), ``A Experimental
Search for Lepton Flavor Violating \muec Conversion at Sensitivity of
$10^{-16}$ with A Slow-Extracted Bunched Proton Beam'', J-PARC Proposal, 2007
and J-PARC Conceptual Design Report, 2009.
\bibitem{sindrumii} W.~Bertl {\it et al.} (SINDRUM-II collaboration), ``A
search for $\mu - e$ conversion in muonic gold'', The European Physical
Journal C 47 (2006).
337-346.\\
%
\bibitem{phaseI12} Y.~Kuno {\it et al.} (COMET collaboration), ``Letter of
Intent of Phase--I for the COMET Experiment at J-PARC'', March