custom styled thesis
17
thesis2/.gitignore
vendored
Normal file
@@ -0,0 +1,17 @@
|
||||
#Compilation byproducts
|
||||
*.aux
|
||||
*.bbl
|
||||
*.toc
|
||||
*.lof
|
||||
*.lot
|
||||
*.blg
|
||||
*.log
|
||||
*.nav
|
||||
*.out
|
||||
*.snm
|
||||
thesis.pdf
|
||||
comment.cut
|
||||
feyngraph.mp
|
||||
|
||||
*.pdf
|
||||
refs/*
|
||||
15
thesis2/Makefile
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@@ -0,0 +1,15 @@
|
||||
DOCNAME = thesis
|
||||
|
||||
INPUT = $(DOCNAME).tex
|
||||
TARGET= $(DOCNAME).pdf
|
||||
|
||||
default: $(TARGET)
|
||||
|
||||
$(TARGET): $(INPUT) Makefile chapters/*.tex custom_macro.tex mythesis.sty
|
||||
@rm -f $(DOCNAME).{aux,toc,lof,lot}
|
||||
pdflatex $< && bibtex $(DOCNAME) && pdflatex $< && pdflatex $<
|
||||
|
||||
clean:
|
||||
@rm -f *.{aux,toc,lof,lot}
|
||||
@rm -f *.{aux,bbl,blg,log,nav,out,snm,toc,lot,lof} $(TARGET)
|
||||
@rm -f comment.cut
|
||||
28
thesis2/chapters/appendices.tex
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@@ -0,0 +1,28 @@
|
||||
%% The "\appendix" call has already been made in the declaration
|
||||
%% of the "appendices" environment (see thesis.tex).
|
||||
\chapter{Pointless extras}
|
||||
\label{app:Pointless}
|
||||
|
||||
\chapterquote{%
|
||||
Le savant n'\'etudie pas la nature parce que cela est utile; \\
|
||||
\indent il l'\'etudie parce qu'il y prend plaisir, \\
|
||||
\indent et il y prend plaisir parce qu'elle est belle.}%
|
||||
{Henri Poincar\'e, 1854--1912}
|
||||
|
||||
Appendixes (or should that be ``appendices''?) make you look really clever, 'cos
|
||||
it's like you had more clever stuff to say than could be fitted into the main
|
||||
bit of your thesis. Yeah. So everyone should have at least three of them\dots
|
||||
|
||||
\section{Like, duh}
|
||||
\label{sec:Duh}
|
||||
Padding? What do you mean?
|
||||
|
||||
\section{$y = \alpha x^2$}
|
||||
\label{sec:EqnTitle}
|
||||
See, maths in titles automatically goes bold where it should (and check the
|
||||
table of contents: it \emph{isn't} bold there!) Check the source: nothing
|
||||
needs to be specified to make this work. Thanks to Donald Arsenau for the
|
||||
teeny hack that makes this work.
|
||||
|
||||
%% Big appendixes should be split off into separate files, just like chapters
|
||||
%\input{app-myreallybigappendix}
|
||||
19
thesis2/chapters/backmatter.tex
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|
||||
%\begin{colophon}
|
||||
%This thesis was made in \LaTeXe{} using the ``hepthesis'' class~\cite{hepthesis}.
|
||||
%\end{colophon}
|
||||
|
||||
%% You're recommended to use the eprint-aware biblio styles which
|
||||
%% can be obtained from e.g. www.arxiv.org. The file mythesis.bib
|
||||
%% is derived from the source using the SPIRES Bibtex service.
|
||||
\bibliographystyle{h-physrev}
|
||||
\bibliography{thesis}
|
||||
|
||||
%% I prefer to put these tables here rather than making the
|
||||
%% front matter seemingly interminable. No-one cares, anyway!
|
||||
%\listoffigures
|
||||
%\listoftables
|
||||
|
||||
%% If you have time and interest to generate a (decent) index,
|
||||
%% then you've clearly spent more time on the write-up than the
|
||||
%% research ;-)
|
||||
%\printindex
|
||||
88
thesis2/chapters/chap1_intro.tex
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|
||||
\chapter*{Introduction}
|
||||
\thispagestyle{empty}
|
||||
\addcontentsline{toc}{chapter}{Introduction}
|
||||
\label{cha:introduction}
|
||||
|
||||
%% Restart the numbering to make sure that this is definitely page #1!
|
||||
\pagenumbering{arabic}
|
||||
|
||||
|
||||
%\begin{itemize}
|
||||
%\item CLFV in 3 lines
|
||||
%\item COMET in 3 lines
|
||||
%\item structure of the thesis:
|
||||
%\begin{itemize}
|
||||
%\item physics motivation of CLFV, COMET (chap 1)
|
||||
%\item overview of COMET, Phase-I, requirements for detectors (chap 2)
|
||||
%\item details of the proton measurements:
|
||||
%\begin{itemize}
|
||||
%\item physics (chap 3)
|
||||
%\item method (chap 4)
|
||||
%\item experimental set up, calibration (chap 4? or 5 )
|
||||
%\item data analysis (chap 5)
|
||||
%\item results, impact (chap 6)
|
||||
%\end{itemize}
|
||||
%\end{itemize}
|
||||
%\end{itemize}
|
||||
%\begin{comment}
|
||||
%The Standard Model (SM) is the most successful theory of particle physics
|
||||
%as it could account for almost all experimental data from high energy
|
||||
%experiments. The discovery of a Higgs-like boson at the LHC in 2012 is another
|
||||
%triumph of the theory. However, it is known that the SM has its limitations,
|
||||
%one example is there is no explanation for the existence of lepton
|
||||
%flavours and flavour conservation.
|
||||
%theory. For example, it does not explain the origin of mass, the nature of dark
|
||||
%matter, or neutrino oscillations.
|
||||
|
||||
%The lepton flavour conservation in the SM is assured by assuming neutrinos are
|
||||
%massless. But, extensive experiments with atmospheric, solar, accelerator,
|
||||
%reactor neutrinos have shown that neutrinos have non-zero masses, and they do
|
||||
%mix between flavours~\cite{BeringerArguin.etal.2012}. In other words, lepton
|
||||
%flavour violation (LFV) does occur in neutrino oscillations.
|
||||
|
||||
%While lepton flavour is totally violated in the neutrino sector, no charged
|
||||
%lepton flavour violation (CLFV) has ever been observed. Therefore, any
|
||||
%experimental evidence of lepton flavour violation with charged lepton would be
|
||||
%a breakthrough that leads to new physics beyond the SM.
|
||||
%\end{comment}
|
||||
%The Standard Model (SM) is the most successful theory of particle physics
|
||||
%as it could account for almost all experimental data from high energy
|
||||
%experiments. However, it is also known that the SM has its
|
||||
%TODO: wording /duplicaitons
|
||||
The COMET experiment~\cite{COMET.2007}, proposed at the Japan Proton
|
||||
Accelerator Research Complex (J-PARC), is a next-generation-experiment that
|
||||
searches for evidence of charged lepton flavour violation (CLFV) with muons.
|
||||
The branching ratio of CLFV in the Standard Model, even with massive neutrinos,
|
||||
is prohibitively small, at the order of $10^{-54}$. Therefore, any experimental
|
||||
observation of CLFV would be a clear signal of new physics beyond the SM.
|
||||
|
||||
The COMET (\textbf{CO}herent \textbf{M}uon to \textbf{E}lectron
|
||||
\textbf{T}ransition) Collaboration aims to probe the conversion of a muon to
|
||||
an electron in a nucleus field at a sensitivity of $6\times10^{-17}$, pushing
|
||||
for a four orders of magnitude improvement from the current limit set by the
|
||||
SINDRUM-II~\cite{Bertl.etal.2006}. A staging approach is adopted at the COMET
|
||||
to achieve an intermediate physics result, as well as to gain operational
|
||||
experience. The first stage, COMET Phase I, is scheduled to start in 2016 with
|
||||
the goal sensitivity of $3\times 10^{-15}$ after a three-month-running period.
|
||||
|
||||
A cylindrical drift chamber being developed by the Osaka University group
|
||||
will be the main tracking detector in the COMET Phase I. It is anticipated that
|
||||
the chamber will be heavily occupied by protons emitted after nuclear muon
|
||||
capture in the stopping target, and thus an absorber will be installed to
|
||||
reduce the proton hit rate to a tolerable level. A study of proton emission
|
||||
following nuclear muon capture for optimisation of the proton absorber is
|
||||
presented in this thesis.
|
||||
|
||||
The thesis is structured as follows:
|
||||
firstly,
|
||||
the physics motivation of the COMET experiment, with muon's normal decays and
|
||||
CLFV decays, is described in Chapter~\ref{cha:clfv}.
|
||||
Chapter~\ref{cha:comet_overview} gives an overview of the
|
||||
COMET experiment: beam lines, detectors and their requirements, and expected
|
||||
sensitivities. Details of the study on proton emission are described in
|
||||
Chapters~\ref{cha:alcap_phys},~\ref{cha:the_alcap_run_2013},~\ref{cha:data_analysis}:
|
||||
physics, method, experimental set up, data analysis. The results and impacts of
|
||||
the study on COMET Phase-I design is discussed in
|
||||
Chapter~\ref{cha:discussions}.
|
||||
|
||||
% chapter introduction (end)
|
||||
369
thesis2/chapters/chap2_mu_e_conv.tex
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@@ -0,0 +1,369 @@
|
||||
\chapter{Lepton flavour and $\mu-e$ conversion}
|
||||
\thispagestyle{empty}
|
||||
\label{cha:clfv}
|
||||
|
||||
\section{Lepton flavour}
|
||||
\label{sec:lepton_flavour}
|
||||
According to the SM, all matter is built from a small set of fundamental
|
||||
spin one-half particles, called fermions: six quarks and six leptons.
|
||||
The six leptons form three generations (or flavours), namely:
|
||||
\begin{equation*}
|
||||
\binom{\nu_e}{e^-}, \quad \binom{\nu_\mu}{\mu^-} \quad \textrm{ and } \quad
|
||||
\binom{\nu_\tau}{\tau^-}
|
||||
\end{equation*}
|
||||
|
||||
Each lepton is assigned a lepton flavour quantum number, $L_e$, $L_\mu$,
|
||||
$L_\tau$, equals to $+1$ for each lepton and $-1$ for each antilepton of the
|
||||
appropriate generation. The lepton flavour number is conserved in the SM, for
|
||||
example in the decay of a positive pion:
|
||||
\begin{align*}
|
||||
&\pi^+ \rightarrow \mu^+ + \nu_\mu \\
|
||||
L_\mu \quad &0\quad \textrm{ }-1 \quad +1
|
||||
\end{align*}
|
||||
or, the interaction of an electron-type antineutrino with a proton (inverse
|
||||
beta decay):
|
||||
\begin{align*}
|
||||
&\quad \overline{\nu}_e + p \rightarrow e^+ + n \\
|
||||
L_e \quad &-1 \quad \textrm{ }0 \quad -1 \textrm{ } \quad 0
|
||||
\end{align*}
|
||||
|
||||
The decay of a muon to an electron and a photon, where lepton flavour numbers
|
||||
are violated by one unit or more, is forbidden:
|
||||
%(the limit
|
||||
%on this branching ratio is \meglimit~at 90\% confidence level
|
||||
%(C.L.)~\cite{Adam.etal.2013}).
|
||||
\begin{equation}
|
||||
\begin{aligned}
|
||||
&\quad \mu^+ \rightarrow e^+ + \gamma\\
|
||||
L_\mu \quad &-1 \qquad 0 \qquad 0\\
|
||||
L_e \quad &\quad 0 \quad -1 \qquad 0
|
||||
\end{aligned}
|
||||
\label{eq:mueg}
|
||||
\end{equation}
|
||||
%One more decay?
|
||||
|
||||
%\hl{TODO: Why massless neutrinos help lepton flavour conservation??}
|
||||
%\hl{TODO: copied from KunoOkada}
|
||||
%In the minimal version of the SM, where only one Higgs doublet is included and
|
||||
%massless neutrinos are assumed, lepton flavor conservation is an automatic
|
||||
%consequence of gauge invariance and the renormalizability of the SM
|
||||
%Lagrangian. It is the basis of a natural explanation for the smallness of
|
||||
%lepton flavor violation (LFV) in charged lepton processes.
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
|
||||
\section{Muon and its decays in the Standard Model}
|
||||
\label{sec:muon_decay_in_the_standard_model}
|
||||
|
||||
\subsection{Basic properties of the muon}
|
||||
\label{sub:basic_properties_of_the_muon}
|
||||
|
||||
The muon is a charged lepton, its static properties have been measured with
|
||||
great precisions and are summarised in the ``Review of Particle Physics'' of
|
||||
the Particle Data Group (PDG)~\cite{BeringerArguin.etal.2012}. Some of the
|
||||
basic properties are quoted as follows:
|
||||
\begin{enumerate}
|
||||
\item The muon mass is given by the muon to electron mass ratio,
|
||||
\begin{align}
|
||||
\frac{m_\mu}{m_e} &= 206.768 2843 \pm 0.000 0052\\
|
||||
m_\mu &= 105.6583715 \pm 0.0000035 \textrm{ MeV/}c^2
|
||||
\end{align}
|
||||
\item The spin of the muon is determined to
|
||||
be $\frac{1}{2}$ as the measurements of the muon's gyromagnetic give
|
||||
$g_\mu = 2$ within an overall accuracy better than 1 ppm. It is common to
|
||||
quoted the result of $g_\mu$ as muon magnetic moment anomaly:
|
||||
\begin{equation}
|
||||
\frac{g-2}{2} = (11659209 \pm 6)\times 10^{-10}
|
||||
\end{equation}
|
||||
\item The charge of the muon is known to be equal to that of the
|
||||
electron within about 3 ppb,
|
||||
\begin{equation}
|
||||
\frac{q_{\mu^+}}{q_{e^-}} + 1 = (1.2 \pm 2.1)\times 10^{-9}
|
||||
\end{equation}
|
||||
\item Electric dipole moment:
|
||||
\begin{equation}
|
||||
d = \frac{1}{2}(d_{\mu^-} - d_{\mu^+})
|
||||
= (-0.1 \pm 0.9) \times 10^{-19} \textrm{ }e\cdot\si{\centi\meter}
|
||||
\end{equation}
|
||||
\item The muon is not stable, average lifetime of the free muon is:
|
||||
\begin{equation}
|
||||
\tau_{\mu} = 2.1969811 \pm 0.0000022 \textrm{ }\si{\micro\second}
|
||||
\end{equation}
|
||||
\end{enumerate}
|
||||
|
||||
% subsection basic_properties_of_the_muon (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
\subsection{Decays of the muon}
|
||||
\label{sub:decays_of_the_muon}
|
||||
Because of charge and lepton flavour conservations, the simplest possible decay
|
||||
of muons is:
|
||||
\begin{equation}
|
||||
\mu^- \rightarrow e^- \nu_\mu \overline{\nu}_e
|
||||
\label{eq:micheldecay}
|
||||
\end{equation}
|
||||
Muons can also decay in the radiative mode:
|
||||
\begin{equation}
|
||||
\mu^- \rightarrow e^- \nu_\mu \overline{\nu}_e \gamma
|
||||
\label{eq:mue2nugamma}
|
||||
\end{equation}
|
||||
or with an associated $e^+ e^-$ pair:
|
||||
\begin{equation}
|
||||
\mu \rightarrow e^- \nu_\mu \overline{\nu}_e e^+ e^-
|
||||
\label{eq:mu3e2nu}
|
||||
\end{equation}
|
||||
|
||||
The dominant process, \micheldecay is commonly called Michel decay. It can be
|
||||
described by the V-A interaction which is a special case of a local,
|
||||
derivative-free, lepton-number-conserving four-fermion interaction.
|
||||
%using $V-A$
|
||||
%inteaction, a special case of four-fermion interaction, by Louis
|
||||
%Michel~\cite{Michel.1950}.
|
||||
The model contains independent real parameters that can be determined from
|
||||
measurements of muon life time, muon decay and inverse muon
|
||||
decay. Experimental results from extensive measurements of Michel parameters
|
||||
are consistent with the predictions of the V-A
|
||||
theory~\cite{Michel.1950,FetscherGerber.etal.1986,BeringerArguin.etal.2012}.
|
||||
|
||||
The radiative decay~\eqref{eq:mue2nugamma} is treated as an internal
|
||||
bremsstrahlung process~\cite{EcksteinPratt.1959}.
|
||||
%It occurs at the rate of about 1\% of all muon decays.
|
||||
Since it is not possible to clearly separated this mode
|
||||
from Michel decay in the soft-photon limit, the radiative mode is regarded as
|
||||
a subset of the Michel decay. An additional parameter is included to describe
|
||||
the electron and photon spectra in this decay channel. Like the case of
|
||||
Michel decay, experiments results on the branching ratio and the parameter are
|
||||
in agreement with the SM's predictions~\cite{BeringerArguin.etal.2012}.
|
||||
|
||||
There is a small probability (order of $10^{-4}$~\cite{EcksteinPratt.1959})
|
||||
that the photon in \muenng would internally convert to an
|
||||
$e^+e^-$ pair, resulting in the decay mode \muennee.
|
||||
%\hl{TODO: more?}
|
||||
|
||||
The branching ratios for decay modes of muons, compiled by the PDG, are
|
||||
listed in Table~\ref{tab:SM_muon_decays}.
|
||||
|
||||
\begin{table}[htb!]
|
||||
\begin{center}
|
||||
\begin{tabular}{l l l}
|
||||
\toprule
|
||||
Decay mode & Branching ratio & Remarks\\
|
||||
\midrule
|
||||
\micheldecay & $\simeq 1$ & commonly called Michel decay\\
|
||||
|
||||
\muenng & $0.014 \pm 0.004$ &
|
||||
subset of Michel decay, $E_\gamma > 10 \textrm{ MeV}$ \\
|
||||
|
||||
\muennee & $(3.4 \pm 0.2 \pm 0.3)\times 10^{-5}$ &
|
||||
transverse momentum cut $p_T>17 \textrm{ MeV/c}$\\
|
||||
\bottomrule
|
||||
\end{tabular}
|
||||
\end{center}
|
||||
\caption{Decay modes and branching ratios of muon listed by
|
||||
PDG~\cite{BeringerArguin.etal.2012}}
|
||||
\label{tab:SM_muon_decays}
|
||||
\end{table}
|
||||
%\hl{TODO: Michel spectrum}
|
||||
% subsection decays_of_the_muon (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
|
||||
% section muon_decay_in_the_standard_model (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
\section{Lepton flavour violated decays of muons}
|
||||
\label{sec:lepton_flavour_violation}
|
||||
%Historically, the ideas of lepton flavours and lepton flavour conservation
|
||||
%emerged from null-result experiments, such as a series of searches for \mueg in
|
||||
%1950s and 1960s
|
||||
%The fact that there is no convincing fundamental symmetry that leads to the
|
||||
%conservation, and
|
||||
%The fact that no underlying symmetry leads to this
|
||||
%conservation has been found, and mixing between generations does happen in the
|
||||
%quark sector make experimental searches for lepton flavour violation (LFV)
|
||||
%interesting.
|
||||
|
||||
%The decay \mueg and \mueee were of great interest in the 1950s and 1960s when
|
||||
%it is believed that the muon is an excited state of the electron.
|
||||
|
||||
The existence of the muon has always been a puzzle. At first, people thought
|
||||
that it would be an excited state of the electron. Therefore, the searches for
|
||||
\mueg was performed by Hincks and Pontercorvo~\cite{HincksPontecorvo.1948}; and
|
||||
Sard and Althaus~\cite{SardAlthaus.1948}. Those searches failed to find the
|
||||
photon of about 50 MeV that would have accompanied the decay electron in case
|
||||
the two-body decay \mueg had occurred. From the modern point of view, those
|
||||
experiments were the first searches for charged lepton flavour violation (LFV).
|
||||
|
||||
Since then, successive searches for LFV with the muon have been carried out. All
|
||||
the results were negative and the limits of the LFV branching ratios had been
|
||||
more and more stringent. Those null-result experiments suggested the lepton
|
||||
flavours - muon flavour $L_\mu$ and electron flavour $L_e$. The notion of lepton
|
||||
flavour was experimentally verified in the Nobel Prize-winning experiment of
|
||||
Danby et al. at Brookhaven National Laboratory
|
||||
(BNL)~\cite{DanbyGaillard.etal.1962}. Then the concepts of generations of
|
||||
particles was developed~\cite{MakiNakagawa.etal.1962}, and integrated into the
|
||||
SM, in which the lepton flavour conservation is guaranteed by and exact
|
||||
symmetry, owing to massless neutrinos.
|
||||
|
||||
Following the above LFV searches with muons, searches with various particles,
|
||||
such as kaons, taus, and others have been done. The upper limit have been
|
||||
improved at a rate of two orders of magnitude per decade. %TODO(Fig).
|
||||
|
||||
While all of those searches yielded negative results, LFV with neutrinos is
|
||||
confirmed with observations of neutrino oscillations; i.e. neutrino
|
||||
of one type changes to another type when it travels in space-time. The
|
||||
phenomenon means that there exists a mismatch between the flavour and
|
||||
mass eigenstates of neutrinos; and neutrinos are massive. Therefore, the SM
|
||||
must be modified to accommodate the massive neutrinos.
|
||||
|
||||
With the massive neutrinos charged lepton flavour violation (CLFV) must occur
|
||||
through oscillations in loops. But, CLFV processes are highly suppressed in the
|
||||
SM.
|
||||
For example, Marciano and Mori ~\cite{MarcianoMori.etal.2008} calculated the
|
||||
branching ratio of the process \mueg to be \brmeg$<10^{-54}$. Other
|
||||
CLFV processes with muons are also suppressed to similar practically
|
||||
unmeasurable levels.%\hl{TODO: Feynman diagram}
|
||||
Therefore, any experimental
|
||||
observation of CLFV would be an unambiguous signal of the physics beyond the
|
||||
SM. Many models for physics beyond the SM, including supersymmetric (SUSY)
|
||||
models, extra dimensional models, little Higgs models, predict
|
||||
significantly larger CLFV
|
||||
~\cite{MarcianoMori.etal.2008, MiharaMiller.etal.2013, BernsteinCooper.2013}.
|
||||
%\hl{TODO: DNA of CLFV charts}
|
||||
%A comprehensive list of predictions from various models, compiled by
|
||||
%Altmannshofer and colleagues ~\cite{AltmannshoferBuras.etal.2010a} is
|
||||
%reproduced in Table~\ref{tab:clfv_dna}.
|
||||
|
||||
%\begin{table}[htb!]
|
||||
%\begin{center}
|
||||
%\begin{tabular}{l l l}
|
||||
%\toprule
|
||||
%Decay mode & Branching ratio & Remarks\\
|
||||
%\midrule
|
||||
%\micheldecay & $\simeq 1$ & commonly called Michel decay\\
|
||||
|
||||
%\muenng & $0.014 \pm 0.004$ &
|
||||
%subset of Michel decay, $E_\gamma > 10 \textrm{ MeV}$ \\
|
||||
|
||||
%\muennee & $(3.4 \pm 0.2 \pm 0.3)\times 10^{-5}$ &
|
||||
%transverse momentum cut $p_T>17 \textrm{ MeV/c}$\\
|
||||
%\bottomrule
|
||||
%\end{tabular}
|
||||
%\end{center}
|
||||
%\caption{CLFV rates from various models~\cite{AltmannshoferBuras.etal.2010a}}
|
||||
%\label{tab:clfv_dna}
|
||||
%\end{table}
|
||||
|
||||
%It can be seen from the table that there are two CLFV processes with muons are
|
||||
%predicted to occur at large rates by all new physics models, namely \mueg and
|
||||
|
||||
%It is calculated that there are two CLFV processes that would
|
||||
%occur at large rates by many new physics models,
|
||||
Among the CLFV processes, the \mueg and
|
||||
the \muec are expected to have large effect by many models. The current
|
||||
experimental limits on these two decay modes are set by MEG
|
||||
experiment~\cite{Adam.etal.2013} and SINDRUM-II
|
||||
experiment~\cite{Bertl.etal.2006}:
|
||||
\begin{equation}
|
||||
\mathcal{B}(\mu^+ \rightarrow e^+ \gamma) < 5.7 \times 10^{-13}
|
||||
\end{equation}
|
||||
, and:
|
||||
\begin{equation}
|
||||
\mathcal{B} (\mu^- + Au \rightarrow e^- +Au) < 7\times 10^{-13}
|
||||
\end{equation}
|
||||
|
||||
%\hl{TODO: mueg and muec relations, Lagrangian \ldots}
|
||||
%The observation of one CLFV process may indicate the mass scale of the physics
|
||||
%beyond the SM, but it would not be enough to distinguish between different
|
||||
%models correspond to that physics.
|
||||
|
||||
% section lepton_flavour_violation (end)
|
||||
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
\section{Phenomenology of \mueconv}
|
||||
\label{sec:phenomenoly_of_muec}
|
||||
The conversion of a captured muon into an electron in the field of a nucleus
|
||||
has been one of the most powerful probe to search for CLFV. This section
|
||||
highlights phenomenology of the \muec.
|
||||
|
||||
\subsection{What is \mueconv}
|
||||
\label{sub:what_is_muec}
|
||||
When a muon is stopped in a material, it is quickly captured by atoms
|
||||
into a high orbital momentum state, forming a muonic atom, then
|
||||
it rapidly cascades to the lowest state 1S. There, it undergoes either:
|
||||
\begin{itemize}
|
||||
\item normal Michel decay: \micheldecay; or
|
||||
\item weak capture by the nucleus: $\mu^- p \rightarrow \nu_\mu n$
|
||||
\end{itemize}
|
||||
|
||||
In the context of physics beyond the SM, the exotic process of \mueconv where
|
||||
a muon decays to an electron without neutrinos is also
|
||||
expected, but it has never been observed.
|
||||
\begin{equation}
|
||||
\mu^{-} + N(A,Z) \rightarrow e^{-} + N(A,Z)
|
||||
\end{equation}
|
||||
The emitted electron in this decay
|
||||
mode , the \mueconv electron, is mono-energetic at an energy far above the
|
||||
endpoint
|
||||
of the Michel spectrum (52.8 MeV):
|
||||
\begin{equation}
|
||||
E_{\mu e} = m_\mu - E_b - \frac{E^2_\mu}{2m_N}
|
||||
\end{equation}
|
||||
where $m_\mu$ is the muon mas; $E_b \simeq Z^2\alpha^2 m_\mu/2$ is the binding
|
||||
energy of the muonic atom; and the last term is the nuclear recoil energy
|
||||
neglecting high order terms. For Al ($Z = 13$), the target of choice in the new
|
||||
\mueconv experiments, the outgoing electron has energy of $E_{\mu e} \simeq
|
||||
104.96$ MeV.
|
||||
% subsection what_is_muec (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
|
||||
\subsection{Measurement of \mueconv}
|
||||
\label{sub:measurement_of_mueconv}
|
||||
The quantity measured in searches for \mueconv is the ratio between the rate of
|
||||
\mueconv, and the rate of all muons captured:
|
||||
\begin{equation}
|
||||
R_{\mu e} =
|
||||
\frac{\Gamma(\mu^-N \rightarrow e^-N)}{\Gamma(\textrm{capture})}
|
||||
\label{eq:muerate_def}
|
||||
\end{equation}
|
||||
The normalisation to captures has advantages when one does calculation since
|
||||
many details of the nuclear wavefunction cancel out in the ratio.
|
||||
%Detailed
|
||||
%calculations have been performed by Kitano et al.~\cite{KitanoKoike.etal.2002a,
|
||||
%KitanoKoike.etal.2007}, and Cirigliano et al.~\cite{Cirig}
|
||||
The muon capture rate can be measured by observing the characteristic X-rays
|
||||
emitted when the muon stops, and cascades to the 1S orbit. Since the stopped
|
||||
muon either decays or be captured, the stopping rate is:
|
||||
\begin{equation}
|
||||
\Gamma_{\textrm{stop}} = \Gamma_{\textrm{decay}} + \Gamma_{\textrm{capture}}
|
||||
\end{equation}
|
||||
The mean lifetime $\tau = 1/\Gamma$, then:
|
||||
\begin{equation}
|
||||
\frac{1}{\tau_{\textrm{stop}}} = \frac{1}{\tau_{\textrm{decay}}} +
|
||||
\frac{1}{\tau_{\textrm{capture}}}
|
||||
\end{equation}
|
||||
The mean lifetimes of free muons and muons in a material are well-known,
|
||||
therefore the number of captures can be inferred from the number of stops. For
|
||||
aluminium, $\frac{\Gamma_{\textrm{capture}}}{\Gamma_{\textrm{stop}}} = 0.609$
|
||||
and the mean lifetime of stopped muons is 864
|
||||
ns~\cite{SuzukiMeasday.etal.1987}.
|
||||
|
||||
The core advantages of the \mueconv searches compares to other CLFV searches
|
||||
(\mueg or \mueee) are:
|
||||
\begin{itemize}
|
||||
\item the emitted electron is the only product, so the measurement is simple,
|
||||
no coincidence is required; and
|
||||
\item the electron is mono-energetic, its energy is far above
|
||||
the endpoint of the Michel spectrum (52.8 MeV) where the background is very
|
||||
clean. Essentially, the only intrinsic physics background comes from decay
|
||||
of the muon orbiting the nucleus.
|
||||
\end{itemize}
|
||||
% subsection measurement_of_mueconv (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
|
||||
%\hl{TODO}
|
||||
%\subsection{Signal and backgrounds of \mueconv experiments}
|
||||
%\label{sub:signal_and_backgrounds_of_mueconv_experiments}
|
||||
|
||||
|
||||
|
||||
% subsection signal_and_backgrounds_of_mueconv_experiments (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
% section phenomenoly_of_muec (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
726
thesis2/chapters/chap3_comet.tex
Normal file
@@ -0,0 +1,726 @@
|
||||
\chapter{The COMET experiment}
|
||||
\label{cha:comet_overview}
|
||||
\thispagestyle{empty}
|
||||
|
||||
This chapter describes the new experimental search for \mueconv, namely COMET -
|
||||
(\textbf{CO}herent \textbf{M}uon to \textbf{E}lectron \textbf{T}ransition). The
|
||||
experiment will be carried out at the Japan Proton Accelerator Research Complex
|
||||
(J-PARC), aims at a sensitivity of \sn{6}{-17} i.e. 10,000 times better than the
|
||||
current best limit.
|
||||
|
||||
%At the Japan Proton Accelerator Research Complex (J-PARC), an experiment to
|
||||
%search for \muec~conversion, which is called
|
||||
%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.
|
||||
|
||||
%\begin{itemize}
|
||||
%\item present status of mueconv experiments
|
||||
%\begin{itemize}
|
||||
%\item SINDRUM-II description, results, short comings
|
||||
%\item new ideas: MECO, Mu2e, COMET
|
||||
%\end{itemize}
|
||||
%\item Concepts of COMET
|
||||
%\begin{itemize}
|
||||
%\item highly intense muon beam
|
||||
%\item pulsed proton beam
|
||||
%\item curved solenoids
|
||||
%\end{itemize}
|
||||
%\item COMET's beam lines and detectors
|
||||
%\begin{itemize}
|
||||
%\item proton beam: energy, time structure, planned operations
|
||||
%\item pion production: yields, target, capture solenoids
|
||||
%\item muon transportation: requirements, field
|
||||
%\item stopping target: material, geometry, field, energy loss
|
||||
%\item electron transportation:
|
||||
%\item detectors: electron tracker and calorimeter
|
||||
%\item DAQ
|
||||
%\end{itemize}
|
||||
%\end{itemize}
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
|
||||
\section{Experimental status of \mueconv searches}
|
||||
\label{sec:experimental_status_of_mueconv_searches}
|
||||
|
||||
\subsection{Experimental history}
|
||||
\label{sub:experimental_history}
|
||||
|
||||
The searches for \mueconv has been ongoing for more than 50 years, started in
|
||||
1952 with cosmic rays~\cite{LagarriguePeyrou.1952} and then moved to
|
||||
accelerators. The list in the Table~\ref{tab:mueconv_history} is reproduced
|
||||
from a recent review of Bernstein and Cooper~\cite{BernsteinCooper.2013}.
|
||||
\begin{table}[htb]
|
||||
\begin{center}
|
||||
\begin{tabular}{l l l c}
|
||||
\toprule
|
||||
\textbf{Year} & \textbf{Limit} (90\% C.L.) & \textbf{Material}
|
||||
& \textbf{Reference}\\
|
||||
\midrule
|
||||
1952 & \sn{1.0}{-1} & Sn, Sb & \cite{LagarriguePeyrou.1952} \\
|
||||
1955 & \sn{5.0}{-4} & Cu & \cite{SteinbergerWolfe.1955} \\
|
||||
1961 & \sn{4.0}{-6} & Cu & \cite{SardCrowe.etal.1961}\\
|
||||
1961 & \sn{5.9}{-6} & Cu & \cite{ConversiLella.etal.1961}\\
|
||||
1962 & \sn{2.2}{-7} & Cu & \cite{ConfortoConversi.etal.1962}\\
|
||||
1964 & \sn{2.2}{-7} & Cu & \cite{ConversiLella.etal.1961}\\
|
||||
1972 & \sn{2.6}{-8} & Cu & \cite{ConversiLella.etal.1961}\\
|
||||
1977 & \sn{4.0}{-10} & S & \cite{ConversiLella.etal.1961}\\
|
||||
1982 & \sn{7.0}{-11} & S & \cite{ConversiLella.etal.1961}\\
|
||||
1988 & \sn{4.6}{-12} & Ti & \cite{ConversiLella.etal.1961}\\
|
||||
1993 & \sn{4.3}{-12} & Ti & \cite{ConversiLella.etal.1961}\\
|
||||
1995 & \sn{6.5}{-13} & Ti & \cite{ConversiLella.etal.1961}\\
|
||||
1996 & \sn{4.6}{-11} & Pb & \cite{ConversiLella.etal.1961}\\
|
||||
2006 & \sn{7.0}{-13} & Au & \cite{ConversiLella.etal.1961}\\
|
||||
\bottomrule
|
||||
\end{tabular}
|
||||
\end{center}
|
||||
\caption{History of \mueconv experiments, reproduced
|
||||
from~\cite{BernsteinCooper.2013}}
|
||||
\label{tab:mueconv_history}
|
||||
\end{table}
|
||||
|
||||
The most recent experiments were the SINDRUM and SINDRUM-II at the Paul
|
||||
Scherrer Institute (PSI), Switzerland. The SINDRUM-II measured the branching
|
||||
ratio of \mueconv on a series of heavy targets: Ti, Pb and Au. The proton beam
|
||||
at PSI is a continuous wave beam, with a time structure of 0.3 ns bursts every
|
||||
19.75 \nano\second. An 8-\milli\meter-thick CH$_2$ degrader was used to reduce
|
||||
the radiative pion capture and other prompt backgrounds. Cosmic backgrounds are
|
||||
rejected using a combination of
|
||||
passive shielding, veto counters and reconstruction cuts. The momenta of muons
|
||||
were 52 \mega\electronvolt\per\cc and 53 \mega\electronvolt\per\cc, and the
|
||||
momentum spread was 2\%.
|
||||
\begin{figure}[htbp] \centering
|
||||
\includegraphics[width=0.85\textwidth]{figs/sindrumII_setup}
|
||||
\caption{SINDRUM-II set up}
|
||||
\label{fig:sindrumII_setup}
|
||||
\end{figure}
|
||||
|
||||
Electrons emitted from the target were tracked in a 0.33 T solenoid field.
|
||||
Detector system consisted of a superconducting solenoid, two plastic
|
||||
scintillation hodoscopes, a plexiglass Cerenkov hodoscope, and two drift
|
||||
chambers. In the latest measurement, the SINDRUM-II collaboration have not
|
||||
found any conversion electron from captured muons in a gold target, hence set
|
||||
the upper limit for
|
||||
the branching ratio of \mueconv in gold with 90 \% C.L. at \sn{7.0}{-13}.
|
||||
|
||||
The reconstructed momenta of electrons around the signal region from SINDRUM-II
|
||||
is shown in the Figure~\ref{fig:sindrumII_result}. It can be seen that the muon
|
||||
decay in orbit background falls steeply near the endpoint as expected, but, the
|
||||
prompt background induced by pions still remains even after the cut in timing
|
||||
and track angle. This indicates the problem of pion contamination is very
|
||||
important in probing lower sensitivity.
|
||||
\begin{figure}[htbp]
|
||||
\centering
|
||||
\includegraphics[width=0.55\textwidth]{figs/sindrumII_Au_result}
|
||||
\caption{SINDRUM-II result}
|
||||
\label{fig:sindrumII_result}
|
||||
\end{figure}
|
||||
% subsection experimental_history (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
\subsection{New generation of \mueconv~experiments}
|
||||
\label{sub:new_generation_of_mueconv_experiments}
|
||||
|
||||
A new generation of \mueconv experiments have been proposed with scenarios to
|
||||
overcome pion induced background in the SINDRUM-II. Lobashev and collaborators
|
||||
first suggested the basic idea for new \mueconv at the Moscow Muon Factory;
|
||||
this idea was used to develop the MECO experiment at Brookhaven National
|
||||
Laboratory. The MECO experiment was cancelled due to budget constraints. The two
|
||||
modern experiments, COMET at J-PARC and Mu2e at Fermilab use the initial idea
|
||||
with more upgrades and modifications.
|
||||
|
||||
The basic ideas of the modern experiments are:
|
||||
\begin{enumerate}
|
||||
\item Highly intense muon source: the total number of muons needed is of the
|
||||
order of $10^{18}$ in order to achieve a sensitivity of $10^{-16}$. This
|
||||
can be done by producing more pions using a high power proton beam, and
|
||||
having a high efficiency pion collection system;
|
||||
\item Pulsed proton beam with an appropriate timing: the proton pulse should
|
||||
be short compares to the lifetime of muons in the stopping target material,
|
||||
and the period between pulses should be long enough for prompt backgrounds
|
||||
from pion to decay before beginning the measurement. It is also crucial
|
||||
that there is no proton leaks into the measuring interval;
|
||||
\item Curved solenoids for charge and momentum selection: at first, the curved
|
||||
solenoids remove the line of sight backgrounds. A charged particle travels
|
||||
through a curved solenoidal field will have the centre of the helical
|
||||
motion drifted up or down depends on the sign of the charge, and the
|
||||
magnitude of the drift is proportional to its momentum. By using this
|
||||
effect and placing suitable collimators, charge and momentum selection can
|
||||
be made.
|
||||
\end{enumerate}
|
||||
% subsection new_generation_of_mueconv_experiments (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
|
||||
% section experimental_status_of_mueconv_searches (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
|
||||
\section{Concepts of the COMET experiment}
|
||||
\label{sec:concepts_of_the_comet_experiment}
|
||||
This section elaborates the design choices of the COMET to realise the basic
|
||||
ideas mentioned above. Figures and numbers, other than noted, are taken from
|
||||
the COMET's documentations:
|
||||
\begin{itemize}
|
||||
%TODO citations
|
||||
\item Conceptual design report for the COMET experiment~\cite{COMET.2009}
|
||||
\item Proposal Phase-I 2012
|
||||
\item TDR 2014
|
||||
\end{itemize}
|
||||
|
||||
|
||||
\subsection{Proton beam}
|
||||
\label{sub:proton_beam}
|
||||
A high power pulsed proton beam is of utmost importance to achieve the desired
|
||||
sensitivity of the COMET experiment. A slow-extracted proton beam from
|
||||
the J-PARC main ring (MR), which is designed to deliver \sn{3.6}{15} protons per
|
||||
cycle at a frequency of 0.45 Hz, will be used for the COMET experiment. The
|
||||
proton beam power of the current design is 8 GeV$\times$7 $\mu$A, or
|
||||
\sn{4.4}{13} protons/s. The beam energy 8 \giga\electronvolt~ helps to minimise
|
||||
the production of antiprotons.
|
||||
|
||||
The proton pulse width is chosen to be 100 ns, and the pulse period to be
|
||||
$1 \sim 2 \textrm{ }\mu\textrm{s}$. This time structure is sufficient for the
|
||||
search for \mueconv in an aluminium target where the lifetime of muons is 864
|
||||
ns. A plan of accelerator operation to realise the scheme is shown in
|
||||
the Figure~\ref{fig:comet_mr_4filled}, where 4 out of 9 MR buckets are filled.
|
||||
|
||||
As mentioned, it is very important that there is no stray proton arrives in the
|
||||
measuring period between two proton bunches. An extinction factor is defined as
|
||||
the ratio between number of protons in between two pulses and the number of
|
||||
protons in the main pulse. In order to achieve the goal sensitivity of the
|
||||
COMET, an extinction factor of \sn{}{-9} is required.
|
||||
|
||||
Requirements for the proton beam are summarised in the
|
||||
Table~\ref{tab:comet_proton_beam}.
|
||||
|
||||
\begin{figure}[htb]
|
||||
\centering
|
||||
\includegraphics[width=0.8\textwidth]{figs/comet_mr_4filled}
|
||||
\caption{The COMET proton bunch structure in the RCS (rapid cycle
|
||||
synchrotron) and MR where 4 buckets
|
||||
are filled producing 100 \nano\second~bunches separated by 1.2
|
||||
\micro\second.}
|
||||
\label{fig:comet_mr_4filled}
|
||||
\end{figure}
|
||||
|
||||
\begin{table}[htb]
|
||||
\begin{center}
|
||||
\begin{tabular}{l l}
|
||||
\toprule
|
||||
Beam power & 56 \kilo\watt\\
|
||||
Energy & 8 \giga\electronvolt\\
|
||||
Average current & 7 \micro\ampere\\
|
||||
Beam emittance & 10 $\pi$\cdot mm\cdot mrad\\
|
||||
Protons per bunch & $<10^{11}$\\
|
||||
Extinction & \sn{}{-9}\\
|
||||
Bunch separation & $1 \sim 2$ \micro\second\\
|
||||
Bunch length & 100 \nano\second\\
|
||||
\bottomrule
|
||||
\end{tabular}
|
||||
\end{center}
|
||||
\caption{Pulsed proton beam for the COMET experiment}
|
||||
\label{tab:comet_proton_beam}
|
||||
\end{table}
|
||||
|
||||
% subsection proton_beam (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
\subsection{Pion production and capture solenoid}
|
||||
\label{sub:pion_production_can_capture_solenoid}
|
||||
Muons for the COMET experiment are produced by colliding the proton beam with
|
||||
a pion production target, made of either platinum, gold or tungsten, collecting
|
||||
pions and then letting them decay. To collect as many pions (and cloud muons)
|
||||
as possible, the pions are captured
|
||||
using a high solenoidal magnetic field with a large solid angle. Since muons
|
||||
will be stopped in a conversion target, low energy muons, and thus low energy
|
||||
pions, are preferred. It is known from other measurements that backward
|
||||
scattered pions (with respect to proton beam direction) of high energy are
|
||||
suppressed, and the yield of low energy pions in the backward direction is not
|
||||
too low compares to that of the forward direction (see
|
||||
Figure~\ref{fig:pion_yield}). For these reasons, the COMET
|
||||
decided to collect backward pions.
|
||||
\begin{figure}[htb]
|
||||
\centering
|
||||
\includegraphics[width=0.95\textwidth]{figs/pion_yield}
|
||||
\caption{Comparison between backward and forward pions production in a gold
|
||||
target.}
|
||||
\label{fig:pion_yield}
|
||||
\end{figure}
|
||||
|
||||
The pion capture system is composed of several superconducting solenoids:
|
||||
capture solenoids and matching solenoids. The magnetic field distribution along
|
||||
the beam axis of the COMET is shown in the Figure~\ref{fig:comet_Bfield}. The
|
||||
peak field of 5 T is created by the capture solenoid, and the matching
|
||||
solenoids provide a smooth transition from that peak field to the 3 T field in
|
||||
the pions/muons transportation region. The superconducting solenoids are
|
||||
cooled by liquid helium, and a radiation shield composed of copper and tungsten
|
||||
will be installed inside the cryostat to reduce radiation heat load.
|
||||
\begin{figure}[htb]
|
||||
\centering
|
||||
\includegraphics[width=0.85\textwidth]{figs/comet_Bfield}
|
||||
\caption{Magnetic field distribution along the COMET beam line.}
|
||||
\label{fig:comet_Bfield}
|
||||
\end{figure}
|
||||
|
||||
% subsection pion_production_can_capture_solenoid (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
\subsection{Pions and muons transportation solenoids}
|
||||
\label{sub:pion_and_muon_transportation}
|
||||
Muons and pions are transported to the muon stopping target through a muon
|
||||
beam line, which includes several curved and straight superconducting solenoid
|
||||
magnets. A schematic layout of the muon beam line, include the capture and
|
||||
detector sections, is shown in Figure~\ref{fig:comet_beamline_layout}.
|
||||
\begin{figure}[htb]
|
||||
\centering
|
||||
\includegraphics[width=0.95\textwidth]{figs/comet_beamline_layout}
|
||||
\caption{Schematic layout of the COMET beam line.}
|
||||
\label{fig:comet_beamline_layout}
|
||||
\end{figure}
|
||||
|
||||
The requirements for the muon transportation beam line are:
|
||||
\begin{itemize}
|
||||
\item being long enough for pions to decay, for instance, the survival rate
|
||||
of pions will be about \sn{2}{-3} after 20 m;
|
||||
\item being able to select low momentum negative muons with momentum of around
|
||||
40 MeV/$c$, and eliminate high momentum muons ($> 75\textrm{ MeV/}c$),
|
||||
since they can decay in flight and produce spurious signals of $\sim$ 105
|
||||
MeV electrons.
|
||||
\end{itemize}
|
||||
The selection of charge and momentum is done by the curved solenoids. It is
|
||||
know that, in a curved solenoidal field, the centre of the helical trajectory
|
||||
of a charged particle drifts perpendicularly to the curved plane. The magnitude
|
||||
of the drift is given by:
|
||||
\begin{align}
|
||||
D &= \frac{1}{qB} \frac{s}{R} \frac{p_L^2 + \frac{1}{2}p_T^2}{p_L}\\
|
||||
&= \frac{1}{qB} \frac{s}{R} \frac{p}{2}
|
||||
\left( \textrm{cos}\theta + \frac{1}{\textrm{cos}\theta} \right)\\
|
||||
&= \frac{1}{qB} \theta_{bend} \frac{p}{2}
|
||||
\left( \textrm{cos}\theta + \frac{1}{\textrm{cos}\theta} \right)
|
||||
\end{align}
|
||||
where $q$ is the electric charge of the particle; $B$ is the magnetic field at
|
||||
the axis; $s$ and $R$ are the path length and the radius of the curvature; $p$,
|
||||
$p_T$ and $p_L$ are total momentum, transversal momentum and longitudinal
|
||||
momentum of the particles, respectively; $\theta = \textrm{atan}(p_T/p_L)$ is
|
||||
the pitch angle of the helical trajectory; and $\theta_{bend} = s/R$ is called
|
||||
the bending angle.
|
||||
It is clear that $D$ is proportional to $\theta_{bend}$, to total momentum $p$.
|
||||
Charged particles with opposite signs move in opposite directions. Therefore it
|
||||
is possible to select muons around 40 MeV/$c$ by using suitable collimator
|
||||
after the curved solenoid.
|
||||
|
||||
In order to keep the centre of the helical trajectories of the muons with
|
||||
a reference momentum $p_0$ in the vertical plane, a compensating dipole field
|
||||
parallel to the drift direction is needed. In the COMET, the dipole fields are
|
||||
produced by additional coils winded around the solenoid coils. The magnitude of
|
||||
the compensating field is:
|
||||
\begin{equation}
|
||||
B_{\textrm{comp}} = \frac{1}{qR} \frac{p_0}{2}
|
||||
\left( \textrm{cos}\theta_0 + \frac{1}{\textrm{cos}\theta_0} \right)
|
||||
\end{equation}
|
||||
where the trajectories of charged particles with momentum $p_0$ and pitch angle
|
||||
$\theta_0$ are corrected to be on-axis. An average dipole field of 0.03 T is
|
||||
needed to select 40 MeV/$c$ muons as required by the COMET design.
|
||||
% subsection pion_and_muon_transportation (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
\subsection{Muon stopping target}
|
||||
\label{sub:muon_stopping_target}
|
||||
Muon stopping target is place at 180\degree~bending after the pion production
|
||||
target (Figure~\ref{fig:comet_beamline_layout}) in its own solenoid. The target
|
||||
is designed to maximise the muon stopping efficiency and minimise the energy
|
||||
loss of signal electrons.
|
||||
|
||||
%\hl{TODO: Target choice: separation, product, lifetime, energy loss\ldots}
|
||||
It is calculated that the branching ratio of \mueconv increases with atomic
|
||||
number $Z$, and plateaus above $Z \simeq 30$, then decreases as $Z>60$. The
|
||||
lifetime of muons inside a material decreases quickly as $Z$ increases.
|
||||
Tracking wise, lower $Z$ material provides better reconstructed momentum
|
||||
resolution. Therefore, light material is preferable as muon stopping target.
|
||||
|
||||
The first choice for the muon stopping target material in the COMET is
|
||||
aluminium. A titanium target is also considered. Configuration of the target is
|
||||
shown in the Table~\ref{tab:comet_al_target}. Monte Carlo studies with this
|
||||
design showed that net stopping efficiency is 0.29, and average energy loss
|
||||
of signal electrons is about 400 \kilo\electronvolt.
|
||||
\begin{table}[htb]
|
||||
\begin{center}
|
||||
\begin{tabular}{l l}
|
||||
\toprule
|
||||
\textbf{Item} & \textbf{Specification}\\
|
||||
\midrule
|
||||
Material & Aluminium\\
|
||||
Shape & Flat disks\\
|
||||
Disk radius & 100 \milli\meter\\
|
||||
Disk thickness & 200 \micro\meter\\
|
||||
Number of disks & 17\\
|
||||
Disk spacing & 50 \milli\meter\\
|
||||
\bottomrule
|
||||
\end{tabular}
|
||||
\end{center}
|
||||
\caption{Configuration of the muon stopping target.}
|
||||
\label{tab:comet_al_target}
|
||||
\end{table}
|
||||
|
||||
A graded magnetic field (reduces from 3 T to 1 T) is produced at the
|
||||
location of the stopping target (see Figure~\ref{fig:comet_target_Bfield}) to
|
||||
maximise the acceptance for \mueconv signals, since electrons emitted in the
|
||||
backward
|
||||
direction would be reflected due to magnetic mirroring. The graded field also
|
||||
helps optimising the transmission efficiency to the subsequent electron
|
||||
transport section.
|
||||
\begin{figure}[htb]
|
||||
\centering
|
||||
\includegraphics[width=0.85\textwidth]{figs/comet_target_Bfield}
|
||||
\caption{The graded magnetic field near the stopping target region.}
|
||||
\label{fig:comet_target_Bfield}
|
||||
\end{figure}
|
||||
|
||||
% subsection muon_stopping_target (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
\subsection{Electron transportation beam line}
|
||||
\label{sub:electron_transportation_beam_line}
|
||||
The 180\degree~bending electron transport solenoids help remove line-of-sight
|
||||
between the target and the detector system. It works similarly to the muon
|
||||
transportation section, but is tuned differently to accept electrons of about
|
||||
105 \mega\electronvolt\per\cc. A compensation field of 0.17 T along the
|
||||
vertical direction will be applied. Electrons with momentum less than 80
|
||||
\mega\electronvolt\per\cc are blocked at the exit of this section by
|
||||
a collimator to reduce DIO electrons rate. The net acceptance of signals of
|
||||
\mueconv is about 0.32, and the detector hit rate will be in the order of
|
||||
1 \kilo\hertz~for \sn{}{11} stopped muons\per\second.
|
||||
% subsection electron_transportation_beam_line (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
\subsection{Electron detectors}
|
||||
\label{sub:electron_detectors}
|
||||
The \mueconv signal electrons is measured by an electron detector system, which
|
||||
consists of straw-tube trackers and an electromagnetic calorimeter - shown in
|
||||
Figure~\ref{fig:comet_detector_system}. The
|
||||
requirements for the detector system is to distinguish electrons from other
|
||||
particles, and measure their momenta, energy and timings. The whole detector
|
||||
system is in a uniform solenoidal magnetic field under vacuum. Passive and
|
||||
active shielding against cosmic rays is considered.
|
||||
|
||||
The tracking detector has to provide a momentum resolution less than 350
|
||||
\kilo\electronvolt\per\cc in order to achieve a sensitivity of \sn{3}{-17}.
|
||||
There are five stations of straw-tube gas chambers, each provides two
|
||||
dimensional information. Each straw tube is 5 \milli\meter in diameter and has
|
||||
a 25 \micro\meter-thick wall. According to a GEANT4 Monte Carlo simulation,
|
||||
a position resolution of 250 \micro\meter can be obtained, which is enough for
|
||||
350 \kilo\electronvolt\per\cc momentum resolution. The DIO background of 0.15
|
||||
events is estimated.
|
||||
|
||||
The electromagnetic calorimeter serves three purposes: a) to measure electrons
|
||||
energy with high energy resolution; b) to provide timing information and
|
||||
trigger timing for the detector system; and c) to provide additional data on
|
||||
hit positions. Two candidate crystals, GSO and LYSO, are under consideration.
|
||||
\begin{figure}[htb]
|
||||
\centering
|
||||
\includegraphics[width=0.75\textwidth]{figs/comet_detector_system}
|
||||
\caption{Layout of the electron detectors.}
|
||||
\label{fig:comet_detector_system}
|
||||
\end{figure}
|
||||
|
||||
The requirements for \mueconv signals are:
|
||||
\begin{itemize}
|
||||
\item from the 350 \kilo\electronvolt\per\cc~momentum resolution, the signal
|
||||
region is determined to be 103.5 \mega\electronvolt\per\cc~to 105.2
|
||||
\mega\electronvolt\per\cc;
|
||||
\item transversal momentum of signal electrons is required to be greater than
|
||||
52 \mega\electronvolt\per\cc to remove backgrounds from beam electrons and
|
||||
muons decay in flight;
|
||||
\item timing wise, conversion electrons should arrive in the time window of
|
||||
detection which is about 700 \nano\second~after each proton pulses
|
||||
(Figure~\ref{fig:comet_meas_timing}). The acceptance in this detection
|
||||
window is about 0.39 for aluminium.
|
||||
\end{itemize}
|
||||
|
||||
\begin{figure}[htb]
|
||||
\centering
|
||||
\includegraphics[width=0.7\textwidth]{figs/comet_meas_timing}
|
||||
\caption{Timing window of detection.}
|
||||
\label{fig:comet_meas_timing}
|
||||
\end{figure}
|
||||
% subsection electron_detectors (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
\subsection{Signal sensitivity and background estimation}
|
||||
\label{sub:signal_sensitivity_and_background_estimation}
|
||||
The single event sensitivity (SES) of the \mueconv search is defined as:
|
||||
\begin{equation}
|
||||
\mathcal{B}(\mu^-Al\rightarrow e^- Al) =
|
||||
\frac{1}{N^{\textrm{stop}}_{\mu}\cdot f_{\textrm{cap}} \cdot A_e}
|
||||
\label{eq:mue_sensitivity}
|
||||
\end{equation}
|
||||
where $N^{\textrm{stop}}_{\mu}$ is the number of muons stopping in the muon
|
||||
target; $f_{\textrm{cap}}$ is the fraction of captured muons; and $A_e$ is the
|
||||
detector acceptance. The total number of stopped muons is projected as
|
||||
$N^{\textrm{stop}}_{\mu} = 2\times 10^{18}$ for a \sn{2}{7}\second~run time;
|
||||
$f_{\textrm{cap}} = 0.61$ for aluminium; and the total acceptance for the COMET
|
||||
detector system is $A_e =0.031$. Using these
|
||||
numbers, the SES of the COMET is calculated to be
|
||||
\sn{2.6}{-17}. The 90\% CL upper limit is given by $2.3\times\mathcal{B}$:
|
||||
\begin{equation}
|
||||
\mathcal{B}(\mu^-Al\rightarrow e^- Al) < 6 \times 10^{-17} \quad
|
||||
\textrm{(90\% C.L.)}
|
||||
\end{equation}
|
||||
|
||||
Potential backgrounds for the COMET are:
|
||||
\begin{enumerate}
|
||||
\item Intrinsic physics backgrounds: originates from muons stopped in the
|
||||
stopping target, including muon decays in orbit, radiative muon capture and
|
||||
particles such as protons and neutrons emitted after muon capture;
|
||||
\item Beam related backgrounds: caused by particles (electrons, pions, muons
|
||||
and antiprotons) in the beam. They are either prompt or late-arriving.
|
||||
A beam pulsing with high proton extinction factor is required to reject
|
||||
this type of backgrounds;
|
||||
\item Accidental background from cosmic rays
|
||||
\end{enumerate}
|
||||
The expected background rates for the COMET at an SES of
|
||||
\sn{3}{-17} is summarised in Table~\ref{tab:comet_background_estimation}.
|
||||
\begin{table}[htb]
|
||||
\begin{center}
|
||||
%\begin{tabular}{l l}
|
||||
\begin{tabular}{l r@{.}l}
|
||||
\toprule
|
||||
\textbf{Background} & \multicolumn{2}{l}{\textbf{Events}}\\
|
||||
\midrule
|
||||
%\end{tabular}{l l}
|
||||
%\begin{tabular}{l r@{.}l}
|
||||
Radiative pion capture & 0&05\\
|
||||
Beam electrons & $<$0&1\\
|
||||
Muon decay in flight & $<$0&0002\\
|
||||
Pion decay in flight & $<$0&0001\\
|
||||
Neutron induced & 0&024\\
|
||||
Delayed pion radiative capture & 0&002\\
|
||||
Antiproton induced & 0&007\\
|
||||
Muon decay in orbit & 0&15\\
|
||||
Radiative muon capture & $<$0&001\\
|
||||
Muon capture with neutron emission & $<$0&001\\
|
||||
Muon capture with proton emission & $<$0&001\\
|
||||
Cosmic ray muons & 0&002\\
|
||||
Electron cosmic ray muons & 0&002\\
|
||||
\midrule
|
||||
\textbf{Total} &0&34\\
|
||||
\bottomrule
|
||||
\end{tabular}
|
||||
\end{center}
|
||||
\caption{Backgrounds of the COMET experiment.}
|
||||
\label{tab:comet_background_estimation}
|
||||
\end{table}
|
||||
|
||||
% subsection signal_sensitivity_and_background_estimation (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
% section concepts_of_the_comet_experiment (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
|
||||
\section{The COMET Phase-I}
|
||||
\label{sec:the_comet_phase_i}
|
||||
The techniques of beam pulsing and curved solenoids that the COMET will utilise
|
||||
are believed to greatly reduce potential backgrounds, by several orders of
|
||||
magnitude, for the \mueconv search. That also means that backgrounds are being
|
||||
extrapolated over four orders of magnitude from existing data. In order to
|
||||
obtain data-driven estimates of backgrounds, and inform the detailed design for
|
||||
the ultimate COMET experiment, and initial phase is desirable. Also, the 5-year
|
||||
mid-term plan from 2013 of J-PARC includes the construction of the COMET beam
|
||||
line. For these reasons, the COMET collaboration considers a staged approach
|
||||
with the first stage, so called COMET Phase-I, with a shorter muon
|
||||
transportation solenoid, up to the first 90\degree.
|
||||
|
||||
%\begin{wrapfigure}{r}{0.5\textwidth}
|
||||
%\centering
|
||||
%\includegraphics[width=0.49\textwidth]{figs/comet_phase1_layout}
|
||||
%\caption{Lay out of the COMET Phase-I, the target and detector solenoid are
|
||||
%placed after the first 90\degree~bend.}
|
||||
%\label{fig:comet_phase1_layout}
|
||||
%\end{wrapfigure}
|
||||
\begin{SCfigure}
|
||||
\centering
|
||||
\caption{Lay out of the COMET Phase-I, the target and detector solenoid are
|
||||
placed after the first 90\degree~bend.}
|
||||
\includegraphics[width=0.4\textwidth]{figs/comet_phase1_layout}
|
||||
\label{fig:comet_phase1_layout}
|
||||
\end{SCfigure}
|
||||
|
||||
The COMET Phase-I has two major goals:
|
||||
\begin{enumerate}
|
||||
\item Direct measurements of the proton extinction factor, and other potential
|
||||
backgrounds for the full COMET experiment. These include backgrounds due to
|
||||
beam particles such as pions, neutrons, antiprotons, photons and electrons;
|
||||
and physics background from muon DIO. Straw tube trackers and crystal
|
||||
calorimeter with the same technology in the full COMET will be used, thus
|
||||
these detectors can be regarded as the final prototype.
|
||||
\item Search for \mueconv with an intermediate sensitivity of \sn{3.1}{-15},
|
||||
a two orders of magnitude improvement from the SINDRUM-II limit. To realise
|
||||
this goal, two options for detectors are being considered, either a reused
|
||||
of the detectors for background measurements, or a dedicated detector.
|
||||
The latter will be described in detail later.
|
||||
\end{enumerate}
|
||||
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
\subsection{Proton beam for the COMET Phase-I}
|
||||
\label{sub:proton_beam_for_the_comet_phase_i}
|
||||
Proton beam for the Phase-I differs only in beam power compares to that of the
|
||||
full COMET. It is estimated that a beam power of
|
||||
3.2~\kilo\watt~$=$~8~\giga\electronvolt~$\times$~0.4~\micro\ampere~(or
|
||||
\sn{2.5}{12} protons\per\second) will be enough for beam properties
|
||||
study and achieving the physics goal of this stage.
|
||||
Starting from a lower intensity is also suitable for performing accelerator
|
||||
studies that are needed to realise 8 \giga\electronvolt beam extraction from
|
||||
the J-PARC main ring.
|
||||
% subsection proton_beam_for_the_comet_phase_i (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
\subsection{Pion production and transportation solenoids}
|
||||
\label{sub:pion_production_and_transportation_solenoids}
|
||||
|
||||
Since the beam power will be lower, it is proposed to use a graphite target in
|
||||
the Phase-I. This will minimise the activation of the target station and heat
|
||||
shield which will be easier for necessary upgrading for Phase-II operation.
|
||||
A target length of 600 \milli\meter~(1.5 radiation length) and target radius of
|
||||
20 \milli\meter~are chosen. The target is located at the centre of the pion
|
||||
capture solenoid where the peak magnetic field of 5 T is achieved.
|
||||
A correction dipole filed of 0.05 T is also applied to improve the pion yield.
|
||||
|
||||
The pion/muon beam line for COMET Phase-I consists of the pion capture solenoid
|
||||
section (CS), muon transport solenoid section (TS) up to the first
|
||||
90\degree~bending, and a set of matching solenoids (see
|
||||
Figure~\ref{fig:comet_phase1_magnets}). At the end of the muon beam line, the
|
||||
detectors and the detector solenoid (DS) are installed. To reduce beam
|
||||
backgrounds, a beam collimator is placed upstream of the detector solenoid.
|
||||
\begin{figure}[htb]
|
||||
\centering
|
||||
\includegraphics[width=0.85\textwidth]{figs/comet_phase1_magnets}
|
||||
\caption{A schematic view of the superconducting solenoid magnet system for
|
||||
the COMET Phase-I. Prefix CS is for capture solenoids, MS is for matching
|
||||
solenoids, and TS is for transport solenoids. BS and DS are beam collimation
|
||||
system and detector solenoid, respectively.}
|
||||
\label{fig:comet_phase1_magnets}
|
||||
\end{figure}
|
||||
% subsection pion_production_and_transportation_solenoids (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
\subsection{Detectors for \mueconv search in the Phase-I}
|
||||
\label{sub:detectors_for_mueconv_search_in_the_phase_i}
|
||||
|
||||
As mentioned, two types of detectors are considered for physics measurements in
|
||||
the Phase-I. The dedicated detector system consists of a cylindrical drift
|
||||
chamber (CDC), a trigger hodoscope, a proton absorber and a detector solenoid
|
||||
(Figure~\ref{fig:comet_phase1_cydet}).
|
||||
The whole system is referred as cylindrical detector system (CyDet) in the
|
||||
COMET's documentation. The CyDet has advantages that low momentum particles for
|
||||
the stopping target will not reach the detector, thus the hit rates are kept
|
||||
manageable even at high beam currents. Furthermore, the majority of beam
|
||||
particles, except those scattering at large angles, will not directly hit the
|
||||
CyDet.
|
||||
|
||||
\begin{figure}[htb]
|
||||
\centering
|
||||
\includegraphics[width=0.85\textwidth]{figs/comet_phase1_cydet}
|
||||
\caption{Schematic layout of the CyDet.}
|
||||
\label{fig:comet_phase1_cydet}
|
||||
\end{figure}
|
||||
|
||||
The CDC is the main tracking detector that provides information for
|
||||
reconstruction of charged particle tracks and measuring their momenta. The key
|
||||
parameters for the CDC are listed in the
|
||||
Table~\ref{tab:comet_phase1_cdc_params}.
|
||||
Trigger hodoscopes are placed at both upstream and downstream ends of the CDC.
|
||||
An absorber is placed concentrically with respect to the CDC axis to
|
||||
reduce potential high rates caused by protons emitted after nuclear muon
|
||||
capture in the stopping target.
|
||||
|
||||
The CDC covers the region
|
||||
from 500 \milli\meter~to 831 \milli\meter~in the radial direction. The length
|
||||
of the CDC is 1500 \milli\meter. The inner wall is made of a 100
|
||||
\micro\meter~thick aluminised Mylar. The end-plates will be conical in shape
|
||||
and about 10 \milli\meter~thick to support the feedthroughs. The outer wall is
|
||||
made of 5 \milli\meter~carbon fibre reinforced plastic (CFRP).
|
||||
|
||||
The CDC is arranged in 20 concentric sense layers with alternating positive and
|
||||
negative stereo angles. The sense wires are made of gold-plated tungsten, 30
|
||||
\micro\meter~in diameter, tensioned to 50 \gram. The field wires are uncoated
|
||||
aluminium wires with a diameter of 80 \micro\meter, at the same tension of 50
|
||||
\gram. A high voltage of $1700\sim1900$ \volt~will be applied to the sense
|
||||
wires with the field wires at ground potential, giving an avalanche gain of
|
||||
approximately \sn{4}{4}. A gas mixture of helium:isobutane(90:10) is preferred
|
||||
since the CDC momentum resolution is dominated by multiple scattering. With
|
||||
these configurations, an intrinsic momentum resolution of 197
|
||||
\kilo\electronvolt\per\cc~is achievable according to our tracking study.
|
||||
|
||||
\begin{table}[htb]
|
||||
\begin{center}
|
||||
\begin{tabular}{l l l}
|
||||
\toprule
|
||||
\textbf{Inner wall} & Length & 1500 \milli\meter\\
|
||||
& Radius & 500 \milli\meter\\
|
||||
\midrule
|
||||
\textbf{Outer wall} & Length & 1740.9 \milli\meter\\
|
||||
& Radius & 831 \milli\meter\\
|
||||
\midrule
|
||||
\textbf{Sense wire} & Number of layers & 20\\
|
||||
& Material & Gold-plated tungsten\\
|
||||
& Diameter & 30 \micro\meter\\
|
||||
& Number of wires & 4986\\
|
||||
& Tension & 50 \gram\\
|
||||
%& Radius of the innermost wire at the EP & 530 mm\\
|
||||
%& Radius of the outermost wire at the EP & 802 mm\\
|
||||
\midrule
|
||||
\textbf{Field wire} & Material & Aluminium\\
|
||||
& Diameter & 80 \micro\meter\\
|
||||
& Number of wires & 14562\\
|
||||
& Tension & 50 \gram\\
|
||||
\midrule
|
||||
\textbf{Gas} & & Helium:Isobutane (90:10)\\
|
||||
\bottomrule
|
||||
\end{tabular}
|
||||
\end{center}
|
||||
\caption{Main parameters of the CDC for the COMET Phase-I.}
|
||||
\label{tab:comet_phase1_cdc_params}
|
||||
\end{table}
|
||||
|
||||
The maximum usable muon beam intensity will be limited by the detector hit
|
||||
occupancy. Charge particles with transversal momentum greater than 70
|
||||
\mega\electronvolt\per\cc~are expected to reach the CDC. Those particles are:
|
||||
protons emitted from nuclear muon capture, and electrons from muon decay in
|
||||
orbit. It is calculated that the hit rate due to proton emission dominates,
|
||||
where the highest rate is 11 \kilo\hertz\per cell compares to 5 \kilo\hertz\per
|
||||
cell contribution from DIO electrons. Another potential issue caused by protons
|
||||
is the ageing effect on the CDC as they leave about a 100 times larger
|
||||
energy deposit than the minimum ionisation particles.
|
||||
|
||||
For those reasons, we plan to install an absorber to reduce the rate of protons
|
||||
reaching the CDC. However, there is no experimental data available for the rate
|
||||
of protons emitted after muon capture in aluminium. In the design of the COMET
|
||||
Phase-I, we use a conservative estimation of the rate of protons from energy
|
||||
spectrum of charged particles emitted from muon capture in
|
||||
$^{28}$Si~\cite{SobottkaWills.1968}. The baseline design for the proton
|
||||
absorber is 1.0 \milli\meter~thick CFRP, which contributes 195
|
||||
\kilo\electronvolt\per\cc~to the momentum resolution of reconstructed track.
|
||||
|
||||
In order to obtain a better understanding of the protons emission, and then
|
||||
further optimisation of the CDC, a dedicated experiment to measure proton
|
||||
emission rate and energy spectrum is being carried out at PSI. This experiment
|
||||
is described in detail in next chapters.
|
||||
% subsection detectors_for_mueconv_search_in_the_phase_i (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
\subsection{Sensitivity of the \mueconv search in the Phase-I}
|
||||
\label{sub:sensitivity_of_the_mueconv_search_in_the_phase_i}
|
||||
The SES for the Phase-I is given by
|
||||
the Equation~\ref{eq:mue_sensitivity}. Using $N_{\mu} = 1.3\times 10^{16}$,
|
||||
$f_{\textrm{cap}} = 0.61$, and $A_e = 0.043$ from MC study for the Phase-I, the
|
||||
SES becomes:
|
||||
\begin{equation}
|
||||
\mathcal{B}(\mu^-Al\rightarrow e^- Al) = 3.1\times 10^{-15}
|
||||
\end{equation}
|
||||
% subsection sensitivity_of_the_mueconv_search_in_the_phase_i (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
\subsection{Time line of the COMET Phase-I and Phase-II}
|
||||
\label{sub:time_line_of_the_phase_i}
|
||||
We are now in the construction stage of the COMET Phase-I, which is planned to
|
||||
be finished by the end of 2016. We will carry out engineering run in 2016,
|
||||
and subsequently, physics run in 2017. A beam time of 90 days is expected to
|
||||
achieve the goal sensitivity of the Phase-I. An anticipated schedule for the
|
||||
COMET, both Phase-I and Phase-II, is shown in Figure~\ref{fig:sched}.
|
||||
\begin{figure}[tbh]
|
||||
\centering
|
||||
\includegraphics[width=0.8\textwidth]{figs/sched}
|
||||
\caption{The anticipated schedule of the COMET experiment.}
|
||||
\label{fig:sched}
|
||||
\end{figure}
|
||||
|
||||
% subsection time_line_of_the_phase_i (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
% section the_comet_phase_i (end)
|
||||
901
thesis2/chapters/chap4_alcap_phys.tex
Normal file
@@ -0,0 +1,901 @@
|
||||
\chapter
|
||||
[Proton emission following nuclear muon capture - The AlCap experiment]
|
||||
{Proton emission following \\nuclear muon capture \\and the AlCap experiment}
|
||||
\label{cha:alcap_phys}
|
||||
\thispagestyle{empty}
|
||||
As mentioned earlier, the emission rate of protons
|
||||
following nuclear muon capture on aluminium is of interest to the COMET Phase-I
|
||||
since protons can cause a very high hit rate on the proposed cylindrical drift
|
||||
chamber. Another \mueconv experiment, namely Mu2e at Fermilab, which aims at
|
||||
a similar goal sensitivity as that of the COMET, also shares the same interest
|
||||
on proton emission. Therefore, a joint COMET-Mu2e project was formed to carry
|
||||
out the measurement of proton, and other charged particles, emission. The
|
||||
experiment, so-called AlCap, has been proposed and approved to be carried out
|
||||
at PSI in 2013~\cite{AlCap.2013}. In addition to proton, the AlCap
|
||||
experiment will also measure:
|
||||
\begin{itemize}
|
||||
\item neutrons, because they can cause backgrounds on other detectors and
|
||||
damage the front-end electronics; and
|
||||
\item photons, since they provide ways to normalise number of stopped muons
|
||||
in the stopping target.
|
||||
\end{itemize}
|
||||
|
||||
The emission of particles following muon capture in nuclei
|
||||
%Historically, the emission of protons, as well as other particles, has
|
||||
has been studied thoroughly for several nuclei in the context of ``intermediate
|
||||
energy nuclear physics'' where it is postulated that the weak interaction is
|
||||
well understood and muons are used as an additional probe to investigate the
|
||||
nuclear structure~\cite{Singer.1974, Measday.2001}.
|
||||
Unfortunately, the proton emission rate for aluminium in the energy range of
|
||||
interest is not available. This chapter reviews the current knowledge on
|
||||
emission of particles with emphasis on proton.
|
||||
%theoretically and experimentally, hence serves as the motivation for the AlCap
|
||||
%experiment.
|
||||
|
||||
\begin{comment}
|
||||
\begin{itemize}
|
||||
%\item Motivation: why looked for protons in COMET, what is the status in
|
||||
%theory and experiment
|
||||
%\begin{itemize}
|
||||
%\item COMET Phase-I need
|
||||
%\item lack of experimental data
|
||||
%\item addition to protons: neutrons and photons
|
||||
%\end{itemize}
|
||||
\item Atomic capture of muon
|
||||
\begin{itemize}
|
||||
\item formation of the muonic atom
|
||||
\end{itemize}
|
||||
\item Nuclear muon capture
|
||||
\begin{itemize}
|
||||
\item physics: capture on proton
|
||||
\item energy
|
||||
\item de-excitation modes: mostly neutrons, other may occur
|
||||
\end{itemize}
|
||||
\item Charged particles/protons
|
||||
\begin{itemize}
|
||||
\item general
|
||||
\item alpha, protons
|
||||
\item
|
||||
\end{itemize}
|
||||
\item Plan and goals of the AlCap experiment
|
||||
\end{itemize}
|
||||
\end{comment}
|
||||
|
||||
\section{Atomic capture of the negative muon}
|
||||
\label{sec:atomic_capture_of_the_negative_muon}
|
||||
Theoretically, the capturing process can be described in the following
|
||||
stages~\cite{FermiTeller.1947, WuWilets.1969}:
|
||||
\begin{enumerate}
|
||||
\item High to low (a few \kilo\electronvolt) energy: the muon velocity are
|
||||
greater than the velocity of the valence electrons of the atom. Slowing
|
||||
down process is similar to that of fast heavy charged particles. It takes
|
||||
about \sn{}{-9} to \sn{}{-10} \second~to slow down from a relativistic
|
||||
\sn{}{8} \electronvolt~energy to 2000 \electronvolt~in condensed matter,
|
||||
and about 1000 times as long in air.
|
||||
\item Low energy to rest: in this phase, the muon velocity is less than that
|
||||
of the valence electrons, the muon is considered to be moving inside
|
||||
a degenerate electron gas. The muon rapidly comes to a stop either in
|
||||
condensed matters ($\sim$\sn{}{-13} \second) or in gases ($\sim$\sn{}{-9}
|
||||
\second).
|
||||
\item Atomic capture: the muon has no kinetic energy, it is captured by the
|
||||
host atom into one of high orbital states, forming a muonic atom. The
|
||||
distribution of initial states is not well known. The details depend on
|
||||
whether the material is a solid or gas, insulator or material
|
||||
\item Electromagnetic cascade: since all muonic states are unoccupied, the
|
||||
muon cascades down to states of low energy. The transition is accompanied
|
||||
by the emission of Auger electrons or characteristic X-rays, or excitation
|
||||
of the nucleus. The time taken for the muon to enter the lowest possible
|
||||
state, 1S, from the instant of its atomic capture is
|
||||
$\sim$\sn{}{-14}\second.
|
||||
\item Muon disappearance: after reaching the 1S state, the muons either
|
||||
decays with a half-life of \sn{2.2}{-6} \second~or gets captured by the
|
||||
nucleus. In hydrogen, the capture to decay probability ratio is about
|
||||
\sn{4}{-4}. Around $Z=11$, the capture probability is roughly equal to the
|
||||
decay probability. In heavy nuclei ($Z\sim50$), the ratio of capture to
|
||||
decay probabilities is about 25.
|
||||
|
||||
The K-shell muon will be $m_\mu/m_e \simeq 207$ times nearer the nucleus
|
||||
than a K-shell electron. The close proximity of the K-shell muon in the
|
||||
Coulomb field of a nuclear, together with its weak interaction with the
|
||||
nucleus, allows the muon to spend a significant fraction of time (\sn{}{-7}
|
||||
-- \sn{}{-6} \second) within the nucleus, serving as an ideal probe for the
|
||||
distribution of nuclear charge and nuclear moments.
|
||||
|
||||
\end{enumerate}
|
||||
% section atomic_capture_of_the_negative_muon (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
\section{Nuclear capture of the negative muon}
|
||||
\label{sec:nuclear_muon_capture}
|
||||
The nuclear capture process is written as:
|
||||
\begin{equation}
|
||||
\mu^- + A(N, Z) \rightarrow A(N, Z-1) + \nu_\mu
|
||||
\label{eq:mucap_general}
|
||||
\end{equation}
|
||||
The resulting nucleus can be either in its ground state or in an excited state.
|
||||
The reaction is manifestation of the elementary ordinary muon capture on the
|
||||
proton:
|
||||
\begin{equation}
|
||||
\mu^- + p \rightarrow n + \nu_\mu
|
||||
\label{eq:mucap_proton}
|
||||
\end{equation}
|
||||
If the resulting nucleus at is in an excited state, it could cascade to lower
|
||||
states by emitting light particles and leaving a residual heavy nucleus. The
|
||||
light particles are mostly neutrons and (or) photons. Neutrons can also be
|
||||
directly knocked out of the nucleus via the reaction~\eqref{eq:mucap_proton}.
|
||||
Charged particles are emitted with probabilities of a few percent, and are
|
||||
mainly protons, deuterons and alphas have been observed in still smaller
|
||||
probabilities. Because of the central interest on proton emission, it is covered
|
||||
in a separated section.
|
||||
|
||||
\subsection{Muon capture on the proton}
|
||||
\label{sub:muon_capture_on_proton}
|
||||
%It is theoretically
|
||||
%very important in understanding the structure of the Lagrangian for the
|
||||
%strangeness-preserving semileptonic weak interaction. But it is also the
|
||||
%hardest one experimentally. The first reason is the rate is small ($\sim$460
|
||||
%\reciprocal\second) compares to the decay rate
|
||||
%($\sim$\sn{455}{3}~\reciprocal\second)~\cite{Measday.2001}. Secondly, the
|
||||
%$\mu p$ atom is quite active, so it is likely to form muonic molecules like
|
||||
%$p\mu p$, $p\mu d$ and $p\mu t$, which complicate the study of weak
|
||||
%interaction.
|
||||
The underlying interaction in proton capture in Equation~\eqref{eq:mucap_proton}
|
||||
at nucleon level and quark level
|
||||
are depicted in the Figure~\ref{fig:feyn_protoncap}. The flow of time is from
|
||||
the left to the right hand side, as an incoming muon and an up quark
|
||||
exchange a virtual $W$ boson to produce a muon neutrino and a down quark, hence
|
||||
a proton transforms to a neutron.
|
||||
|
||||
\begin{figure}[htb]
|
||||
\centering
|
||||
\includegraphics[width=0.4\textwidth]{figs/mucap_proton}
|
||||
\hspace{10mm}
|
||||
\includegraphics[width=0.4\textwidth]{figs/mucap_quark}
|
||||
\caption{A tree-level Feynman diagram of muon capture on the proton, at the
|
||||
nucleon-level (left), and at the quark-level (right).}
|
||||
\label{fig:feyn_protoncap}
|
||||
\end{figure}
|
||||
|
||||
The four-momentum transfer in the interaction is fixed at
|
||||
$q^2 = (q_n - q_p)^2 = -0.88m_\mu^2 \ll m_W^2$. The smallness of the momentum
|
||||
transfer in comparison to the $W$ boson's mass makes it possible to treat the
|
||||
interaction as a four-fermion interaction with Lorentz-invariant transition
|
||||
amplitude:
|
||||
\begin{equation}
|
||||
\mathcal{M} = \frac{G_F V_{ud}}{\sqrt{2}}J^\alpha j_\alpha
|
||||
\label{eq:4fermion_trans_amp}
|
||||
\end{equation}
|
||||
where $J$ is the nucleon current $p\rightarrow n$, and $j$ is the lepton
|
||||
current $\mu \rightarrow \nu_\mu$, $G_F$ is the Fermi coupling constant, and
|
||||
$V_{ud}$ is the matrix element of the Cabibbo-Kobayashi-Maskawa
|
||||
(CKM) matrix. The lepton current is expressed as a purely $V-A$ coupling of
|
||||
lepton states:
|
||||
\begin{equation}
|
||||
j_\alpha = i\bar{\psi}_\nu \gamma_\alpha (1 - \gamma_5) \psi_\mu
|
||||
\label{eq:weakcurrent_lepton}
|
||||
\end{equation}
|
||||
The weak current of individual quarks is similar to that of leptons with the
|
||||
only modification is an appropriate element of the CKM matrix ($V_{ud}$, which
|
||||
is factored out in Eq.~\eqref{eq:4fermion_trans_amp}):
|
||||
\begin{equation}
|
||||
J^\alpha = i\bar{\psi}_d (1 - \gamma_5) \psi_u
|
||||
\label{eq:weakcurrent_ud}
|
||||
\end{equation}
|
||||
If the nucleon were point-like, the nucleon current would have the same form as
|
||||
in Eq.~\eqref{eq:weakcurrent_ud} with suitable wavefunctions of the proton and
|
||||
neutron. But that is not the case, in order to account for the complication of
|
||||
the nucleon, the current must be modified by six real form factors
|
||||
$g_i(q^2), i = V, M, S, A, T, P$:
|
||||
\begin{align}
|
||||
J_\alpha &= i\bar{\psi}_n(V^\alpha - A^\alpha)\psi_p,\\
|
||||
V^\alpha &= g_V (q^2) \gamma^\alpha + i \frac{g_M(q^2)}{2m_N}
|
||||
\sigma^{\alpha\beta} q_\beta + g_S(q^2)q^\alpha,\\
|
||||
A^\alpha &= g_A(q^2)\gamma^\alpha \gamma_5 + ig_T(q^2)
|
||||
\sigma^{\alpha\beta} q_\beta\gamma_5 + \frac{g_P(q^2)}{m_\mu}\gamma_5
|
||||
q^\alpha,
|
||||
\end{align}
|
||||
where the $V^\alpha$ and $A^\alpha$ are the vector and axial currents, $m_\mu$
|
||||
and $m_N$ are the muon and nucleon mass, respectively. The scaling by the muon
|
||||
and nucleon mass is by convention in Mukhopadhyay's
|
||||
review~\cite{Mukhopadhyay.1977}.
|
||||
|
||||
Among the six form factors, the so-called second class currents, $g_T$ and
|
||||
$g_S$, vanish under the symmetry of G-parity, which is the product of charge
|
||||
conjugation and isospin rotation. Experimental limits for non-zero $g_T$ and
|
||||
$g_S$ are not very tight, but are negligible with respect to other
|
||||
uncertainties in muon capture~\cite{Measday.2001}.
|
||||
|
||||
The vector form factor $g_V$, and the weak-magnetic form factor $g_M$ are
|
||||
equivalent to the electromagnetic form factors of the nucleon according the
|
||||
conserved vector current (CVC) hypothesis. The values of these couplings are
|
||||
determined from elastic electron-nucleon scattering experiments, then
|
||||
extrapolated to the momentum transfer $q^2$.
|
||||
|
||||
Using $\mu - e$ universality, the axial form factor $g_A$ in this case is
|
||||
related to that of electron as: $(g_A/g_V)^\mu = (g_A/g_V)^e$ at zero momentum
|
||||
transfer. This equality has been checked using results from muon decay and beta
|
||||
decay experiments. The $q^2$-dependence of $g_A$ is deducted from neutrino
|
||||
scattering experiments.
|
||||
|
||||
The pseudoscalar form factor $g_P$ is determined by measuring the capture rate
|
||||
of the process in Eq.~\eqref{eq:mucap_proton}. However, because of the smallness
|
||||
capture rate in comparison to muon decay rate, and other complications due to
|
||||
muonic molecules $p\mu p$, $d\mu p$ and $t\mu p$, $g_P$ is the least
|
||||
well-defined form factor. Only recently, it is measured with a reasonable
|
||||
precision~\cite{AndreevBanks.etal.2013a}.
|
||||
The values of the six form factors at $q^2 = -0.88m^2_\mu$ are listed in
|
||||
Table~\ref{tab:formfactors}.
|
||||
\begin{table}[htb]
|
||||
\begin{center}
|
||||
\begin{tabular}{l l l}
|
||||
\toprule
|
||||
\textbf{Form factor} & \textbf{Value at $-0.88m^2_\mu$}\\
|
||||
\midrule
|
||||
$g_S$ & $0$\\
|
||||
$g_T$ & $0$\\
|
||||
$g_V$ & $0.976 \pm 0.001$\\
|
||||
$g_M$ & $3.583 \pm 0.003$\\
|
||||
$g_A$ & $1.247 \pm 0.004$\\
|
||||
$g_P$ & $8.06 \pm 0.55$\\
|
||||
\bottomrule
|
||||
\end{tabular}
|
||||
\end{center}
|
||||
\caption{Values of the weak form factors of the nucleon at $q^2
|
||||
= -0.88m^2_\mu$}
|
||||
\label{tab:formfactors}
|
||||
\end{table}
|
||||
|
||||
%\hl{Radiative capture}
|
||||
% subsection muon_capture_on_proton (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
\subsection{Total capture rate}
|
||||
\label{sub:total_capture_rate}
|
||||
The captured muon at the 1S state has only two choices, either to decay or to
|
||||
be captured on the nucleus. Thus, the total capture rate for negative muon,
|
||||
$\Lambda_t$ is given by:
|
||||
\begin{equation}
|
||||
\Lambda_t = \Lambda_c + Q \Lambda_d
|
||||
\label{eq:mu_total_capture_rate}
|
||||
\end{equation}
|
||||
where $\Lambda_c$ and $\Lambda_d$ are partial capture rate and decay rate,
|
||||
respectively, and $Q$ is the Huff factor, which is corrects for the fact that
|
||||
muon decay rate in a bound state is reduced because of the binding energy
|
||||
reduces the available energy.
|
||||
%The total capture rates for several selected
|
||||
%elements are compiled by Measday~\cite{Measday.2001},
|
||||
%and reproduced in
|
||||
%Table~\ref{tab:total_capture_rate}.
|
||||
%\begin{table}[htb]
|
||||
%\begin{center}
|
||||
%\begin{tabular}{l l r@{.}l r@{.}l@{$\pm$}l l}
|
||||
%\toprule
|
||||
%\textbf{$Z$ ($Z_{\textrm{eff}}$)} &
|
||||
%\textbf{Element} &
|
||||
%\multicolumn{2}{l}{\textbf{Mean lifetime}} &
|
||||
%\multicolumn{3}{l}{\textbf{Capture rate}} &
|
||||
%\textbf{Huff factor}\\
|
||||
%& &
|
||||
%\multicolumn{2}{c}{\textbf{(\nano\second)}} &
|
||||
%\multicolumn{3}{l}{\textbf{$\times 10^3$ (\reciprocal\second)}} &\\
|
||||
%\midrule
|
||||
%1 (1.00) & $^1$H & 2194&90 $\pm$0.07 & 0&450 &0.020 & 1.00\\
|
||||
%& $^2$H & 2194&53 $\pm$0.11 & 0&470 &0.029 & \\
|
||||
%2 (1.98) & $^3$He & 2186&70 $\pm$0.10 & 2&15 &0.020 & 1.00\\
|
||||
%& $^4$He & 2195&31 $\pm$0.05 & 0&470&0.029 & \\
|
||||
%\bottomrule
|
||||
%\end{tabular}
|
||||
%\end{center}
|
||||
%\caption{Total capture rate of the muon in nuclei for several selected
|
||||
%elements, compiled by Measday~\cite{Measday.2001}}
|
||||
%\label{tab:total_capture_rate}
|
||||
%\end{table}
|
||||
|
||||
Theoretically, it is assumed that the muon capture rate on a proton of the
|
||||
nucleus depends only on the overlap of the muon with the nucleus. For light
|
||||
nuclei where the point nucleus concept is applicable, there are $Z$ protons and
|
||||
the radius of the muon orbital decreases as $Z^{-1}$, the probability of
|
||||
finding the muon at the radius increases as $Z^3$, therefore the capture rate
|
||||
increases as $Z^4$. Because the muon radius soon becomes comparable to that of
|
||||
the nucleus, corrections are needed, so $Z_{\textrm{eff}}$ is used instead of
|
||||
$Z$.
|
||||
|
||||
The effect of the nucleus for higher $Z$ is more profound, there is no
|
||||
theoretical model that provides a satisfied explanation for all experimental
|
||||
data. One simple formula from Primakoff gives a reasonable,
|
||||
and of course not perfect, description of the existing data~\cite{Measday.2001}:
|
||||
\begin{equation}
|
||||
\Lambda_c(A,Z) = Z^4_{\textrm{eff}} X_1 \left[1
|
||||
- X_2\left(\frac{A-Z}{2A}\right)\right]
|
||||
\label{eq:primakoff_capture_rate}
|
||||
\end{equation}
|
||||
where $X_1 = 170$ \reciprocal\second~is the muon capture rate for hydrogen, but
|
||||
reduced because a smaller phase-space in the nuclear muon capture compares to
|
||||
that of a nucleon; and $X_2 = 3.125$ takes into account the fact that it is
|
||||
harder for protons to transforms into neutrons due to the Pauli exclusion
|
||||
principle in heavy nuclei where there are more neutrons than protons.
|
||||
% subsection total_capture_rate (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
\subsection{Neutron emission}
|
||||
\label{sub:neutron_emission}
|
||||
The average number of neutrons emitted per muon capture generally increases
|
||||
with $Z$, but there are large deviations from the trend due to particular
|
||||
nuclear structure effects. The trend is shown in Table~\ref{tab:avg_neutron}
|
||||
and can be expressed by a simple empirical function
|
||||
$n_{avg} = (0.3 \pm 0.02)A^{1/3}$~\cite{Singer.1974}.
|
||||
\begin{table}[htb]
|
||||
\begin{center}
|
||||
\begin{tabular}{c c}
|
||||
\toprule
|
||||
\textbf{Elements} & \textbf{Average number of }\\
|
||||
& \textbf{neutrons per capture}\\
|
||||
\midrule
|
||||
Al & 1.262 $\pm$ 0.059\\
|
||||
Si & 0.864 $\pm$ 0.072\\
|
||||
Ca & 0.746 $\pm$ 0.032\\
|
||||
Fe & 1.125 $\pm$ 0.041\\
|
||||
Ag & 1.615 $\pm$ 0.060\\
|
||||
I & 1.436 $\pm$ 0.056\\
|
||||
Au & 1.662 $\pm$ 0.044\\
|
||||
Pb & 1.709 $\pm$ 0.066\\
|
||||
\bottomrule
|
||||
\end{tabular}
|
||||
\end{center}
|
||||
\caption{Average number of neutrons emitted per muon capture compiled by
|
||||
Measday~\cite{Measday.2001}}
|
||||
\label{tab:avg_neutron}
|
||||
\end{table}
|
||||
|
||||
The neutron emission can be explained by several mechanisms:
|
||||
\begin{enumerate}
|
||||
\item Direct emission follows reaction~\eqref{eq:mucap_proton}: these neutrons
|
||||
have fairly high energy, from a few \mega\electronvolt~to as high as 40--50
|
||||
\mega\electronvolt.
|
||||
\item Indirect emission through an intermediate compound nucleus: the energy
|
||||
transferred to the neutron in the process~\eqref{eq:mucap_proton} is 5.2
|
||||
\mega\electronvolt~if the initial proton is at rest, in nuclear
|
||||
environment, protons have a finite momentum distribution, therefore the
|
||||
mean excitation energy of the daughter nucleus is around 15 to 20
|
||||
\mega\electronvolt~\cite{Mukhopadhyay.1977}. This is above the nucleon
|
||||
emission threshold in all complex nuclei, thus the daughter nucleus can
|
||||
de-excite by emitting one or more neutrons. In some actinide nuclei, that
|
||||
excitation energy might trigger fission reactions. The energy of indirect
|
||||
neutrons are mainly in the lower range $E_n \le 10$ \mega\electronvolt~with
|
||||
characteristically exponential shape of evaporation process. On top of that
|
||||
are prominent lines might appear where giant resonances occur.
|
||||
\end{enumerate}
|
||||
Experimental measurement of neutron energy spectrum is technically hard, and it
|
||||
is difficult to interpret the results. Due to these difficulties, only a few
|
||||
energy spectrum measurements were made, none of them covers the full energy
|
||||
range and mostly at high energy region~\cite{Measday.2001}.
|
||||
% subsection neutron_emission_after_muon_capture (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
\section[Proton emission]
|
||||
{Proton emission}
|
||||
\label{sec:proton_emission}
|
||||
\subsection{Experimental status}
|
||||
\label{sub:experimental_status}
|
||||
The measurement of charged particle emission is quite difficult and
|
||||
some early measurements with nuclear emulsion are still the best available
|
||||
data. There are two reasons for that:
|
||||
\begin{enumerate}
|
||||
\item The emission rate is small: the de-excitation of the nucleus through
|
||||
charged particle is possible, but occurs at very low rate compares to
|
||||
neutron emission. The rate is about 15\% for light nuclei and
|
||||
reduces to a few percent for medium and heavy nuclei.
|
||||
\item The charged particles are short ranged: the emitted protons,
|
||||
deuterons and alphas are typically low energy (2--20~\mega\electronvolt).
|
||||
But a relatively thick target is normally needed in order to achieve
|
||||
a reasonable muon stopping rate and charged particle statistics. Therefore,
|
||||
emulsion technique is particularly powerful.
|
||||
\end{enumerate}
|
||||
The first study was done by Morigana and Fry~\cite{MorinagaFry.1953} where
|
||||
24,000 muon tracks were stopped in their nuclear emulsion which contains silver,
|
||||
bromine, and other light elements, mainly nitrogen, carbon, hydrogen and
|
||||
oxygen. The authors identified a capture on a light element as it would leave
|
||||
a recoil
|
||||
track of the nucleus. They found that for silver bromide AgBr, $(2.2 \pm
|
||||
0.2)\%$ of the captures produced protons and $(0.5 \pm 0.1)\%$ produced alphas.
|
||||
For light elements, the emission rate for proton and alpha are respectively
|
||||
$(9.5 \pm 1.1)\%$ and $(3.4 \pm 0.7)\%$. Subsequently, Kotelchuk and
|
||||
Tyler~\cite{KotelchuckTyler.1968} had a result which was about 3 times more
|
||||
statistics and in fair agreement with Morigana and Fry
|
||||
(Figure~\ref{fig:kotelchuk_proton_spectrum})
|
||||
\begin{figure}[htb]
|
||||
\centering
|
||||
\includegraphics[width=0.65\textwidth]{figs/kotelchuk_proton_spectrum}
|
||||
\caption{Early proton spectrum after muon capture in silver bromide AgBr
|
||||
recorded using nuclear emulsion. Image is taken from
|
||||
Ref.~\cite{KotelchuckTyler.1968}}
|
||||
\label{fig:kotelchuk_proton_spectrum}
|
||||
\end{figure}
|
||||
|
||||
Protons with higher energy are technically easier to measure, but because of
|
||||
the much lower rate, they can only be studied at meson facilities. Krane and
|
||||
colleagues~\cite{KraneSharma.etal.1979} measured proton emission from
|
||||
aluminium, copper and lead in the energy range above 40 \mega\electronvolt~and
|
||||
found a consistent exponential shape in all targets. The integrated yields
|
||||
above 40 \mega\electronvolt~are in the \sn{}{-4}--\sn{}{-3} range (see
|
||||
Table~\ref{tab:krane_proton_rate}), a minor contribution to total proton
|
||||
emission rate.
|
||||
\begin{table}[htb]
|
||||
\begin{center}
|
||||
\begin{tabular}{c c r@{$\pm$}l@{$\times$}r}
|
||||
\toprule
|
||||
\textbf{Target} & \textbf{Exponential constant}&
|
||||
\multicolumn{3}{c}{\textbf{Integrated yield}}\\
|
||||
& \textbf{$E_0$ (MeV)}
|
||||
& \multicolumn{3}{c}{\textbf{$E_p\ge 40$ MeV}}\\
|
||||
\midrule
|
||||
Al & $7.5 \pm 0.4$ & (1.38&0.09)&\sn{}{-3}\\
|
||||
Cu & $8.3 \pm 0.5$ & (1.96&0.12)&\sn{}{-3}\\
|
||||
Pb & $9.9 \pm 1.1$ & (0.171&0.028)&\sn{}{-3}\\
|
||||
\bottomrule
|
||||
\end{tabular}
|
||||
\end{center}
|
||||
\caption{Proton integrated yields and exponential constants measured by Krane
|
||||
et al.~\cite{KraneSharma.etal.1979}. The yields are assumed to be
|
||||
proportional to exp($-E/E_0$).}
|
||||
\label{tab:krane_proton_rate}
|
||||
\end{table}
|
||||
|
||||
Their result on aluminium, the only experimental data existing for this target,
|
||||
is shown in Figure~\ref{fig:krane_proton_spec} in comparison with spectra from
|
||||
neighbouring elements, namely silicon measured by Budyashov et
|
||||
al.~\cite{BudyashovZinov.etal.1971} and magnesium measured Balandin et
|
||||
al.~\cite{BalandinGrebenyuk.etal.1978}. The authors noted aluminium data and
|
||||
silicon data are in reasonable agreement both in the yield and the energy
|
||||
dependence, while magnesium data shows significant drop in intensity. They then
|
||||
suggested the possibility of an interesting nuclear structure dependency that
|
||||
might be at work in this mass range.
|
||||
|
||||
\begin{figure}[htb]
|
||||
\centering
|
||||
\includegraphics[width=0.65\textwidth]{figs/krane_proton_spec}
|
||||
\caption{Yield of charged particles following muon capture in aluminium
|
||||
target (closed circle) in the energy range above 40 MeV and an exponential
|
||||
fit. The open squares are silicon data from Budyashov et
|
||||
al.~\cite{BudyashovZinov.etal.1971}, the open triangles are magnesium data
|
||||
from Balandin et al.~\cite{BalandinGrebenyuk.etal.1978}.}
|
||||
\label{fig:krane_proton_spec}
|
||||
\end{figure}
|
||||
|
||||
The aforementioned difficulties in charged particle measurements could be
|
||||
solved using an active target, just like nuclear emulsion. Sobottka and
|
||||
Wills~\cite{SobottkaWills.1968} took this approach when using a Si(Li) detector
|
||||
to stop muons. They obtained a spectrum of charged particles up to 26
|
||||
\mega\electronvolt~in Figure~\ref{fig:sobottka_spec}. The peak below 1.4
|
||||
\mega\electronvolt~is due to the recoiling $^{27}$Al. The higher energy events
|
||||
including protons, deuterons and alphas constitute $(15\pm 2)\%$ of capture
|
||||
events, which is consistent with a rate of $(12.9\pm1.4)\%$ from gelatine
|
||||
observed by Morigana and Fry. This part has an exponential
|
||||
decay shape with a decay constant of 4.6 \mega\electronvolt. Measday
|
||||
noted~\cite{Measday.2001} the fractions of events in
|
||||
the 26--32 \mega\electronvolt~range being 0.3\%, and above 32
|
||||
\mega\electronvolt~range being 0.15\%. This figure is in agreement with the
|
||||
integrated yield above 40 \mega\electronvolt~from Krane et al.
|
||||
|
||||
In principle, the active target technique could be applied to other material
|
||||
such as germanium, sodium iodine, caesium iodine, and other scintillation
|
||||
materials. The weak point of this method is that there is no particle
|
||||
identification like in nuclear emulsion, the best one can achieve after all
|
||||
corrections is a sum of all charged particles. It should be noted here
|
||||
deuterons can contribute significantly, Budyashov et
|
||||
al.~\cite{BudyashovZinov.etal.1971} found deuteron components to be
|
||||
$(34\pm2)\%$ of the charged particle yield above 18 \mega\electronvolt~in
|
||||
silicon, and $(17\pm4)\%$ in copper.
|
||||
\begin{figure}[htb]
|
||||
\centering
|
||||
\includegraphics[width=0.75\textwidth]{figs/sobottka_spec}
|
||||
\caption{Charged particle spectrum from muon capture in a silicon detector,
|
||||
image taken from Sobottka and Wills~\cite{SobottkaWills.1968}.}
|
||||
\label{fig:sobottka_spec}
|
||||
\end{figure}
|
||||
|
||||
Another technique had been used to study proton emission is the activation
|
||||
method where the residual nucleus is identified by its radioactivity. This
|
||||
method can provide the rate of charged particles emission by adding up the
|
||||
figures from all channels such as $(\mu^-,\nu p)$, $(\mu^-,\nu p(xn))$,
|
||||
$(\mu^-, \nu \alpha)$, $(\mu^-, \nu \alpha(xn))$. The number of elements that
|
||||
can be studied using this method is limited by several requirements: (a)
|
||||
mono-isotopic element is preferable; (b) the radioactive daughter should emit
|
||||
gamma-rays with a reasonable half-life; (c) the $(\mu^-,\nu xn)$ reactions
|
||||
should lead to either stable daughters, or daughters with very short
|
||||
half-lives. The last condition is important in ensuring the dominating neutron
|
||||
emission processes do not interfere with counting of the much less frequent
|
||||
proton emission reactions.
|
||||
|
||||
Vil'gel'mova et al.~\cite{VilgelmovaEvseev.etal.1971} found the single proton
|
||||
(unaccompanied by any neutron)
|
||||
emission rates in the $^{28}\textrm{Si}(\mu^-,\nu p)^{27}\textrm{Mg}$ and
|
||||
$^{39}\textrm{K}(\mu^-,\nu p)^{38}\textrm{Cl}$ reactions are $(5.3 \pm 1.0)$\%
|
||||
and $(3.2 \pm 0.6)$\%, respectively.
|
||||
Singer~\cite{Singer.1974} compared the figure for silicon and the result from
|
||||
active target measurement and found that the reaction
|
||||
$^{28}\textrm{Si}(\mu^-,\nu pn)^{26}\textrm{Mg}$ could occur at a similar rate
|
||||
to that of the $^{28}\textrm{Si}(\mu^-,\nu p)^{27}\textrm{Mg}$. That also
|
||||
indicates that the deuterons and alphas might constitute a fair amount in the
|
||||
spectrum in Figure~\ref{fig:sobottka_spec}.
|
||||
|
||||
Wyttenbach et al.~\cite{WyttenbachBaertschi.etal.1978} studied $(\mu^-,\nu p)$,
|
||||
$(\mu^-,\nu pn)$, $(\mu^-,\nu p2n)$, $(\mu^-,\nu p3n)$ and $(\mu^-,\nu\alpha)$
|
||||
in a wide range of 18 elements from sodium to bismuth.Their results plotted
|
||||
against the Coulomb barrier for the outgoing protons are given in
|
||||
Figure~\ref{fig:wyttenbach_rate_1p}, ~\ref{fig:wyttenbach_rate_23p}. The
|
||||
classical Coulomb barrier $V$ they used are given by:
|
||||
\begin{equation}
|
||||
V = \frac{zZe^2}{r_0A^{\frac{1}{3}} + \rho},
|
||||
\label{eqn:classical_coulomb_barrier}
|
||||
\end{equation}
|
||||
where $z$ and $Z$ are the charges of the outgoing particle and of the residual
|
||||
nucleus, values $r_0 = 1.35 \textrm{ fm}$, and $\rho = 0 \textrm{ fm}$ for
|
||||
protons were taken.
|
||||
\begin{figure}[htb]
|
||||
\centering
|
||||
\includegraphics[width=0.85\textwidth]{figs/wyttenbach_rate_1p}
|
||||
\caption{Activation results from Wyttenbach et
|
||||
al.~\cite{WyttenbachBaertschi.etal.1978} for the $(\mu^-,\nu p)$ and
|
||||
$(\mu^-,\nu pn)$ reactions.}
|
||||
\label{fig:wyttenbach_rate_1p}
|
||||
\end{figure}
|
||||
\begin{figure}[htb]
|
||||
\centering
|
||||
\includegraphics[width=0.85\textwidth]{figs/wyttenbach_rate_23p}
|
||||
\caption{Activation results from Wyttenbach et
|
||||
al.~\cite{WyttenbachBaertschi.etal.1978} for the $(\mu^-,\nu p2n)$ and
|
||||
$(\mu^-,\nu p3n)$ reactions.}
|
||||
\label{fig:wyttenbach_rate_23p}
|
||||
\end{figure}
|
||||
|
||||
Wyttenbach et al.\ saw that the cross section of each reaction decreases
|
||||
exponentially with increasing Coulomb barrier. The decay constant for all
|
||||
$(\mu^-,\nu pxn)$ is about 1.5 per \mega\electronvolt~of Coulomb barrier. They
|
||||
also commented a ratio for different de-excitation channels:
|
||||
\begin{equation}
|
||||
(\mu^-,\nu p):(\mu^-,\nu pn):(\mu^-,\nu p2n):(\mu^-,\nu p3n) = 1:6:4:4,
|
||||
\label{eqn:wyttenbach_ratio}
|
||||
\end{equation}
|
||||
The authors compared their results with many preceded works and rejected
|
||||
the results from Vil'gel'mova et al.~\cite{VilgelmovaEvseev.etal.1971} as being
|
||||
too high, but Measday~\cite{Measday.2001} noted it it is not
|
||||
necessarily true since there has been suggestion from other experiments that
|
||||
$(\mu^-, \nu p)$ reactions might become more important for light nuclei.
|
||||
Measday also commented that the ratio~\eqref{eqn:wyttenbach_ratio} holds over
|
||||
a broad range of mass, but below $A=40$ the $(\mu^-,\nu p)$ reaction can vary
|
||||
significantly from nucleus to nucleus.
|
||||
% subsection experimental_status (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
\subsection{Theoretical models}
|
||||
\label{sub:theoretical_models}
|
||||
|
||||
The first attempt to explain the result of Morigana and Fry was done by
|
||||
Ishii~\cite{Ishii.1959}. He assumed a two-step scenario: firstly a compound
|
||||
nucleus is formed, and then it releases energy by statistical emission of
|
||||
various particles. Three models for momentum distribution of protons in the
|
||||
nucleus were used: (I) the Chew-Goldberger distribution
|
||||
$\rho(p) \sim A/(B^2 + p^2)^2$; (II) Fermi gas at zero temperature; and (III)
|
||||
Fermi gas at a finite temperature ($kT = 9$ \mega\electronvolt).
|
||||
|
||||
A very good agreement with the experimental result for the alpha emission was
|
||||
obtained with distribution (III), both in the absolute percentage and the energy
|
||||
distribution (curve (III) in the left hand side of
|
||||
Figure~\ref{fig:ishii_cal_result}). However, the calculated emission of protons
|
||||
at the same temperature falls short by about 10
|
||||
times compares to the data. The author also found that the distribution
|
||||
(I) is unlikely to be suitable for proton emission, and using that distribution
|
||||
for alpha emission resulted in a rate 15 times larger than observed.
|
||||
|
||||
\begin{figure}[htb]
|
||||
\centering
|
||||
\includegraphics[width=.49\textwidth]{figs/ishii_cal_alpha}
|
||||
%\hspace{10mm}
|
||||
\includegraphics[width=.49\textwidth]{figs/ishii_cal_proton}
|
||||
\caption{Alpha spectrum (left) and proton spectrum (right) from Ishii's
|
||||
calculation~\cite{Ishii.1959} in comparison with experimental data from
|
||||
Morigana and Fry. Image is taken from Ishii's paper.}
|
||||
\label{fig:ishii_cal_result}
|
||||
\end{figure}
|
||||
Singer~\cite{Singer.1974} noted that by assuming a reduced effective mass for
|
||||
the nucleon, the average excitation energy will increase, but the proton
|
||||
emission rate does not significantly improve and still could not explain the
|
||||
large discrepancy. He concluded that the evaporation mechanism can account
|
||||
for only a small fraction of emitted protons. Moreover, the high energy protons
|
||||
of 25--50 \mega\electronvolt~cannot be explained by the evaporation mechanism.
|
||||
He and Lifshitz~\cite{LifshitzSinger.1978, LifshitzSinger.1980} proposed two
|
||||
major corrections to Ishii's model:
|
||||
\begin{enumerate}
|
||||
\item A new description of the nucleon momentum in the nucleus with more high
|
||||
momentum components. This helps explaining the high momentum part of the
|
||||
proton spectrum.
|
||||
\item Pre-equilibrium emission of proton is included: both pre-equilibrium
|
||||
and statistical emission were taken into account. The equilibrium state is
|
||||
achieved through a series of intermediate states, and at each state there
|
||||
is possibility for particles to escape from the nucleus.
|
||||
\end{enumerate}
|
||||
With these improvements, the calculated proton spectrum agreed reasonably with
|
||||
data from Morigana and Fry in the energy range $E_p \le 30$ \mega\electronvolt.
|
||||
Lifshitz and Singer noted the pre-equilibrium emission is more important for
|
||||
heavy nuclei. Its contribution in light nuclei is about a few percent,
|
||||
increasing to several tens of percent for $100<A<180$, then completely
|
||||
dominating in very heavy nuclei. This trend is also seen in other nuclear
|
||||
reactions at similar excitation energies. The pre-equilibrium emission also
|
||||
dominates the higher-energy part, although it falls short at energies higher
|
||||
than 30 \mega\electronvolt. The comparison between the calculated proton
|
||||
spectrum and experimental data is shown in
|
||||
Fig.~\ref{fig:lifshitzsinger_cal_proton}.
|
||||
\begin{figure}[htb]
|
||||
\centering
|
||||
\includegraphics[width=0.85\textwidth]{figs/lifshitzsinger_cal_proton}
|
||||
\caption{Proton energy spectrum from muon capture in AgBr, the data in
|
||||
histogram is from Morigana and Fry, calculation by Lifshitz and
|
||||
Singer~\cite{LifshitzSinger.1978} showed contributions from the
|
||||
pre-equilibrium emission and the equilibrium emission.}
|
||||
\label{fig:lifshitzsinger_cal_proton}
|
||||
\end{figure}
|
||||
|
||||
The authors found their corrections accounts well for the observed data in
|
||||
a wide range of elements $23 \le A \le 209$. They calculated both the single
|
||||
proton emission rate $(\mu^-, \nu p)$ and the inclusive emission rate:
|
||||
\begin{align*}
|
||||
\sum(\mu^-, \nu p) = &(\mu^-, \nu p) + (\mu^-, \nu pn) + (\mu^-, \nu p2n)\\
|
||||
&+ \ldots + (\mu^-, \nu d) + (\mu^-, \nu dn)) + \ldots
|
||||
\end{align*}
|
||||
The deuteron emission channels are included to comparisons with activation
|
||||
data where there is no distinguish between $(\mu^-, \nu pn)$ and $(\mu^-,d)$,
|
||||
\ldots Their calculated emission rates together with available experimental
|
||||
data is reproduced in Table~\ref{tab:lifshitzsinger_cal_proton_rate}.
|
||||
\begin{table}[htb]
|
||||
\begin{center}
|
||||
\begin{tabular}{c c c c c}
|
||||
\toprule
|
||||
Target nucleus & Calculation & Experiment & Estimate & Comments \\
|
||||
%\textbf{Col1}\\
|
||||
\midrule
|
||||
$^{27}_{13}$Al & 40 & $>28 \pm 4$ & (70) & 7.5 for $T>40$ MeV \\
|
||||
$^{28}_{14}$Si & 144 & $150\pm30$ & & 3.1 and 0.34 $d$ for $T>18$ MeV \\
|
||||
$^{31}_{15}$P & 35 & $>61\pm6$ & (91) & \\
|
||||
$^{46}_{22}$Ti & & & & \\
|
||||
$^{51}_{23}$V & 25 & $>20\pm1.8$ & (32) & \\
|
||||
%item1\\
|
||||
\bottomrule
|
||||
\end{tabular}
|
||||
\end{center}
|
||||
\caption{Calculated of the single proton emission rate and the inclusive
|
||||
proton emission rate. The experimental data are mostly from Wyttenbach et
|
||||
al.\cite{WyttenbachBaertschi.etal.1978}}
|
||||
\label{tab:lifshitzsinger_cal_proton_rate}
|
||||
\end{table}
|
||||
A generally good agreement between calculation and experiment can be seen from
|
||||
Table~\ref{tab:lifshitzsinger_cal_proton_rate}. The rate of $(\mu^-,\nu p)$
|
||||
reactions for $^{28}\textrm{Al}$ and $^{39}\textrm{K}$ are found to be indeed
|
||||
higher than average, though not as high as Vil'gel'mora et
|
||||
al.~\cite{VilgelmovaEvseev.etal.1971} observed.
|
||||
|
||||
For protons with higher energies in the range of
|
||||
40--90 \mega\electronvolt~observed in the emulsion data as well as in later
|
||||
experiments~\cite{BudyashovZinov.etal.1971,BalandinGrebenyuk.etal.1978,
|
||||
KraneSharma.etal.1979}, Lifshitz and Singer~\cite{LifshitzSinger.1988}
|
||||
suggested another contribution from capturing on correlated two-nucleon
|
||||
cluster, an idea that had been proposed earlier by Singer~\cite{Singer.1961}.
|
||||
In this calculation, the authors considered the captures on cluster in which
|
||||
two nucleons interact with each other via meson exchange current. There is
|
||||
experimental evidence that the nuclear surface is reach in nucleon clusters,
|
||||
and it had been shown that the meson exchange current increases the total
|
||||
capture rate in deuterons by 6\%. The result of this model was a mix, it
|
||||
accounted well for Si, Mg and Pb data, but predicted rates about 4 times
|
||||
smaller in cases of Al and Cu, and about 10 times higher in case of AgBr
|
||||
(Table~\ref{tab:lifshitzsinger_cal_proton_rate_1988}).
|
||||
\begin{table}[htb]
|
||||
\begin{center}
|
||||
\begin{tabular}{l l c}
|
||||
\toprule
|
||||
\textbf{Nucleus} & \textbf{Exp.$\times 10^3$} & \textbf{MEC cal.$\times
|
||||
10^3$}\\
|
||||
\midrule
|
||||
Al & $1.38 \pm 0.09$ & 0.3\\
|
||||
Si & $0.87 \pm 0.14$ & 0.5\\
|
||||
Mg & $0.17 \pm 0.05$ & 0.2\\
|
||||
Cu & $1.96 \pm 0.12$ & 0.5\\
|
||||
AgBr & $(4.7 \pm 1.1)\times 10^{-2}$ & 0.4\\
|
||||
Pb & $0.17 \pm 0.03$ & 0.3\\
|
||||
\bottomrule
|
||||
\end{tabular}
|
||||
\end{center}
|
||||
\caption{Probability of proton emission with $E_p \ge 40$
|
||||
\mega\electronvolt~as calculated by Lifshitz and
|
||||
Singer~\cite{LifshitzSinger.1988} in comparison with available data.}
|
||||
\label{tab:lifshitzsinger_cal_proton_rate_1988}
|
||||
\end{table}
|
||||
% subsection theoretical_models (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
\subsection{Summary on proton emission from aluminium}
|
||||
\label{sub:summary_on_proton_emission_from_aluminium}
|
||||
There is no direct measurement of proton emission following
|
||||
muon capture in the relevant energy for the COMET Phase-I of 2.5--10
|
||||
\mega\electronvolt:
|
||||
\begin{enumerate}
|
||||
\item Spectrum wise, only one energy spectrum (Figure~\ref{fig:krane_proton_spec})
|
||||
for energies above 40 \mega\electronvolt~is available from Krane et
|
||||
al.~\cite{KraneSharma.etal.1979},
|
||||
where an exponential decay shape with a decay constant of
|
||||
$7.5 \pm 0.4$~\mega\electronvolt. At low energy range, the best one can get is
|
||||
the charged particle spectrum, which includes protons, deuterons and alphas,
|
||||
from the neighbouring element silicon (Figure~\ref{fig:sobottka_spec}).
|
||||
This charged particle spectrum peaks around 2.5 \mega\electronvolt~and
|
||||
reduces exponentially with a decay constant of 4.6 \mega\electronvolt.
|
||||
\item The activation data from Wyttenbach et
|
||||
al.~\cite{WyttenbachBaertschi.etal.1978} only gives rate of
|
||||
$^{27}\textrm{Al}(\mu^-,\nu pn)^{25}\textrm{Na}$ reaction, and set a lower
|
||||
limit for proton emission rate at $(2.8 \pm 0.4)\%$ per muon capture. If
|
||||
the ratio~\eqref{eqn:wyttenbach_ratio} holds true for aluminium, then the
|
||||
inclusive proton rate would be $7\%$, higher than the calculated rate of
|
||||
$4\%$ by Lifshitz and Singer~\cite{LifshitzSinger.1980}.
|
||||
Both activation technique and inclusive rate calculation do not distinguish
|
||||
between different channels that give the same final state, such as between
|
||||
$^{27}\textrm{Al}(\mu^-,\nu pn)^{25}\textrm{Na}$ and
|
||||
$^{27}\textrm{Al}(\mu^-,\nu d)^{25}\textrm{Na}$ reactions.
|
||||
\end{enumerate}
|
||||
|
||||
In short, the knowledge on proton emission from aluminium at low energy is
|
||||
limited. The rate estimation does not separate protons from deuterons,
|
||||
and experimentally, there is a lower limit of $(2.8\pm0.4)\%$ per muon capture.
|
||||
A spectrum shape at this energy range is not available.
|
||||
% subsection summary_on_proton_emission_from_aluminium (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
% section proton_emission (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
\section{The AlCap experiment}
|
||||
\label{sec:the_alcap_experiment}
|
||||
\subsection{Motivation of the AlCap experiment}
|
||||
\label{sub:motivation_of_the_alcap_experiment}
|
||||
As mentioned, protons from muon capture on aluminium might cause a very high
|
||||
rate in the COMET Phase-I CDC. The detector is designed to accept particles
|
||||
with momenta in the range of 75--120 \mega\electronvolt\per\cc.
|
||||
Figure~\ref{fig:proton_impact_CDC} shows that protons with kinetic energies of
|
||||
2.5--8 \mega\electronvolt~will hit the CDC. Such events are troublesome due to
|
||||
their large energy deposition. Deuterons and alphas at that momentum range is
|
||||
not of concern because they have lower kinetic energy and higher stopping
|
||||
power, thus are harder to escape the muon stopping target.
|
||||
\begin{figure}[htb]
|
||||
\centering
|
||||
\includegraphics[width=0.85\textwidth]{figs/proton_impact_CDC}
|
||||
\caption{Momentum-kinetic energy relation of protons, deuterons and alphas
|
||||
below 10\mega\electronvolt. Shaded area is the acceptance of the COMET
|
||||
Phase-I's CDC. Protons with energies in the range of 2.5--8
|
||||
\mega\electronvolt~are in the acceptance of the CDC. Deuterons and alphas at
|
||||
low energies should be stopped inside the muon stopping target.}
|
||||
\label{fig:proton_impact_CDC}
|
||||
\end{figure}
|
||||
|
||||
The COMET plans to introduce a thin, low-$Z$ proton absorber in between the
|
||||
target and the CDC to produce proton hit rate. The absorber will be effective
|
||||
in removing low energy protons. The high energy protons that are moderated by
|
||||
the absorber will fall into the acceptance range of the CDC, but because of the
|
||||
exponential decay shape of the proton spectrum, the hit rate caused by these
|
||||
protons should be affordable.
|
||||
|
||||
The proton absorber solves the problem of hit rate, but it degrades the
|
||||
reconstructed momentum resolution. Therefore its thickness and geometry should
|
||||
be carefully designed. The limited information available makes it difficult to
|
||||
arrive at a conclusive detector design. The proton emission rate could be 4\%
|
||||
as calculated by Lifshitz and Singer~\cite{LifshitzSinger.1980}; or 7\% as
|
||||
estimated from the $(\mu^-,\nu pn)$ activation data and the ratio
|
||||
\eqref{eqn:wyttenbach_ratio}~\cite{WyttenbachBaertschi.etal.1978}; or as high
|
||||
as 15-20\% from silicon and neon.
|
||||
|
||||
For the moment, design decisions in the COMET Phase-I are made based on
|
||||
conservative assumptions: emission rate of 15\% and an exponential decay shape
|
||||
are adopted follow the silicon data from Sobottka and Will
|
||||
~\cite{SobottkaWills.1968}. The spectrum shape is fitted with an empirical
|
||||
function given by:
|
||||
\begin{equation}
|
||||
p(T) = A\left(1-\frac{T_{th}}{T}\right)^\alpha e^{-(T/T_0)},
|
||||
\label{eqn:EH_pdf}
|
||||
\end{equation}
|
||||
where $T$ is the kinetic energy of the proton, and the fitted parameters are
|
||||
$A=0.105\textrm{ MeV}^{-1}$, $T_{th} = 1.4\textrm{ MeV}$, $\alpha = 1.328$ and
|
||||
$T_0 = 3.1\textrm{ MeV}$. The baseline
|
||||
design of the absorber is 1.0 \milli\meter~thick
|
||||
carbon-fibre-reinforced-polymer (CFRP) which contributes
|
||||
195~\kilo\electronvolt\per\cc~to the momentum resolution. The absorber also
|
||||
down shifts the conversion peak by 0.7 \mega\electronvolt. This is an issue as
|
||||
it pushes the signal closer to the DIO background region. For those reasons,
|
||||
a measurement of the rate and spectrum of proton emission after muon capture is
|
||||
required in order to optimise the CDC design.
|
||||
% subsection motivation_of_the_alcap_experiment (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
\subsection{Experimental method for proton measurement}
|
||||
\label{sub:experimental_method}
|
||||
We planned to use a low energy, narrow momentum spread available at PSI to
|
||||
fight the aforementioned difficulties in measuring protons. The beam momentum
|
||||
is tunable from 28 to 45~\mega\electronvolt\ so that targets at different
|
||||
thickness from 25 to 100 \micro\meter\ can be studied. The $\pi$E1 beam line
|
||||
could provide about \sn{}{3} muons\per\second\ at 1\% momentum spread, and
|
||||
\sn{}{4} muons\per\second\ at 3\% momentum spread. With this tunable beam, the
|
||||
stopping distribution of the muons is well-defined.
|
||||
|
||||
The principle of the particle identification used in the AlCap experiment is
|
||||
that for each species, the function describes the relationship between energy
|
||||
loss per unit length (dE/dx) and the particle energy E is uniquely defined.
|
||||
With a simple system of two detectors, dE/dx can be obtained by
|
||||
measuring energy deposit $\Delta$E in one detector of known thickness
|
||||
$\Delta$x, and E is the sum of energy deposit in both detector if the particle
|
||||
is fully stopped.
|
||||
|
||||
In the AlCap, we realise the idea with a pair of silicon detectors: one thin
|
||||
detector of 65~\micron\ serves as the $\Delta$E counter, and one thick detector
|
||||
of 1500~\micron\ that can fully stop protons up to about 12~MeV. Since the
|
||||
$\Delta \textrm{d}=65$~\micron\ is known, the function relates dE/dx to
|
||||
E reduces to a function between $\Delta$E and E. Figure~\ref{fig:pid_sim} shows
|
||||
that the function of protons can be clearly distinguished from other charged
|
||||
particles in the energy range of interest.
|
||||
\begin{figure}[htbp]
|
||||
\centering
|
||||
\includegraphics[width=0.75\textwidth]{figs/pid_sim}
|
||||
\caption{Simulation study of PID using a pair of silicon detectors}
|
||||
\label{fig:pid_sim}
|
||||
\end{figure}
|
||||
|
||||
The AlCap uses two pairs of detector with large area, placed symmetrically with
|
||||
respect to the target provide a mean to check for muon stopping distribution.
|
||||
The absolute number of stopped muons are inferred
|
||||
from the number of muonic X-rays recorded by a germanium detector. For
|
||||
aluminium, the $(2p-1s)$ line is at 346 \kilo\electronvolt. The acceptances of
|
||||
detectors will be assessed by detailed Monte Carlo study using Geant4.
|
||||
|
||||
% subsection experimental_method (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
\subsection{Goals and plan of the experiment}
|
||||
\label{sub:goals_of_the_experiment}
|
||||
|
||||
Our experimental program is organised in three distinct work packages (WP),
|
||||
directed by different team leaders, given in parentheses.
|
||||
|
||||
\begin{itemize}
|
||||
\item[WP1:] (Kammel (Seattle), Kuno(Osaka)) \textbf{Charged
|
||||
Particle Emission after Muon Capture.}\\ Protons emitted after nuclear muon
|
||||
capture in the stopping target dominate the single-hit rates in the tracking
|
||||
chambers for both the Mu2e and COMET Phase-I experiments. We plan to measure
|
||||
both the total rate and the energy spectrum to a precision of 5\% down to
|
||||
proton energies of 2.5 MeV.
|
||||
\item[WP2:] (Lynn(PNNL), Miller(BU))
|
||||
\textbf{Gamma and X-ray Emission after Muon Capture.}\\ A Ge detector will
|
||||
be used to measure X-rays from the muonic atomic cascade, in order to provide
|
||||
the muon-capture normalization for WP1, and is essential for very thin
|
||||
stopping targets. It is also the primary method proposed for calibrating the
|
||||
number of muon stops in the Mu2e and COMET experiments. Two additional
|
||||
calibration techniques will also be explored; (1) detection of delayed gamma
|
||||
rays from nuclei activated during nuclear muon capture, and (2) measurement
|
||||
of the rate of photons produced in radiative muon decay. The first of these
|
||||
would use a Ge detector and the second a NaI detector. The NaI
|
||||
calorimeter will measure the rate of high energy photons from radiative muon
|
||||
capture (RMC), electrons from muon decays in orbit (DIO), and photons from
|
||||
radiative muon decay (RMD), as potential background sources for the
|
||||
conversion measurement. As these rates are expected to be extremely low near
|
||||
the conversion electron energy, only data at energies well below 100 MeV will
|
||||
be obtained.
|
||||
\item[WP3:] (Hungerford(UH), Winter(ANL)) \textbf{Neutron
|
||||
Emission after Muon Capture.}\\ Neutron rates and spectra after capture in
|
||||
Al and Ti are not well known. In particular, the low energy region below 10
|
||||
MeV is important for determining backgrounds in the Mu2e/COMET detectors and
|
||||
veto counters as well as evaluating the radiation damage to electronic
|
||||
components. Carefully calibrated liquid scintillation detectors, employing
|
||||
neutron-gamma discrimination and spectrum unfolding techniques, will measure
|
||||
these spectra. The measurement will attempt to obtain spectra as low or lower
|
||||
than 1 MeV up to 10 MeV. \\
|
||||
\end{itemize}
|
||||
|
||||
WP1 is the most developed
|
||||
project in this program. Most of the associated apparatus has been built and
|
||||
optimized. We are ready to start this experiment in 2013, while preparing and
|
||||
completing test measurements and simulations to undertake WP2 and WP3.
|
||||
|
||||
The measurement of proton has been carried out in November and December 2013,
|
||||
the details are described in following chapters.
|
||||
% subsection goals_of_the_experiment (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
% section the_alcap_experiment (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
|
||||
% section nuclear_muon_capture (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
% chapter alcap_phys (end)
|
||||
638
thesis2/chapters/chap5_alcap_setup.tex
Normal file
@@ -0,0 +1,638 @@
|
||||
\chapter{The AlCap Run 2013}
|
||||
\label{cha:the_alcap_run_2013}
|
||||
\thispagestyle{empty}
|
||||
The first run of the AlCap experiment was performed at the $\pi$E1 beam line
|
||||
area, PSI (Figure~\ref{fig:psi_exp_hall_all}) from November 26 to December 23,
|
||||
2013. The goal of the run was to measure protons rate and spectrum following
|
||||
muon capture on aluminium.
|
||||
|
||||
\begin{figure}[p]
|
||||
\centering
|
||||
\includegraphics[height=0.85\textheight]{figs/psi_exp_hall_all}
|
||||
\caption{Layout of the PSI experimental hall, $\pi$E1 experimental area is
|
||||
marked with the red circle. \\Image taken from
|
||||
\url{http://www.psi.ch/num/FacilitiesEN/HallenplanPSI.png}}
|
||||
\label{fig:psi_exp_hall_all}
|
||||
\end{figure}
|
||||
|
||||
\section{Experimental set up}
|
||||
\label{sec:experimental_set_up}
|
||||
The low energy muons from the $\pi$E1 beam line were stopped in thin aluminium
|
||||
and silicon targets, and charged particles emitted were measured by two pairs
|
||||
of silicon detectors inside of a vacuum vessel
|
||||
(Figure~\ref{fig:alcap_setup_detailed}). A stopped muon event is defined by
|
||||
a group of upstream detectors and a muon veto plastic scintillator.
|
||||
The number of stopped muons is monitored by a germanium detector placed outside
|
||||
of the vacuum chamber. In addition, several plastic scintillators were used to
|
||||
provide veto signals for the silicon and germanium detectors. Two liquid
|
||||
scintillators for neutron measurements were also tested in this run.
|
||||
\begin{figure}[htbp]
|
||||
\centering
|
||||
\includegraphics[width=0.65\textwidth]{figs/alcap_setup_detailed}
|
||||
\caption{AlCap detectors: two silicon packages inside the vacuum vessel,
|
||||
muon beam detectors including plastic scintillators and a wire chamber,
|
||||
germanium detector and veto plastic scintillators.}
|
||||
\label{fig:alcap_setup_detailed}
|
||||
\end{figure}
|
||||
|
||||
\subsection{Muon beam and vacuum chamber}
|
||||
Muons in the $\pi$E1 beam line are decay products of pions created
|
||||
as a 590~\mega\electronvolt\ proton beam hit a thick carbon target
|
||||
(E-target in Figure~\ref{fig:psi_exp_hall_all}). The beam line was designed to
|
||||
deliver muons with momenta ranging from 10 to 500~\mega\electronvolt\per\cc\
|
||||
and
|
||||
momentum spread from 0.26 to 8.0\%. These parameters can be selected by
|
||||
changing various magnets and slits shown in
|
||||
Figure~\ref{fig:psi_piE1_elements}~\cite{Foroughli.1997}.
|
||||
|
||||
\begin{figure}[htb]
|
||||
\centering
|
||||
\includegraphics[width=0.7\textwidth]{figs/psi_piE1_elements}
|
||||
\caption{The $\pi$E1 beam line}
|
||||
\label{fig:psi_piE1_elements}
|
||||
\end{figure}
|
||||
|
||||
One of the main requirements of the AlCap experiment was a low energy muon beam
|
||||
with narrow momentum bite in order to achieve a high fraction of stopping muons
|
||||
in the very thin targets. In this Run 2013, muons from 28 to
|
||||
45~\mega\electronvolt\per\cc\ and momentum spread of 1\% and 3\%were used.
|
||||
|
||||
For part of the experiment the target was replaced with one of the silicon
|
||||
detector packages allowed an accurate momentum and range calibration
|
||||
%(via range-energy relations)
|
||||
of the beam at the target. Figure~\ref{fig:Rates} shows the measured muon rates
|
||||
as a function of momentum for two different momentum bites.
|
||||
Figure~\ref{fig:Beam} shows an example of the resulting energy spectra.
|
||||
\begin{figure}[htbp]
|
||||
\centering
|
||||
\includegraphics[width=0.6\textwidth]{figs/Rates.png}
|
||||
\caption{Measured muon rate (kHz) at low momenta. Momentum bite of 3 and 1 \%
|
||||
FWHM, respectively.}
|
||||
\label{fig:Rates}
|
||||
\end{figure}
|
||||
|
||||
\begin{figure}[htbp]
|
||||
\centering
|
||||
\includegraphics[width=0.9\textwidth]{figs/beam.pdf}
|
||||
\caption{Energy deposition at 36.4 MeV/c incident muon beam in an
|
||||
1500-\micron-active
|
||||
target. The peak at low energy is due to beam electrons, the
|
||||
peaks at higher energies are due to muons. Momentum bite of 1 and 3\% FWHM
|
||||
on left and right hand side, respectively.} \label{fig:Beam}
|
||||
\end{figure}
|
||||
|
||||
The targets and charged particle detectors are installed inside the vacuum
|
||||
chamber as shown in Figure~\ref{fig:alcap_setup_detailed}. The muon beam enters
|
||||
from the right of the image and hits the target, which is placed at the
|
||||
centre of the vacuum chamber and orientated at 45 degrees to the beam axis.
|
||||
The side walls and bottom flange of the vessel provide several
|
||||
vacuum-feedthroughs for the high voltage and signal cables for the silicon and
|
||||
scintillator detectors inside the chamber.
|
||||
In addition, the chamber is equipped with several lead collimators
|
||||
%so that muons that are not captured in the target would quickly decay.
|
||||
to quickly capture muons that do not stop in the actual target.
|
||||
%\begin{figure}[htbp]
|
||||
%\centering
|
||||
%\includegraphics[width=0.55\textwidth]{figs/SetupOverview.jpg}
|
||||
%\caption{Vacuum chamber in beam line}
|
||||
%\label{fig:SetupOverall}
|
||||
%\end{figure}
|
||||
|
||||
%It is known fact that there is a risk of sparkling between the electrodes of
|
||||
%a silicon detector in the low vacuum region of $10^{-3}$ mbar.
|
||||
%An interlock mechanism was installed to prevent the bias of the
|
||||
%silicon detectors from being applied before the safe vacuum level.
|
||||
For a safe operation of the silicon detector, a vacuum of $<10^{-4}$\,mbar was
|
||||
necessary. With the help of the vacuum group of PSI, we could consistently
|
||||
reach $10^{-4}$\,mbar within 45 minutes after closure of the chamber's top
|
||||
flange.
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
|
||||
\subsection{Silicon detectors}
|
||||
The main detectors for proton measurement in the Run 2013 were four large area
|
||||
silicon detectors. The silicon detectors were grouped into two detector
|
||||
packages located symmetrically at 90 degrees of the nominal muon beam path, SiL
|
||||
and SiR in Figure~\ref{fig:alcap_setup_detailed}. Each arm consists of: one
|
||||
$\Delta$E counter, a 65-\micro\meter-thick silicon detector, divided into
|
||||
4 quadrants; one E counter made from 1500-\micron-thick silicon; and one
|
||||
plastic scintillator to identify electrons or high energy protons that pass
|
||||
through the silicon. The area of each of these silicon detectors and the
|
||||
scintillators is $50\times50 \textrm{mm}^2$.
|
||||
|
||||
The detectors were named according to their positions relative to the muon
|
||||
view: the SiL package contains the thin
|
||||
detector SiL1 and thick detector SiL2; the SiR package has SiR1 and SiR2
|
||||
accordingly. Each quadrant of the thin detectors were also numbered from 1 to
|
||||
4, i.e. SiL1-1, SiL1-2, SiL1-3, SiL1-4, SiR1-1, SiR1-2, SiR1-3,
|
||||
SiR1-4.
|
||||
|
||||
Bias for the four silicon detectors was supplied by an ORTEC 710 NIM module,
|
||||
which has a vacuum interlock input to prevent biasing before the safe vacuum
|
||||
level has been reached. Typical voltage to fully depleted the detectors were
|
||||
-300~\volt\ and -10~\volt\ for the thick and thin silicon detectors
|
||||
respectively. The leakage currents at the operating voltages are less than
|
||||
1.5~\micro\ampere\ for the thick detectors, and about 0.05~\micro\ampere\
|
||||
for the thin ones (see Figure~\ref{fig:si_leakage}).
|
||||
\begin{figure}[htb]
|
||||
\centering
|
||||
\includegraphics[width=0.85\textwidth]{figs/si_leakage}
|
||||
\caption{Leakage currents of the silicon detectors under bias.}
|
||||
\label{fig:si_leakage}
|
||||
\end{figure}
|
||||
|
||||
The fact that a detector were fully depleted was checked by putting
|
||||
a calibration source $^{241}\textrm{Am}$ at its ohmic side, and observing the
|
||||
output
|
||||
pulse height on an oscilloscope. One would expect that the maximum pulse height
|
||||
increases as the bias is raised until the voltage of fully depleted. The effect
|
||||
can also be seen on the pulse height spectrum as in
|
||||
Figure~\ref{fig:sir2_bias_alpha}.
|
||||
\begin{figure}[htb]
|
||||
\centering
|
||||
\includegraphics[width=0.75\textwidth]{figs/sir2_bias_alpha}
|
||||
\caption{$^{241}\textrm{Am}$ spectra in cases of fully depleted (top), and
|
||||
partly depleted (bottom).}
|
||||
\label{fig:sir2_bias_alpha}
|
||||
\end{figure}
|
||||
|
||||
%It is known that the noise level of a silicon detector increases linearly with
|
||||
%its capacity. So both noise and pick-up suppression had been carefully
|
||||
%optimised in the real PSI accelerator environment, particularly for the thin
|
||||
%silicon detectors which have a large capacity of 1~\nano\farad~in each
|
||||
%quadrant.
|
||||
%After improving the feed-through flanges during the set-up phase of the
|
||||
%experiment with isolated ground connections, good electronic resolution of
|
||||
%55--76~\kilo\electronvolt\ FWHM was achieved in the thin silicon detectors.
|
||||
%So achieving good energy resolution was particularly challenging
|
||||
%for the thin silicon detector, as each quadrant had a large capacity of
|
||||
%1~\nano\farad. Both
|
||||
%noise and pick-up suppression had been carefully optimized in the real PSI
|
||||
%accelerator environment.
|
||||
%Optimization of the fast timing signals proved another challenge.
|
||||
%The energy calibration for the silicon detectors were done
|
||||
%by several means:
|
||||
%\begin{enumerate}
|
||||
%\item An $^{241}\textrm{Am}$ alpha source: the main alpha
|
||||
%particles have energies of 5.484~\mega\electronvolt\ (85.2\%) and
|
||||
%5.442~\mega\electronvolt\ (12.5\%). The source emits 79.5
|
||||
%$\alpha\per\second$ in 2$\pi$~\steradian.
|
||||
%\item Test pulse with a fixed amplitude: the preamplifiers used for the
|
||||
%silicon detectors are come with the manufacturer's specification on the
|
||||
%response, namely a 66 \milli\volt\ fed into the test input will produce an
|
||||
%output equivalent to that of a 1 \mega\electronvolt\ energy deposition.
|
||||
%\item Minimum ionisation particles
|
||||
%(MIPs): electrons in the beam are MIPs with a nominal deposit energy of
|
||||
%388~\electronvolt\per\micro\meter\ Si. This is only applicable for thick
|
||||
%silicon detectors because the energy deposit is large enough and the energy
|
||||
%resolution is good enough. During the run, this peak was observed to make
|
||||
%sure the stability of the electronics.
|
||||
%\item Muons with different momenta: the thick silicon detectors were placed
|
||||
%at the target position during beam tuning period, allowed an accurate
|
||||
%momentum and range calibration. This also only works with thick silicon
|
||||
%detectors.
|
||||
%\end{enumerate}
|
||||
% subsection silicon_detectors (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
\subsection{Upstream counters}
|
||||
\label{sub:upstream_counters}
|
||||
The upstream detector consists of three counters: a 500~$\mu$m thick
|
||||
scintillator muon trigger counter ($\mu$SC); a muon anti-coincidence counter
|
||||
($\mu$SCA) surrounding the trigger counter with a hole
|
||||
of 35 \milli\meter\ in diameter to define the beam radius; and a multi-wire
|
||||
proportional chamber ($\mu$PC) that uses 24 X wires and 24 Y wires at
|
||||
2~\milli\meter~intervals.
|
||||
|
||||
The upstream detectors provide signal of an incoming muon as coincident hits on
|
||||
the muon trigger and the wire chamber in anti-coincident with the muon
|
||||
anti-coincidence counter.
|
||||
This set of detectors along with their read-out system
|
||||
belong to the MuSun experiment, which operated at the same beam line just
|
||||
before our run. Thanks to the MuSun group, the detectors were well-tuned and
|
||||
ready to be used in our run without any modification.
|
||||
|
||||
% subsection upstream_counters (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
|
||||
\subsection{Germanium detector}
|
||||
%\begin{figure}[htbp]
|
||||
%\centering
|
||||
%\includegraphics[width=0.9\textwidth]{figs/neutron.png}
|
||||
%\caption{Setup of two
|
||||
%liquid scintillators outside the vacuum envelope for neutron detection.}
|
||||
%\label{fig:neutron}
|
||||
%\end{figure}
|
||||
We used a germanium detector to normalise the number of stopped muons by
|
||||
measuring characteristics muon X-rays from the target material. The primary
|
||||
X-rays of interest are the 346.828~keV line for aluminium targets, and the
|
||||
400.177 line for silicon targets. The energies and intensities of the X-rays
|
||||
listed in Table~\ref{tab:xray_ref} follow measurement results from
|
||||
Measday and colleagues~\cite{MeasdayStocki.etal.2007}.
|
||||
\begin{table}[htb]
|
||||
\begin{center}
|
||||
\begin{tabular}{c l l l l }
|
||||
\toprule
|
||||
\textbf{Elements} & \textbf{Transition}
|
||||
& \textbf{Energy} & \textbf{Intensity}\\
|
||||
\midrule
|
||||
$^{27}\textrm{Al}$ & $2p-1s$ & $346.828 \pm 0.002$ & $79.8\pm 0.8$\\
|
||||
& $3p-1s$ & $412.87 \pm 0.05$ & $7.62\pm 0.15$\\
|
||||
\midrule
|
||||
$^{28}\textrm{Si}$ & $2p-1s$ & $400.177 \pm 0.005$ & $80.3\pm 0.8$\\
|
||||
& $3p-1s$ & $476.80 \pm 0.05$ & $7.40 \pm 0.20$\\
|
||||
\bottomrule
|
||||
\end{tabular}
|
||||
\end{center}
|
||||
\caption{Reference values of major muonic X-rays from aluminium and silicon.}
|
||||
\label{tab:xray_ref}
|
||||
\end{table}
|
||||
|
||||
The germanium detector is
|
||||
a GMX20P4-70-RB-B-PL, n-type, coaxial high purity germanium detector produced
|
||||
by ORTEC. The detector was optimised for low energy gamma and X-rays
|
||||
measurement with an ultra-thin entrance window of 0.5-mm-thick beryllium and
|
||||
a 0.3-\micron-thick ion implanted contact (Figure~\ref{fig:ge_det_dimensions}).
|
||||
This detector is equipped with a transistor reset preamplifier which,
|
||||
according to the producer, enables it to work in an ultra-high rate environment
|
||||
up to $10^6$ counts\per\second~ at 1~\mega\electronvolt.
|
||||
\begin{figure}[htb]
|
||||
\centering
|
||||
\includegraphics[width=0.9\textwidth]{figs/ge_det_dimensions}
|
||||
\caption{Dimensions of the germanium detector}
|
||||
\label{fig:ge_det_dimensions}
|
||||
\end{figure}
|
||||
|
||||
The detector was installed outside of the vacuum chamber at 32 cm from the
|
||||
target, seeing the target through a 10-mm-thick aluminium window, behind
|
||||
a plastic scintillator counter used to veto electrons. Liquid nitrogen
|
||||
necessary for the operation of the detector had to be refilled every 8 hours.
|
||||
A timer was set up in the data acquisition system to remind this.
|
||||
|
||||
\subsection{Plastic and liquid scintillators}
|
||||
\label{sub:plastic_scintillators}
|
||||
Apart from the scintillators in the upstream group, there were four other
|
||||
plastic scintillators used as veto counters for:
|
||||
\begin{itemize}
|
||||
\item punch-through-the-target muons, ScVe
|
||||
\item electrons and other high energy charged particles for germanium
|
||||
detector (ScGe) and silicon detectors (ScL and ScR)
|
||||
\end{itemize}
|
||||
The ScL, ScR and ScVe were installed inside the vacuum vessel and were
|
||||
optically connected to external PMTs by light-guides at the bottom flange.
|
||||
|
||||
We also set up two liquid scintillation counters for neutron measurements in
|
||||
preparation for the next beam time where the neutron measurements will be
|
||||
carried out.
|
||||
% subsection plastic_scintillators (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
\section{Front-end electronics and data acquisition system}
|
||||
The front-end electronics of the AlCap experiment was simple since we employed
|
||||
a trigger-less read out system with waveform digitisers and flash ADCs
|
||||
(FADCs). As shown in Figure~\ref{fig:alcapdaq_scheme}, all plastic
|
||||
scintillators signals were amplified by PMTs, then fed into the digitisers. The
|
||||
signals from silicon and germanium detectors were preamplified, and
|
||||
subsequently shaped by spectroscopy amplifiers and timing filter amplifiers
|
||||
(TFAs) to provide energy and timing information.
|
||||
\begin{figure}[htbp]
|
||||
\centering
|
||||
\includegraphics[width=0.99\textwidth]{figs/alcapdaq_scheme}
|
||||
\caption{Schematic diagram of the electronics and DAQ used in the Run 2013}
|
||||
\label{fig:alcapdaq_scheme}
|
||||
\end{figure}
|
||||
|
||||
The germanium detector has its own transistor reset preamplifier
|
||||
installed very close to the germanium crystal. Two ORTEC Model 142
|
||||
preamplifiers were used for the thick silicon detectors. The timing outputs of
|
||||
the preamplifiers were fed into three ORTEC Model 579 TFAs.
|
||||
We used an ORTEC Model 673 to shape the germanium signal with 6~\micro\second
|
||||
shaping time.
|
||||
|
||||
A more modern-style electronics was used for thin silicon detectors where the
|
||||
preamplifier, shaping and timing amplifiers were implemented on one compact
|
||||
package, namely a Mesytec MSI-8 box. This box has 8 channels, each channel
|
||||
consists of one preamplifier board and one shaper-and-timing filter board which
|
||||
can be fine-tuned independently. The shaping time was set to 1~\micro\second\
|
||||
for all channels.
|
||||
|
||||
The detector system produced signals that differs significantly in time scale,
|
||||
ranging from very fast (about 40~\nano\second\ from scintillators) to very slow
|
||||
(several \micro\second\ from shaping outputs of semiconductor detectors). This
|
||||
lead to the use of several sampling frequencies from 17~\mega\hertz\ to
|
||||
250~\mega\hertz, and three types of digitisers were employed:
|
||||
\begin{itemize}
|
||||
\item custom-built 12-bit 170-MHz FADCs which was designed for the
|
||||
MuCap experiment. Each FADC board has dimensions the same as those of
|
||||
a single-width 6U VME module, but is hosted in a custom built crate due to
|
||||
its different power supply mechanical structure. The FADC communicates with
|
||||
a host computer through a 100-Mb/s Ethernet interface using a simple
|
||||
Ethernet-level protocol. The protocol only allows detecting
|
||||
incomplete data transfers but no retransmitting is possible due to the
|
||||
limited size of the module's output buffer. The FADCs accept clock signal
|
||||
at the frequency of 50~\mega\hertz\ then multiply that internally up to
|
||||
170~\mega\hertz. Each channel on one board can run at different sampling
|
||||
frequency not dependent on other channels. The FADC has 8 single-ended
|
||||
LEMO inputs with 1~\volt pp dynamic range.
|
||||
\item a 14-bit 100-MS/s CAEN VME FADC waveform digitiser model V1724. The
|
||||
module houses 8 channels with 2.25~Vpp dynamic range on single-ended MCX
|
||||
coaxial inputs. The digitiser features an optical link for transmission of
|
||||
data to its host computer. All of 8 channels run at the same sampling
|
||||
frequency and have one common trigger.
|
||||
\item a 12-bit 250-MS/s CAEN desktop waveform digitizer model DT5720. This
|
||||
digitiser is similar to the V1724, except for its form factor and maximum
|
||||
sampling frequency. Although there is an optical link available, the module
|
||||
is connected to its host computer through a USB 2.0 interface where data
|
||||
transfer rate of 30 MB/s was determined to be good enough in our run
|
||||
(actual data rate from this digitiser was typically about 5 MB/s during the
|
||||
run). Communication with both CAEN digitisers was based on CAEN's
|
||||
proprietary binary drivers and libraries.
|
||||
\end{itemize}
|
||||
All digitisers were driven by external clocks which were derived from the same
|
||||
500-\mega\hertz\ master clock, a high precision RF signal generator Model SG382
|
||||
of Stanford Research System.
|
||||
|
||||
The silicon detectors were read out by FADC boards feature network-based data
|
||||
readout interface. To maximize the data throughput, each of the four FADC
|
||||
boards was read out through separate network adapter.
|
||||
The CAEN digitisers were used to read out
|
||||
the germanium detector (timing and energy, slow signals) or scintillator
|
||||
detectors (fast signals). For redundancy, all beam monitors ($\mu$SC, $\mu$SCA
|
||||
and $\mu$PC) were also read out by a CAEN time-to-digital converter (TDC)
|
||||
model V767 which was kindly provided by the MuSun experiment.
|
||||
|
||||
The Data Acquisition System (DAQ) of the AlCap experiment, so-called AlCapDAQ,
|
||||
provided the readout of front-end electronics, event assembling, data logging,
|
||||
hardware monitoring and control, and the run database of the experiment
|
||||
(Figure~\ref{fig:alcapdaq_pcs}). It was based on MIDAS framework~\footnote{
|
||||
MIDAS is a general purpose DAQ software system developed at PSI and TRIUMF:\\
|
||||
\url{http://midas.triumf.ca}} and consisted of two circuits, {\em i})
|
||||
a detector circuit for synchronous data readout from the front-end electronics
|
||||
instrumenting detectors, and {\em ii}) a slow control circuit for asynchronous
|
||||
periodic hardware monitoring (vacuum, liquid nitrogen
|
||||
filling). The detector circuit consisted of three computers, two front-end
|
||||
computers and one computer serving both as a front-end and as a back-end
|
||||
processor. The slow circuit consisted of one computer. All computers were
|
||||
running Linux operating system and connected into a private subnetwork.
|
||||
|
||||
%\hl{TODO: storage and shift monitor}
|
||||
|
||||
\begin{figure}[htb]
|
||||
\centering
|
||||
\includegraphics[width=0.95\textwidth]{figs/alcapdaq_pcs}
|
||||
\caption{AlCapDAQ in the Run 2013. The {\ttfamily fe6} front-end is
|
||||
a VME single board computer belongs to the MuSun group, reads out the
|
||||
upstream detectors.}
|
||||
\label{fig:alcapdaq_pcs}
|
||||
\end{figure}
|
||||
|
||||
The data were collected as dead-time-free time segments of 110~ms, called
|
||||
``block'', followed by about 10-ms-long time intervals used to complete data
|
||||
readout and synchronize the DAQ. Such data collection approach was chosen to
|
||||
maximize the data readout efficiency. During each 110-ms-long period, signals
|
||||
from each detector were digitized independently by threshold crossing. The data
|
||||
segment of each detector data were first written into on-board memories of
|
||||
front-end electronics and either read out in a loop (CAEN TDCs and CAEN
|
||||
digitizers) or streamed (FADCs) into the computer memories. The thresholds were
|
||||
adjusted as low as possible and individually for each detector. The time
|
||||
correlation between detectors would be established in the analysis stage.
|
||||
|
||||
At the beginning of each block, the time counter in each digitiser is reset to
|
||||
ensure time alignment across all modules. The period of 110~ms was chosen to be:
|
||||
{\em i} long enough compares to the time scale of several \micro\second\ of the
|
||||
physics of interest, {\em ii} short enough so that there is no timer rollover
|
||||
on any digitiser (a FADC runs at its maximum speed of 170~\mega\hertz\ could
|
||||
handle up to about 1.5 \second\ with its 28-bit time counter).
|
||||
|
||||
To ease the task of handling data, the data collecting period was divided into
|
||||
short runs, each run stopped when the logger had recorded 2 GB of data.
|
||||
The data size effectively made each run last for about 5 minutes. The DAQ
|
||||
automatically starts a new run with the same parameters after about 6 seconds.
|
||||
The short period of each run also allows the detection, and helps to reduce the
|
||||
influence of effects such as electronics drifting, temperature fluctuation.
|
||||
|
||||
\section{Data sets and statistics}
|
||||
\label{sec:data_sets}
|
||||
|
||||
The main goal of this Run 2013 was to measure the rates and energy spectra of
|
||||
protons following muon capture on aluminium. Also for normalisation and cross
|
||||
checking against the existing experimental data, two types of measurements with
|
||||
different targets were carried out for silicon targets:
|
||||
\begin{itemize}
|
||||
\item[(a)] an active, thick target similar to the set up
|
||||
used by Sobottka and Wills~\cite{SobottkaWills.1968}. This provides
|
||||
a cross-check against the existing experimental data. The silicon detector
|
||||
package at the right hand side was moved to the target position with the
|
||||
thick detector facing the muon beam in this set up.
|
||||
\item[(b)] a passive, thin target and heavy charged particles were observed
|
||||
by the two silicon packages. The measurement serves multiple purposes:
|
||||
confirmation that the particle identification by dE/dx actually works,
|
||||
separation of components of heavy charged particles emitted from the
|
||||
silicon target.
|
||||
\end{itemize}
|
||||
|
||||
As the emitted protons deposit a significant amount of energy in the target
|
||||
material, thin targets and thus excellent momentum resolution of the low energy
|
||||
muon beam are critical. Aluminium targets of 50-\micro\meter\ and
|
||||
100~\micron\ thick were used. Although a beam with low momentum spread of
|
||||
1\% is preferable, it was used for only a small portion of the run due to the
|
||||
low beam rate (see Figure~\ref{fig:Rates}). The beam momentum for each target
|
||||
was chosen to maximise the number of stopped muons. The collected data sets are
|
||||
shown in Table~\ref{tb:stat}.
|
||||
|
||||
\begin{table}[htb!]
|
||||
\begin{center}
|
||||
\vspace{0.15cm}
|
||||
\begin{tabular}{l c c c}
|
||||
\toprule
|
||||
\textbf{Target} &\textbf{Momentum} & \textbf{Run time} & \textbf{Number}\\
|
||||
\textbf{and thickness}&\textbf{scaling factor} & \textbf{(h)} &\textbf{of muons}\\
|
||||
\midrule
|
||||
Si 1500 \micro\meter& 1.32& 3.07& $2.78\times 10^7$\\
|
||||
& 1.30& 12.04& $2.89 \times 10^8$\\
|
||||
& 1.10& 9.36& $1.37 \times 10^8$ \\
|
||||
\midrule
|
||||
Si 62 \micro\meter & 1.06& 10.29& $1.72 \times 10^8$\\
|
||||
\midrule
|
||||
Al 100 \micro\meter& 1.09& 14.37&$2.94 \times 10^8$\\
|
||||
& 1.07& 2.56& $4.99 \times 10^7$\\
|
||||
\midrule
|
||||
Al 50 \micro\meter m & 1.07& 51.94& $8.81 \times 10^8$\\
|
||||
\bottomrule
|
||||
\end{tabular}
|
||||
\end{center}
|
||||
\caption{Run statistics. Momentum scaling
|
||||
normalized to 28 MeV/c.}
|
||||
\label{tb:stat}
|
||||
\end{table}
|
||||
|
||||
% section data_sets (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
\section{Analysis framework}
|
||||
\subsection{Concept}
|
||||
\label{sub:concept}
|
||||
Since the AlCapDAQ is a trigger-less system, it stored all waveforms of the
|
||||
hits occured in 100-ms-long blocks without considering their physics
|
||||
significance The analysis code therefore must be able to extract parameters of
|
||||
the waveforms, then organises the pulses into physics events correlated to
|
||||
stopped muons (Figure~\ref{fig:muon_event}). In addition, the analyser is
|
||||
intended to be usable as a real-time component of a MIDAS DAQ, where simple
|
||||
analysis could be done online for monitoring and diagnostic during the run.
|
||||
|
||||
\begin{figure}[htb]
|
||||
\centering
|
||||
\includegraphics[width=0.9\textwidth]{figs/muon_event.pdf}
|
||||
\caption{Concept of the AlCap analysis code: pulses from individual detector
|
||||
in blocks of time are analysed, then sorted centred around stopped muons.}
|
||||
\label{fig:muon_event}
|
||||
\end{figure}
|
||||
|
||||
The analysis framework of the AlCap consists of two separate programs.
|
||||
A MIDAS-based analyser framework, \alcapana{}, processes the raw data and
|
||||
passes its ROOT data output to a second
|
||||
stage, \rootana{}, where most of the physics analysis is performed.
|
||||
Both programs were designed to be modularised, which allowed us to develop
|
||||
lightweight analysis modules that were used online to generate plots quickly,
|
||||
while more sophisticated modules can be applied in offline analysis.
|
||||
|
||||
The DAQ system generated MIDAS files which stores the data as a stream of MIDAS
|
||||
``banks''. In the AlCapDAQ, each bank corresponds to a single channel on
|
||||
a digitizer and was named according to a predefined convention. The map between
|
||||
detector channels and MIDAS bank names was stored in the MIDAS online database
|
||||
(ODB), along with other settings such as sampling frequencies, timing offsets,
|
||||
thresholds and calibration coefficients of each channel.
|
||||
%These can then be
|
||||
%accessed by both \alcapana{} and \rootana{} for either online or offline
|
||||
%analysis.
|
||||
|
||||
The first step
|
||||
of the analysis framework is to convert the raw MIDAS data into waveforms,
|
||||
series of digitised samples continuous in time corresponding to pulses from the
|
||||
detector. The waveform is called \tpulseisland{}s, which contain the bank name,
|
||||
the ADC values of the samples and the time stamp of the first sample. This
|
||||
conversion is performed in \alcapana{} and the resulting objects are stored in
|
||||
a ROOT output file as a {\ttfamily TTree}.
|
||||
|
||||
The next step of the analysis is to obtain summary parameters of the pulses
|
||||
from the digitized samples. The parameters of primary interest are the
|
||||
amplitude and time of the peak and the integral of the pulse. This extraction
|
||||
of parameters is done by a \rootana{} module, and the objects produced by this
|
||||
stage are called \tanalysedpulse{}s. Currently, we have a usable and simple
|
||||
algorithm that takes the pulse parameters from the peak of the waveform. In
|
||||
parallel, a pulse finding and template fitting code is being developed because
|
||||
it would provide more accurate pulse information. The first iteration of this
|
||||
code has been completed and is being tested.
|
||||
\begin{figure}[htb]
|
||||
\centering
|
||||
\includegraphics[width=0.85\textwidth]{figs/analysis_scheme}
|
||||
\caption{Concept of the analysis framework in \rootana{}}
|
||||
\label{fig:rootana_scheme}
|
||||
\end{figure}
|
||||
|
||||
After obtaining pulse parameters for individual channel, the pairing up of
|
||||
fast and slow pulses from the same physical detector needs to be done. This
|
||||
entails looping through all fast and slow pulses from each detector,
|
||||
checking for correlated pulses in time and amplitude, creating
|
||||
{\ttfamily TDetectorPulse}s. The {\ttfamily TDetectorPulse}s allow better
|
||||
understanding of the hits on the detector by combining timing information from
|
||||
the fast channel and amplitude information from the slow channel. It also helps
|
||||
reduce the impact of pile-up on the amplitude measurement, where the
|
||||
improved time resolution of the fast channels can be used to separate the
|
||||
overlapping amplitudes in the slow channels. The pulse pairing are applicable to
|
||||
the silicon and germanium channels only. The scintillator channels provide only
|
||||
fast timing signals which can be used as {\ttfamily TDetectorPulse}s directly.
|
||||
|
||||
The detector pulses are subsequently used to identify particles that hit the
|
||||
detectors. These particle hits are still stored in the time-ordered tree
|
||||
corresponds to the 110 ms block length from the AlCapDAQ. By iterating through
|
||||
the tree to find stopped muons and taking any hits within a certain window
|
||||
around this muon from every detector, a stopped-muon-centred tree shown in
|
||||
Figure~\ref{fig:muon_event} can be produced. This will make it much easier to
|
||||
look for coincidences and apply cuts, thereby bringing the end
|
||||
goal of particle numbers and energy distributions.
|
||||
|
||||
% subsection concept (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
|
||||
\subsection{Online analyser}
|
||||
\label{sub:online_analyser}
|
||||
The online analyser was developed and proved to be very useful during the run.
|
||||
A few basic modules were used to produce plots for diagnostic purposes
|
||||
including: persistency view of waveforms, pulse height
|
||||
spectra, timing correlations with respect to the upstream counters. The
|
||||
modules and their purposes are listed in Table~\ref{tab:online_modules}.
|
||||
\begin{table}[htb]
|
||||
\begin{center}
|
||||
\begin{tabular}{l p{6cm}}
|
||||
\toprule
|
||||
\textbf{Module name} & \textbf{Functions}\\
|
||||
\midrule
|
||||
common/MUnCompressRawData & decompress raw MIDAS data\\
|
||||
\midrule
|
||||
FADC/MOctalFADCProcessRaw & \multirow{3}{6cm}{convert raw data to
|
||||
{\ttfamily TPulseIsland}s}\\ v1724/MV1724ProcessRaw& \\
|
||||
dt5720/MDT5720ProcessRaw&\\
|
||||
\midrule
|
||||
muSC\_muPC/MCaenCompProcessRaw& \multirow{4}{6cm}{decompress data from
|
||||
{\ttfamily fe6}, make coincidence in upstream counters} \\
|
||||
muSC\_muPC/MMuPC1AnalysisC&\\
|
||||
muSC\_muPC/MMuPC1AnalysisMQL&\\
|
||||
muSC\_muPC/MMuSCAnalysisMQL&\\
|
||||
\midrule
|
||||
diagnostics/MCommonOnlineDisplayPlots& produce plots of interest\\
|
||||
\midrule
|
||||
FADC/MOctalFADCBufferOverflow& \multirow{2}{6cm}{diagnostics for FADCs}\\
|
||||
FADC/MOctalFADCPacketLoss&\\
|
||||
\midrule
|
||||
common/MExpectedIslands&\multirow{4}{6cm}{diagnostics in general}\\
|
||||
common/MMuSCTimeDifferences&\\
|
||||
common/MNumberIslands&\\
|
||||
common/MPulseLengths&\\
|
||||
\midrule
|
||||
common/MTreeOutput& save {\ttfamily TPulseIsland}s tree\\
|
||||
\bottomrule
|
||||
\end{tabular}
|
||||
\end{center}
|
||||
\caption{Online analysis modules in the Run 2013.}
|
||||
\label{tab:online_modules}
|
||||
\end{table}
|
||||
|
||||
The \alcapana{} served the plots on port 9090 of the {\ttfamily abner}
|
||||
via the ROOT socket protocol. We then used a ROOT-based program called
|
||||
{\ttfamily online-display} to display the plots on the shift terminal
|
||||
({\ttfamily alcap}). The {\ttfamily online-display} simply executed ROOT macros
|
||||
which retrieved plots from the ROOT server, sorted then drew them in
|
||||
groups such as upstream counters, silicon arms. It could also periodically
|
||||
update the plots to reflect real-time status of the detector system.
|
||||
%Screen
|
||||
%shots of the {\ttfamily online-display} with several plots are shown in
|
||||
%Figure~\ref{fig:online_display}.
|
||||
|
||||
%\hl{Screen shots}
|
||||
\subsection{Offline analyser}
|
||||
\label{sub:offline_analyser}
|
||||
Some offline analysis modules has been developed during the beam time and could
|
||||
provide quick feedback in confirming and guiding the decisions at the time. For
|
||||
example, the X-ray spectrum analysis was done to confirm that we could observe
|
||||
the muon capture process (Figure~\ref{fig:muX}), and to help in choosing optimal
|
||||
momenta which maximised the number of stopped muons.
|
||||
\begin{figure}[htbp]
|
||||
\centering
|
||||
\includegraphics[width=0.7\textwidth]{figs/muX.png}
|
||||
\caption{Germanium
|
||||
detector spectra in the range of 300 - 450 keV with different setups: no
|
||||
target, 62-\micron-thick silicon target, and 100-\micron-thick aluminium
|
||||
target. The ($2p-1s$) lines from
|
||||
aluminium (346.828 keV) and silicon (400.177 keV) are clearly visible,
|
||||
the double peaks at 431 and 438 keV are from the lead shield, the peak at
|
||||
351~keV is a background gamma ray from $^{211}$Bi.}
|
||||
\label{fig:muX}
|
||||
\end{figure}
|
||||
|
||||
Although the offline analyser is still not fully developed yet, several modules
|
||||
are ready. They are described in detailed in the next chapter.
|
||||
% subsection offline_analyser (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
% section analysis_strategy (end)
|
||||
|
||||
|
||||
% chapter the_alcap_run_2013 (end)
|
||||
|
||||
1233
thesis2/chapters/chap6_analysis.tex
Normal file
74
thesis2/chapters/chap7_results.tex
Normal file
@@ -0,0 +1,74 @@
|
||||
\chapter{Discussions}
|
||||
\label{cha:discussions}
|
||||
|
||||
\section{Thick aluminium target measurement}
|
||||
\label{sub:active_target_measurement}
|
||||
With a thick and active silicon target, I have tried to reproduce an existing
|
||||
result from Sobottka and Wills~\cite{SobottkaWills.1968}. This is important in
|
||||
giving confidence in our experimental method. The idea is the same as that of
|
||||
the old measurement, where muons were stopped inside a bulk active target and
|
||||
the capture products were measured. Due to the limitation of the
|
||||
currently available analysis tool, a direct comparison with the result of
|
||||
Sobottka and Wills is not practical at the moment.
|
||||
|
||||
But a partial comparison is available for a part of the spectrum from 8 to
|
||||
10~MeV, where my result of $(1.22 \pm 0.19) \times 10^{-2} $ is consistent with
|
||||
the derived value $(1.28\pm0.19)\times10^{-2}$ from the paper of Sobottka and
|
||||
Wills. The agreement was partly because of large error bars in both results.
|
||||
In my part, the largest error came from the uncertainty on choosing the
|
||||
integration window. This can be solved with a more sophisticated pulse
|
||||
finding/calculating algorithm so that the contribution of muons in the energy
|
||||
spectrum can be eliminated by imposing a cut in pulse timing. The
|
||||
under-testing pulse template fitting module could do this job soon.
|
||||
|
||||
The range of 8--10~MeV was chosen to be large enough so that the uncertainty of
|
||||
integration window would not to be too great; and at the same time be small
|
||||
enough so the protons (and other heavier charged particles) would not escape
|
||||
the active target. This range is also more convenient for calculating the
|
||||
partial rate from the old paper of Sobottka and Wills.
|
||||
|
||||
% section protons_following_muon_capture_on_silicon (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
|
||||
\section{Thin silicon target measurement}
|
||||
\label{sub:thin_and_passive_target_measurement}
|
||||
The charged particles in the low energy region of 2.5--8~MeV were measured by
|
||||
dE/dx method. The particle identification was good in lower energy part, but
|
||||
losing its resolution power as energy increases. The current set up could do
|
||||
the PID up to about 8~MeV for protons. This energy range is exactly the
|
||||
relevant range to the COMET experiment (Figure~\ref{fig:proton_impact_CDC}).
|
||||
|
||||
In that useful energy range, the analysis showed a good separation of protons
|
||||
from other heavy charged particles. The contribution of protons in the total
|
||||
charged particles is 87\%. This is the high limit only since the heavier
|
||||
particles at this energy range are most likely to stopped in the thin
|
||||
detectors. More statistic would be needed to estimate the contributions from
|
||||
other particles.
|
||||
|
||||
The effective emission rate of protons per muon capture in this measurement is
|
||||
4.20\%, with a large uncertainty contribution comes from limitation of the
|
||||
timing determination. The spectral integral in the region 2.5--8~MeV on
|
||||
Figure~\ref{fig:sobottka_spec} is about 70\% of the spectrum from 1.4 to
|
||||
26~\MeV, and corresponds to an emission rate of about 10\% per muon capture.
|
||||
The two figures are not in disagreement.
|
||||
|
||||
In order to have a better comparison, a correction or unfolding for energy
|
||||
loss and more MC simulation study are needed. I am on progress of these study.
|
||||
|
||||
% subsection thin_and_passive_target_measurement (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
\section{Aluminium target measurement}
|
||||
\label{sec:aluminium_target_measurement}
|
||||
The proton emission rate was derived as 2.37\%, but the problem on the SiL1-1
|
||||
channel was not solved yet. One possible cause is the muons captured on other
|
||||
lighter material inside the chamber. More investigation will be made on this
|
||||
matter.
|
||||
|
||||
The rate of 2.37\% on aluminium appears to be smaller on that of silicon but
|
||||
the two results are both effective rates, modified by energy loss inside the
|
||||
target. Unfolding and MC study for the correction are ongoing.
|
||||
% section aluminium_target_measurement (end)
|
||||
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
||||
% chapter discussions (end)
|
||||
|
||||
|
||||
68
thesis2/chapters/frontmatter.tex
Normal file
@@ -0,0 +1,68 @@
|
||||
%% Title
|
||||
\titlepage[\vspace{5mm}of Department of Physics,\\
|
||||
Graduate School of Science]%
|
||||
{A dissertation submitted to the Osaka University\\
|
||||
for the degree of Doctor of Philosophy}
|
||||
|
||||
%% Abstract
|
||||
\begin{abstract}%[\smaller \thetitle\\ \vspace*{1cm} \smaller {\theauthor}]
|
||||
%\thispagestyle{empty}
|
||||
COMET [1] is an experiment that aims to search for a charged lepton flavor
|
||||
violation (CLFV) process, the muon-to-electron conversion in the presence of
|
||||
a nucleus,
|
||||
\muec. The process is forbidden in the Standard Model (SM), however is
|
||||
predicted to occur in various extensions of SM, such as . Current experimental
|
||||
upper limit of the branching ratio is $BR(\mu^{-} + Au \rightarrow e^{-} + Au)
|
||||
< 7 \times 10^{-13}$, set by the SINDRUM II experiment [2].
|
||||
|
||||
Using the J-PARC proton beam and the pion capture by
|
||||
a solenoidal field, COMET will have a single event sensitivity 10,000 times
|
||||
better than the current limit. The COMET collaboration has taken a phased
|
||||
approach in which the first phase, COMET Phase-I [3], starts in 2013 and
|
||||
initial data taking in around 2017.
|
||||
|
||||
In order to optimize detector design for the Phase-I, backgrounds from nuclear
|
||||
muon capture are crucial. We have proposed a dedicated experiment , namely
|
||||
AlCap, at PSI, Switzerland to study the backgrounds, including: heavy charged
|
||||
particles, neutrons and photons. The measurements of heavy charged particles
|
||||
have been carried out in the 2013 run and the preliminary analysis results are
|
||||
presented in this thesis.
|
||||
\end{abstract}
|
||||
|
||||
|
||||
%% Declaration
|
||||
\begin{declaration}
|
||||
This dissertation is the result of my own work, except where explicit
|
||||
reference is made to the work of others, and has not been submitted
|
||||
for another qualification to this or any other university.
|
||||
\vspace*{1cm}
|
||||
\begin{flushright}
|
||||
Nam Hoai Tran
|
||||
\end{flushright}
|
||||
\end{declaration}
|
||||
|
||||
|
||||
% Acknowledgements
|
||||
%\begin{acknowledgements}
|
||||
%Of the many people who deserve thanks, some are particularly prominent,
|
||||
%such as my supervisor Professor Yoshitaka Kuno.
|
||||
%\end{acknowledgements}
|
||||
|
||||
|
||||
%% Preface
|
||||
%\begin{preface}
|
||||
%The thesis is about measurements of products of nuclear muon capture on an
|
||||
%aluminum target, which is important for optimization of a tracking detector
|
||||
%of a search for muon to electron conversion, the E21 experiment - so called
|
||||
%COMET - at Japan Proton Accelerator Complex (J-PARC).
|
||||
%\end{preface}
|
||||
|
||||
%% ToC
|
||||
\tableofcontents
|
||||
|
||||
|
||||
%% Strictly optional!
|
||||
%\frontquote{%
|
||||
%Writing in English is the most ingenious torture\\
|
||||
%ever devised for sins committed in previous lives.}%
|
||||
%{James Joyce}
|
||||
56
thesis2/custom_macro.tex
Normal file
@@ -0,0 +1,56 @@
|
||||
\usepackage[]{xspace}
|
||||
\newcommand{\lagr}{\cal{L}}
|
||||
%%% muon decays
|
||||
\newcommand{\mueg}{$\mu^{+} \rightarrow e^{+}\gamma$\xspace}
|
||||
%\newcommand{\mueg}{\APmuon $\rightarrow$ \APelectron\Pphoton}
|
||||
|
||||
%\newcommand{\mueee}{$\mu \rightarrow eee$}
|
||||
\newcommand{\mueee}{$\mu^+ \rightarrow e^+e^+e^+$\xspace}
|
||||
%\newcommand{\meee}{\Pmuon $\rightarrow$ \Pelectron\Pelectron\Pelectron}
|
||||
|
||||
\newcommand{\muenn}{$\mu \rightarrow e \nu \overline{\nu}$\xspace}
|
||||
%\newcommand{\muenn}{\Pmuon $\rightarrow$ \Pelectron \Pnu \APnu}
|
||||
|
||||
|
||||
\newcommand{\muec}{$\mu^{-} N \rightarrow e^{-} N$\xspace}
|
||||
%\newcommand{\muec}{\Pmuon $\rightarrow$ \Pelectron N}
|
||||
|
||||
\newcommand{\muecaz}{$\mu^{-} + N(A,Z) \rightarrow e^{-} + N(A,Z)$\xspace}
|
||||
|
||||
% some limits
|
||||
\newcommand{\sindrumlimit}
|
||||
{$\mathcal{B} (\mu^- + Au \rightarrow e^- +Au) < 7\times 10^{-13}$\xspace}
|
||||
|
||||
\newcommand{\meglimit}
|
||||
{$\mathcal{B}(\mu^+ \rightarrow e^+ \gamma) < 5.7 \times 10^{-13}$\xspace}
|
||||
|
||||
\newcommand{\brmeg}
|
||||
{$\mathcal{B}(\mu^+ \rightarrow e^+ \gamma)$\xspace}
|
||||
|
||||
\newcommand{\micheldecay}{
|
||||
$\mu^- \rightarrow e^- \nu_\mu \overline{\nu}_e$\xspace
|
||||
}
|
||||
\newcommand{\muenng}{
|
||||
$\mu^- \rightarrow e^- \nu_\mu \overline{\nu}_e \gamma$\xspace
|
||||
}
|
||||
\newcommand{\muennee}{
|
||||
$\mu^- \rightarrow e^- \nu_\mu \overline{\nu}_e e^+ e^-$\xspace
|
||||
}
|
||||
|
||||
\newcommand{\mueconv}{$\mu-e$ conversion\xspace}
|
||||
|
||||
%scientific notation,
|
||||
\usepackage[]{xifthen}
|
||||
\newcommand{\sn}[2]
|
||||
{
|
||||
\ifthenelse{\isempty{#1}}
|
||||
{$10^{#2}$\xspace} %if the 1st argument is blank
|
||||
{$#1 \times 10^{#2}$\xspace} %if both exist
|
||||
}
|
||||
|
||||
\newcommand{\cc}{$c$\xspace}
|
||||
|
||||
\newcommand{\rootana}{{\ttfamily rootana}}
|
||||
\newcommand{\alcapana}{{\ttfamily alcapana}}
|
||||
\newcommand{\tpulseisland}{{\ttfamily TPulseIsland}}
|
||||
\newcommand{\tanalysedpulse}{{\ttfamily TAnalysedPulse}}
|
||||
BIN
thesis2/figs/GroupPhoto.jpg
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|
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BIN
thesis2/figs/Measurements.pptx
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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6811
thesis2/figs/comet_full_and_phase1.eps
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|
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BIN
thesis2/figs/comet_mr_4filled.png
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|
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thesis2/figs/comet_phase1_cydet.png
Normal file
|
After Width: | Height: | Size: 596 KiB |
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thesis2/figs/comet_phase1_layout.png
Normal file
|
After Width: | Height: | Size: 483 KiB |
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thesis2/figs/comet_phase1_magnets.png
Normal file
|
After Width: | Height: | Size: 146 KiB |
BIN
thesis2/figs/comet_target_Bfield.png
Normal file
|
After Width: | Height: | Size: 88 KiB |
BIN
thesis2/figs/daq.png
Normal file
|
After Width: | Height: | Size: 320 KiB |
BIN
thesis2/figs/daq_pcs.png
Normal file
|
After Width: | Height: | Size: 292 KiB |
BIN
thesis2/figs/ge_det_dimensions.png
Normal file
|
After Width: | Height: | Size: 250 KiB |
BIN
thesis2/figs/ge_eu152_spec.png
Normal file
|
After Width: | Height: | Size: 164 KiB |
BIN
thesis2/figs/ishii_cal_alpha.png
Normal file
|
After Width: | Height: | Size: 34 KiB |
BIN
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||||
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||||
,*serkFBhD:E;4_(lhb^('$o/U!C=@1EoYG&>$H$Ge,1+i^,'PS29!(q.t56s/b@_?qEcrZ
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||||
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||||
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||||
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||||
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|
||||
*IZM`%cW61JLo[q?G^e@4a.,3#*TI)8n7WJh@_1:)O&mX#N7s=D*AUCPLJ[LBf(95opgf0W
|
||||
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|
||||
$nofAhmf2="1olVC/4,gOf4OW0>5^\2#mN2/DNu7JL=SIlDqiATH)e$B"7bqghrue5u$BWY
|
||||
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|
||||
@P)Od'?Cde65;eOPo4Y0=a-4#H^E/b7]+rV@<Y](Igd..R0JQ6JmWlmE#dNtR+\7'=f7^VX
|
||||
.%;/_Q(*ZfNis:YFF4[S/K57=i[&#bFI83SJfnI=kB44gR[$TSf-R[=m)BEl^leuT,I6m=n
|
||||
ePVqk)R@EWQ:~>
|
||||
Q
|
||||
Q Q
|
||||
showpage
|
||||
%%Trailer
|
||||
count op_count sub {pop} repeat
|
||||
countdictstack dict_count sub {end} repeat
|
||||
cairo_eps_state restore
|
||||
%%EOF
|
||||
BIN
thesis2/figs/susy_contr.png
Normal file
|
After Width: | Height: | Size: 37 KiB |
BIN
thesis2/figs/tap_maxbin_algo.png
Normal file
|
After Width: | Height: | Size: 171 KiB |
BIN
thesis2/figs/tme_sir_prompt_rational.png
Normal file
|
After Width: | Height: | Size: 79 KiB |
BIN
thesis2/figs/wyttenbach_rate_1p.png
Normal file
|
After Width: | Height: | Size: 98 KiB |
BIN
thesis2/figs/wyttenbach_rate_23p.png
Normal file
|
After Width: | Height: | Size: 117 KiB |
16
thesis2/getNewBibtex
Executable file
@@ -0,0 +1,16 @@
|
||||
#! /usr/bin/env bash
|
||||
|
||||
if [[ -n $( echo $* | egrep -- "-h\>|--help\>" ) ]]; then
|
||||
echo -e "Usage: $0 <myfile.tex>"
|
||||
echo "NB. \\input and \\include operations will only work in"
|
||||
echo "the current directory, rather than via the full TeX path."
|
||||
exit 0
|
||||
fi
|
||||
|
||||
maintexfile=$1
|
||||
tempfile=texmerge.$$
|
||||
|
||||
cat $maintexfile | sed -e s/'\\input{\([^}]*\)}'/'#include "\1\.tex"'/g > $tempfile
|
||||
#cat $tempfile
|
||||
cpp $tempfile 2> /dev/null | mail -s "generate bibtex" slaclib2@slac.stanford.edu
|
||||
rm -f $tempfile
|
||||
1016
thesis2/h-physrev.bst
Normal file
50
thesis2/mythesis.sty
Normal file
@@ -0,0 +1,50 @@
|
||||
\NeedsTeXFormat{LaTeX2e}
|
||||
\ProvidesPackage{mythesis}
|
||||
|
||||
%% -------------------------------------
|
||||
%% Standard packages
|
||||
%% -------------------------------------
|
||||
\RequirePackage[top=1in, bottom=1in,
|
||||
%inner=1.25in, outer=1in]{geometry}
|
||||
inner=1.25in, outer=1in, twoside]{geometry}
|
||||
\RequirePackage{amsmath}
|
||||
|
||||
\RequirePackage[%
|
||||
colorlinks=true,% color links instead of using boxes
|
||||
linkcolor=red,% color for internal (intra-document) links
|
||||
citecolor=green,% color for bibliographic links
|
||||
urlcolor=blue,% color for URL links
|
||||
hyperfootnotes=false,% disable links to footnotes (because feature broken)
|
||||
hyperindex,% make page numbers in index into hyperlinks
|
||||
pdfstartview={FitH},% startup page view: fit width of page to window
|
||||
bookmarks
|
||||
]{hyperref}
|
||||
\RequirePackage[all]{hypcap}
|
||||
|
||||
\RequirePackage{booktabs}
|
||||
\RequirePackage{cite}
|
||||
%\RequirePackage{morefloats}
|
||||
\RequirePackage{mathrsfs} % script font
|
||||
\RequirePackage{afterpage}
|
||||
%% Using Babel allows other languages to be used and mixed-in easily
|
||||
\RequirePackage[ngerman,english]{babel}
|
||||
\selectlanguage{english}
|
||||
\RequirePackage[babel]{csquotes}
|
||||
|
||||
\RequirePackage{subfig}
|
||||
\RequirePackage{tikz}
|
||||
\RequirePackage{fixltx2e}
|
||||
\RequirePackage{setspace}
|
||||
\RequirePackage{verbatim}
|
||||
\RequirePackage{lipsum}
|
||||
\RequirePackage[]{siunitx}
|
||||
\RequirePackage{tabularx}
|
||||
\RequirePackage{color}
|
||||
\RequirePackage{pifont}
|
||||
\RequirePackage{soul}
|
||||
\RequirePackage[]{dcolumn}
|
||||
\RequirePackage[]{sidecap}
|
||||
\RequirePackage[]{wrapfig}
|
||||
\RequirePackage{feynmp}
|
||||
\RequirePackage[]{afterpage}
|
||||
\RequirePackage[]{multirow}
|
||||
2655
thesis2/thesis.bib
Normal file
36
thesis2/thesis.tex
Normal file
@@ -0,0 +1,36 @@
|
||||
%\documentclass[11pt,a4paper, singlespace,oneside]{book}
|
||||
\documentclass[11pt,a4paper, singlespace,openany]{book}
|
||||
%\usepackage[]{uva-seas-thesis}
|
||||
\usepackage[]{mythesis}
|
||||
\input{custom_macro}
|
||||
|
||||
\usepackage{fancyhdr}
|
||||
\pagestyle{fancy}
|
||||
\fancyhf{} % clear all headers and footers
|
||||
%\renewcommand{\headrulewidth}{0pt} % remove rule between header and text
|
||||
\fancyhead[LE,RO]{\thepage} % put page number in left header on even pages,
|
||||
% right header on odd pages
|
||||
\fancyhead[RE]{\nouppercase{\leftmark}} % remove uppercase on chapter title
|
||||
\renewcommand{\chaptermark}[1]{\markboth{#1}{}} % remove "Chapter N." prefix
|
||||
|
||||
\author{John Doe}
|
||||
\title{Awesome title}
|
||||
|
||||
%\degree{Phd}
|
||||
%\documenttype{thesis}
|
||||
%\program{Physics}
|
||||
%\graduationmonth{9}
|
||||
%\graduationyear{2014}
|
||||
|
||||
\begin{document}
|
||||
\maketitle
|
||||
%\include{chapters/chap2_mu_e_conv}
|
||||
%\input{chapters/frontmatter}
|
||||
\input{chapters/chap1_intro}
|
||||
\input{chapters/chap2_mu_e_conv}
|
||||
\lipsum[1-15]
|
||||
|
||||
\begin{backmatter}
|
||||
\input{chapters/backmatter}
|
||||
\end{backmatter}
|
||||
\end{document}
|
||||