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AlCapPSI/Introduction.tex

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+A neutrino and a charged lepton form a natural doublet in the SM, and
+are assigned a flavor quantum number. The
+observation that neutrinos oscillate between flavours and thus have
+mass, means that lepton number is not absolutely conserved in the
+neutral lepton sector and this in
+turn means that lepton number is also not conserved in charged lepton
+interactions. However, due to the large difference in mass between the
+neutrino and the $W$ boson, the branching ratio for charged lepton
+flavour violation (CLFV) in a minimal extension to the SM which
+includes massive neutrinos, is extremely small
+(${\cal{O}}(10^{-50})$). Thus, any experimental observation of CLFV would be
+unambiguous evidence of new physics beyond the SM (BSM), and 
+particle physics experiments searching for CLFV are amongst those with
+highest priority ~\cite{Kuno:1999jp}. 
+
+Most extensions to the SM predict rates for CLFV that are within reach
+of the next generation of experiments. Indeed current experiments such
+as MEG search for the CLFV process, $\mu^{+} \rightarrow
+e^{+}\gamma$, and already place severe constraints on models of BSM
+physics. It is possible to construct a generic Lagrangian for BSM CLFV
+interactions~\cite{deGouvea} comprising dipole and
+contact interaction terms at a certain mass scale
+($\Lambda$). The $\mu^{+} \rightarrow e^{+}\gamma$ being probed by MEG,
+is mainly sensitive to dipole interactions whereas processes such as
+$\mu^{+} \rightarrow e^{+}e^{-}e^{+}$ and the coherent neutrinoless
+transition of a muon in the field of a nucleus, \muec, are mainly
+sensitive to contact interactions. Thus in order to elucidate the
+nature of any BSM physics it is necessary to make measurements of all
+three processes. In Fig.~\ref{fg:deGouvea} the rates of CLFV are shown
+as a function of $\Lambda$ and the relative contribution of the dipole
+and contact interactions ($\kappa$).  
+ 
+\begin{center} 
+\begin{figure}[htbp]
+\hspace{30mm}
+\includegraphics[width=0.6\textwidth]{figs/deGouvea.png}
+\caption{Branching ratios for two CLFV processes in a generic model of
+BSM physics having both dipole and contact interactions as a function
+of the scale ($\Lambda$) of the new physics.} 
+\label{fg:deGouvea}
+%\vspace{-5mm}
+\end{figure}
+\end{center}
+
+It can be seen from Fig.~\ref{fg:deGouvea} that CLFV processes
+potentially probe new physics at scales beyond the LHC. Dipole and
+contact interactions arise in many models of BSM
+physics. Supersymmetric interactions naturally provide dipole
+interactions which could result in significant rates for both the
+$\mu^{+} \rightarrow e^{+}\gamma$ and \muec processes while  
+contact interaction terms arise in type-II Seesaw models that generate
+light neutrino masses through mixing with an additional massive
+neutrino. In general the Seesaw mechanism occurs at a very high mass scales
+(${\cal{O}}(10^{10-14})$~GeV). CLFV effects are only large if
+additional new physics e.g. supersymmetry or extra dimensions are
+present at the TeV scale.  In general CLFV processes probe
+physics both at the GUT-scale and the
+TeV-scale. Fig.~\ref{fg:deGouvea} also highlights the necessity of
+searching for CLFV in both the $\mu^{+} \rightarrow e^{+}\gamma$ and
+\muec process (and also the $\mu^{+} \rightarrow e^{+}e^{-}e^{+}$
+process) where the ratio of the branching ratios can yield information
+on $\kappa$. 
+
+The last search for \muec conversion was performed by the SINDRUM
+II collaboration at PSI.  The SINDRUM II spectrometer consisted of a
+set of concentric cylindrical drift chambers inside a superconducting
+solenoid magnet of 1.2 Tesla. The experiment set an upper limit of \muec in Au
+of $B(\mu^{-} + Au \rightarrow e^{-} + Au) < 7 \times
+10^{-13}$~\cite{SindrumGold}. Two new searches for 
+CLFV in the \muec process are being pursued by
+experiments under construction in the USA (Mu2e~\cite{mu2e08}) and
+Japan (COMET~\cite{come07}), both of which seek to probe the
+\muec process with a sensitivity better than $\sim
+10^{-16}$. Schematic layouts of the experiments are shown in
+Fig.~\ref{fg:mu2ecomet}. For BSM interactions dominated by dipole
+operators these experiments have a similar sensitivity to the upgraded
+MEG experiment but have a far greater sensitivity to contact
+interactions. 
+
+The Mu2e and COMET experiments are seeking to improve on the SINDRUM
+II sensitivity by a factor of 10,000. Both experiments utilise
+multi-kW pulsed proton beams of energy 8--9~GeV produced by the FNAL
+(Mu2e) and J-PARC (COMET) accelerator complexes. Mu2e received CD1 DOE
+approval in July 2012 and the staged construction of COMET was
+approved in March 2012 and construction of the beamline will begin in
+2013. In the Phase-I COMET experiment, shown in Fig.~\ref{fg:phase1},
+there is a reduced beamline and a cylindrical drift chamber will
+immediately surround the target where the muons are captured.  Thus, it is
+very sensitive to the products of muon (and pion) nuclear capture. In
+Mu2e there is a similar sensitivity since the straw tracking detector
+is immediately downstream of the muon target. 
+
+\begin{center} 
+\begin{figure}[h]
+%\vspace{-40mm}
+\includegraphics[width=\textwidth]{figs/comet-mu2e.png}
+\caption{Schematic layouts of the Mu2e (left) and the COMET Phase-II experiments (right).}
+\label{fg:mu2ecomet}
+%\vspace{-5mm}
+\end{figure}
+\end{center} 
+
+ The $\mu^{-}N \rightarrow e^{-}N$ process is particularly
+attractive since the experimental signature is very straightforward: a
+single mono-energetic electron with an energy of approximately $m_\mu
+- B$ where $B$ is the binding energy of the muon in the 1{\em s} level
+in the muonic atom (N). This single particle signature does not suffer
+from the accidental background which occurs when coincidences are
+required; e.g. between $e^{+}$ and $\gamma$ in the $\mu^{+} \rightarrow
+e^{+}\gamma$ process.  This is a particularly important, limiting 
+background at 
+high muon rates. The energy of the mono-energetic electron from the \muec
+process has far higher energy than in Michel decays, and for $\mu$
+decays in orbit (DIO) the phase space for electron energies
+approaching the endpoint rapidly vanishes. 
+
+In addition to the DIO process or the CLFV process a muon in the 1{\em
+s} state in a muonic atom can be captured by the nucleus  via the
+process: $\mu^{-} + N(A,Z) \rightarrow \nu_{\mu} + N(A,Z-1)$. In
+general this process is also accompanied by the emission of photons,
+neutrons and charged particles (particularly protons) and it is the
+measurement of these emitted particles that is the subject of this
+proposal. In order to optimize $\mu \to e$ experiments, it is important
+to accurately know the rate and energy spectra of these particles, 
+since they can form significant backgrounds, degrade the
+efficiency of electron tracking, and damage the readout electronics. 
+Photons can Compton scatter or convert, 
+producing electrons which cause ambiguities in track reconstruction
+and degrade detector resolutions. In addition to these problems, low
+energy protons can
+saturate the electronic amplifiers due to large energy losses, and
+prematurely ``age'' detector components.
+Neutrons cause recoil protons and can get captured,
+producing photons.  Low-energy neutron cross sections are large and
+are difficult to shield. They can cause significant radiation damage.  
+Monte-Carlo simulations of these nuclear
+capture processes  rely on spectra
+data taken over twenty years ago for a limited number of nuclei in
+a restricted energy range. Low energy energy neutron spectra are
+particularly dependent on the nucleus. The
+motivation for this proposal is to make precision measurements for the
+target materials that will be used by Mu2e and COMET over the entire
+relevant energy region.  
+
+
+
+
+
+\begin{center} 
+\begin{figure}[htbp]
+\hspace{15mm}
+\includegraphics[width=0.8\textwidth]{figs/comet_phase1_tracker.pdf}
+\caption{The COMET Phase-I detector where a cylindrical drift chamber immediately surrounds the muon stopping target and is therefore subject to the products of the nuclear muon capture process.}
+\label{fg:phase1}
+%\vspace{-5mm}
+\end{figure}
+\end{center}
+
+
+
+