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{\raggedleft December 10, 2012 \\
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{\bf\LARGE Study of Muon Capture for\\\vspace{2mm}
 Muon to Electron  Conversion Experiments}
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\Large
AlCap Collaboration
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%% ML - move this figure to the Intrduction
% \begin{figure}[h]
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% \includegraphics[width=\textwidth]{figs/comet-mu2e.png}
% \caption{Schematic layouts of the Mu2e (left) and the COMET (right).}
% \label{fg:mu2ecomet}
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\begin{center}
%{\tt Preliminary version, final proposal will be uploaded on 12/14/2012}

%{\bf institutional representatives, please check and update}
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D. Alexander\uh,
D.M. Asner\pnnl,
E. Barnes\bu,	 
R. Bernstein\fnal,	 
R. Bonicalzi\pnnl,
A. Daniel\uh,
A. Edmonds\ucl,
A. Empl\uh, 
D. Hertzog\uw,	 
Y. Hino\osaka,	 
E. Hungerford\uh,
T. Itahashi\osaka,	 
D. Kawall\umass,	 
A. Kolakar\bu,	 
B. Krikler\icl,	
\underline{P. Kammel\uw},
K. Kumar\umass,	
\underline{Y. Kuno\osaka},
A. Kurup\icl, 
M. Lancaster\ucl,
J. Miller\bu,	 
M. Murray\uw,	 
T.H. Nam\osaka,	 
V. Ruso\fnal,	 
H. Sakamoto\osaka,	 
A. Sato\osaka,	
M. Schram\pnnl,
Y. Uchida\icl,	
G. Warren\pnnl,
F. Wauters\uw,
P. Winter\anl,	 
M. Wing\ucl,
L. Wood\pnnl


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\anl Argonne National Laboratory, Illinois (ANL)

\bu Boston University, Boston, Massachusetts (BU)

\fnal Fermilab National Accelerator Laboratory, Batavia, Illinois (FNAL)

\uh University of Houston, Houston, Texas (UH)

\icl Imperial College, London, UK (ICL)

\umass University of Massachusetts, Amhurst, Massachusetts (UMass)

\osaka Osaka University, Osaka, Japan (OU)

\pnnl Pacific Northwest National Laboratory, Richland, Washington (PNNL)

\ucl University College London, London, UK (UCL)

\uw University of Washington, Seattle, Washington (UW)

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Co-spokespersons underlined.

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\begin{center}
{\bf Abstract}
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The recent observation that neutrinos oscillate, change flavour, and so have
mass, requires an extension to the Standard Model (SM) and
demonstrates that lepton flavour is 
not an absolutely conserved quantity. However, even when accommodating
finite neutrino mass in a minimal extension
to the SM, the rate of charged lepton flavour
violating (CLFV) interactions is predicted to be far too
small to be observed, ${\cal{O}}(10^{-50})$. Thus, any experimental 
observation of CLFV would be clear evidence of new physics beyond the SM.

Two new projects will search for the CLFV in 
\muec conversion. They are the Mu2e experiment at FNAL and the
COMET experiment at J-PARC. Both experiments utilise multi-kW
pulsed 8$-$9 GeV proton beams to achieve a branching ratio sensitivity lower
than 10$^{-16}$, that is 10,000 times better than the current best
limit established by SINDRUM II.  

Both COMET Phase-I and Mu2e are subject to significant backgrounds
from the products from muon (and pion) nuclear capture. The
goals of  this joint proposal between both Mu2e and COMET
collaborations are precision measurements of muon
capture reactions in candidate targets, at levels required
for the design and optimization of this new generation of experiments.

The initial focus of this proposal is on the measurement of low energy
charged particles after muon capture on a nucleus.  This process 
constitutes a severe background, and has inadequate
experimental information available. X-rays and
gamma-rays will be also observed, in order to verify that this
technique can be used to determine the number of $\mu$ captures in a
$\mu \to e$ experiment, and to
normalize the proposed capture measurements. Finally we propose to
study both neutron emission after $\mu$ capture and radiative decay of
the $\mu$ in the nuclear environment as these also provide crucial
information to be used in the design and simulation of the experiments.\\

PSI beam quality is critical to provide a pure, low-energy muon beam
with a small momentum spread. This is especially important for 
charged-particle emission measurements, as thin targets of 
aluminum, silicon and titanium are required so that the emitted 
particles penetrate the targets with minimal energy loss. 
This was a major problem in previous experiments and has prevented the
extraction of low-energy spectra. 

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