The Mu2e~\cite{mu2e} (FNAL) and COMET~\cite{comet} (J-PARC) experiments seek
to determine the branching
ratio for the charged lepton flavor violating process \muec to better than
10$^{-16}$, which is a factor of 10,000 improvement compared to the current
best limit established by SINDRUM II~\cite{SindrumGold} (PSI). 

The AlCap experiment is a
combined effort of the Mu2e and COMET collaborations to study important
background reactions from muon capture in candidate target materials (Al, Ti),
which are required to optimise the new muon-electron conversion experiments.

In 2013 the AlCap collaboration performed its first run (R2013) at PSI,
focusing on
work package WP1 with preliminary work done on WP2 and WP3. 

The goal of WP1 is to measure the rate and spectrum of
protons emitted after nuclear muon capture since, in both Mu2e and COMET
Phase-I, the single-hit rate of these in the tracking chamber could be
significant. These protons have never been measured in the relevant low energy
regime of 2.5 to 15 MeV.

This progress report presents the status of the analysis of this run
and our beam request for 2015. In short, the program and plans can be
summarised as follows.

\begin{itemize}

\item
\textbf{WP1: Charged Particle Emission after Muon Capture.}
In spite of the commissioning challenges in R2013, the
preliminary analysis presented in the first AlCap PhD. Thesis~\cite{Nam:2014}
led
to the first physics result. The result was both surprising and of high
impact for  
the Mu2e and COMET Phase-I designs. 

The preliminary emission fraction of protons after
muon capture in aluminium was found to be 1.7\%  in the energy range from 
4 MeV to 8 MeV. The total proton emission fraction is estimated to be 3.5\% if
a simple description of the spectral shape holds.

This is much smaller
than the 15\% emission measured in silicon, and the assumed
10-15\% for aluminium which has been the basis
for both designs up-to-now. If this preliminary result holds up, it will
be possible to reduce the thickness of proton absorbers in COMET and Mu2e,
with a corresponding reduction in energy straggling and therefore
improved energy resolution on conversion electron
candidates. Indeed, no proton absorber might be needed at all
for COMET Phase-I~\cite{Nam:2014}.

The goal of the 2015 run is to corroborate these findings in an
upgraded set-up and to extend the measurement to titanium.

\item
\textbf{WP2: Gamma and X-ray Emission after Muon Capture.}


In R2013, the low energy X-ray and gamma ray spectra were measured with a
resolution of about 2 keV using a germanium detector. The measurement
of the number of \atrn{2p}{1s} muonic X-ray transitions provides the number of
stopped muons for the normalisation of the spectra in R2013.

In addition, the full gamma ray spectra can provide a wealth of information
from the peaks associated with muon capture and are being evaluated
for their use as alternative means of monitoring the number of stopped
muons in the full Mu2e and COMET experiments.

%Gamma lines associated with capture on surrounding materials (mainly lead and stainless steel) could interfere with the lines of interest in aluminum. Indeed, we have found one lead gamma line that is close to the Al $2p \to 1s$ muonic X-ray transition and so, in the next run, we will minimize the number of muons stopping in lead. 

For the planned AlCap run, we will improve the measurement of the branching
ratio of an 844 keV delayed gamma ray in aluminium, which is of particular
interest to COMET and Mu2e.
In addition, we will explore the muonic X-rays and gamma rays in titanium, as
well as in any material that will be present in Mu2e and COMET such as lead,
stainless steel, and plastic. For these measurements,
a vacuum chamber is not required and a thicker target can be used compared
to the proton measurements. This
will allow data collection with substantially less background and higher data
rates.

The INFN group will bring a stand-alone $5\times5$ LYSO array
calorimeter which will allow them to parasitically measure the high
energy photon spectrum produced by stopped muons. This will provide
information on the spectrum that can be expected in the Mu2e and COMET
calorimeters, and also will allow an evaluation of the use of high
energy photons for normalisation in Mu2e or COMET.

\item
\textbf{WP3: Neutron Emission after Muon Capture.}
Neutron emission after muon capture,  ie $\mu^-  + A(Z,N)\rightarrow n
 + [A-1](Z-1,N) $   
 is governed by the weak interaction and is similar to electron
 capture on a nucleus with much larger momentum transfer.  The
 distribution of neutron energies with emission greater than 10 MeV
 can explained by a statistical model with effective temperature, $N =
 N_{0} e^{-(E/T)^2}$. However, at lower energies the residual nuclear
 particle-hole states can excite giant dipole resonances, and these
 potentially dominate emission, although present data quality is
 insufficient to build an accurate nuclear physics
 model \cite{Raphael:1967}. Previous emission spectra were obtained on
 selected nuclear targets 30--40 years ago and are of low quality and
 low statistics.   A FLUKA Monte Carlo (MC) simulation of emission
 probabilities predicts the ratio of proton to neutron emission is
 approximately 24\%, with non-negligible multi-neutron emission. While
 the MC was calibrated at CERN in the NTOF experiment, better emission
 data and nuclear models are important. MC codes need more accurate
 low energy data to develop the models.  Neutron emission from the
 stopping target in the Mu2e experiment is an important background to
 be understood and controlled. For example, the MC predicted gamma
 background from neutron capture in the proton attenuation shield
 surrounding the target caused it to be redesigned. Also as the front end
 electronic systems are placed within the detector solenoid, neutron
 induced single-event-upsets (SEU) in the readout electronics requires
 detailed attention. Time-to-failure in memory and logic components
 has shown to be significant and this must be evaluated using accurate
 neutron flux calculations. 

\end{itemize}

