%[This is not official and for distribution, as we still have to discuss with TRIUMF management.]

An initial test run for measuring protons emitted from Al targets was performed by a subset of this collaboration
at PSI in 2009. It helped to commission part of the apparatus used for the current proposal. It also showed
deficiencies which helped to define and optimize the set-up proposed here. In particular, the beam 
quality, the X-ray detection, the thin dE/dx detectors, the shielding of stray muons etc. need additional
careful attention.

In 2012, a joint COMET/Mu2e project on  charged particle emission was started, when the high relevance of
proton emission for the new COMET Phase-I idea was realized. Due to the geographical advantage of
Vancouver, as a good crossing point for the Fermilab and J-PARC collaboration, a proposal~\cite{triumf12}
 was submitted to  TRIUMF in summer 2012, and accepted with high priority. The PAC recommendation reads:\\
{\it\bf ``The proposed set of muon capture background measurements are deemed crucial to the design and understanding of future neutrino-less muon-to-electron decay experiments, such as COMET and mu2e. Thus the committee approves this proposal with high scientific priority. 
It is noted that it is not yet clear which muon beamline will be available for this study with the desired negative muon properties. 
Once a suitable beamline has been identified that will be ready and available on the required timescales, the committee recommends that the full 36 shifts be granted.''}

Unfortunately, the above-mentioned availability of a suitable beamline for low energy $\mu^-$ turned out to be a problem.
The allocated test beam times in fall of 2012 could not be realized, as the M9B beamline experienced vacuum problems
at the T2 target station as well as difficulties with the superconducting decay channel. Moreover, TRIUMF cloud beamlines
with better momentum bin essential for this experiment, are not foreseen to be operated with negative muons.
In this situation, the collaboration asked the TRIUMF management to put S1371 on standby.

Meanwhile, PSI built a new low energy muon beam for particle physics, initially for the MuSun experiment. This
beamline was successfully commissioned by the MuSun collaboration, including some members of the present
collaboration. The results were extremely positive. In particular, the beamline appears to be the world-wide 
superior channel for experiments needing muon beam \textit{at low energies with very narrow energy spread}.
This advantage is due to the high proton intensity at PSI, combined with a special cloud muon channel,
which selects the momentum with three bending magnets and thus allows for a dispersive focus.
  The initial commissioning results by the MuSun collaboration promise excellent conditions for the thin
targets anticipated in AlCap. Further beam details are discussed in the MuSun progress report, submitted to this BVR.

