## MuSun abstract

We propose to measure the rate Λ_{d} for muon capture on the deuteron to better than 1.5% precision. This process is the simplest weak interaction process on a nucleus that can both be calculated and measured to a high degree of precision. The measurement will provide a benchmark result, far more precise than any current experimental information on weak interaction processes in the two-nucleon system. Moreover, it can impact our understanding of fundamental reactions of astrophysical interest, like solar pp fusion and the ν + d reactions observed by the Sudbury Neutrino Observatory. Recent effective field theory calculations have demonstrated, that all these reactions are related by one axial two-body current term, parameterized by a single low-energy constant. Muon capture on the deuteron is a clean and accurate way to determine this constant. Once it is known, the above mentioned astrophysical, as well as other important two-nucleon reactions, will be determined in a model independent way at the same precision as the measured muon capture reaction.

At the moment the experimental situation on μ + d capture is inconclusive. An experiment with 10% errors agrees with theory, the most precise measurement with 6.2% uncertainty disagrees by three standard deviations from the best recent calculation, which has 1% uncertainty. If true, such a discrepancy would have major ramnifications on the above mentioned astrophysical processes. The required significant improvement in precision expected with the MuSun experiment became feasible by the advanced techniques developed for the MuCap experiment. As in the case of that experiment, utmost care is required to eliminate uncertainties due to muon atomic physics effects. Thus, while the general experimental strategy is based on MuCap, a new cryogenic TPC operating at gas densities of 5% of LH_{2} at 30 K will be developed to achieve optimal conditions for an unambiguous interpretation of the experiment. The TPC will be filled with ultrapure deuterium and operated as a high resolution ionization chamber. The different physics requirements of the new MuSun experiment demand several upgrades to the MuCap detector, including full analog readout of the cryo-TPC, the monitoring the muon chemistry by charged particle, neutron and γ detection and an advanced D_{2} gas purification system.

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