The MuSun Experiment: Muon Capture and Chiral Two-body Currents

Muon capture provides a powerful tool to study currents and forces in few-nucleon systems. In effective field theories of two-nucleon weak interactions, a single poorly-known low energy constant enters. The MuSun experiment will extract this quantity from a high precision measurement of the muon capture rate on deuterium, $\mu + d \rightarrow n + n + \nu_{\mu}$, and calibrate related fundamental astrophysics reactions. A unique cryogenic time projection chamber is employed to measure the capture rate as a deviation of the negative muon lifetime in deuterium gas from the free muon lifetime. Analysis of the $10^{10}$ fully reconstructed muon decay events collected over two ten-week beam periods is coming to a close. Advanced analyses to combat the major systematic issues, including muon catalyzed fusion interference, critical background suppression, and gas purity determination, have been developed and applied to the production data of the first beam period. An experimental error budget, overall dataset consistency and the final steps to unblinding will be presented.