Precision Muon Capture

A new generation of experiments is studying muon capture on hydrogen, deuterium, and3He with significantly improved precision using a novel active-target method. At the same time, chiral effective field theories (EFTs) allow the calculation of low-energy electroweak observables on protons and the simplest nuclei within a rigorous quantum chromodynamics (QCD)-based framework. This experimental and theoretical progress significantly enhances our understanding of the axial current. In the case of the nucleon, the unambiguous experimental determination of the pseudoscalar coupling gPclarifies a long-standing puzzle and provides a basic test of QCD at low energies. In the case of deuterium and3He, the new precision data allow a comprehensive test of EFT calculations of weak few-body reactions and determine an essential low-energy constant relevant for related astrophysics reactions of fundamental interest.