Induced pseudoscalar coupling of the proton weak interaction

The induced pseudoscalar coupling gₚ is the least well known of the weak couplings of the proton's charged-current interaction. Its size is dictated by chiral symmetry arguments, and its measurement represents an important test of quantum chromodynamics at low energies. Experiments over the past decade have produced a large body of new data relevant to the coupling gₚ. These data include measurements of radiative and nonradiative muon capture on targets ranging from hydrogen and few-nucleon systems to complex nuclei. The authors review the theoretical underpinnings of gₚ, the experimental studies, and the procedures and uncertainties in extracting the coupling from data. They also discuss current puzzles and future opportunities.