Aspects of Pion and Muon Capture.

This dissertation reviews particle, nuclear, and atomic physics aspects of negative pion and muon capture, as illustrated by experiment. Atomic-capture of negative pions was studied in an high precision pionic x-ray measurement. The charged -pion mass was determined to seven parts-per-million uncertainty; and the pionic x-ray fine-structure predicted by the Klein -Gordan equation was observed. This experiment also established an upper limit on the muon neutrino mass of 570 keV at the 90% confidence level. Particle-physics aspects have been investigated in the "pion-like" process of nuclear muon capture on the deuteron. By selecting "back-to-back" neutron events where little momentum is carried by the muon neutrino, it was possible to observe the enhancement to the neutron energy spectrum predicted by meson-exchange current corrections to the muon capture-reaction calculations. Nuclear reactions following stopped negative pion capture in ^{165}Ho and ^{181}Ta have been investigated by coincidence measurements of gamma rays and neutrons emitted by the reaction products. The observed neutron spectra are compared to spectra calculated by a hybrid exciton model calculation of pre-equilibrium neutron emission. The model was modified to include the kinematics of pion capture on a pair of correlated nucleons.