Radiative muon capture on carbon, oxygen, and calcium

The photon energy spectra from radiative muon capture on C12, O16, and Ca40 have been measured using a time projection chamber as a pair spectrometer. The branching ratio for radiative muon capture is sensitive to gp, the induced pseudoscalar coupling constant of the weak interaction. Expressed in terms of the axial-vector weak coupling constant ga, values of gp/ga=5.7±0.8 and gp/ga=7.3±0.9 are obtained for Ca40 and O16, respectively, from comparison with phenomenological calculations of the nuclear response. From comparison with microscopic calculations, values of gp/ga=4.6±1.8, 13.6-1.9+1.6, and 16.2-0.7+1.3 for Ca40, O16, and C12, respectively, are obtained. The microscopic results are suggestive of a renormalization of the nucleonic form factors within the nucleus.