Radiative muon capture on Al, Si, Ca, Mo, Sn, and Pb.

Abstract

The branching ratio for radiative muon capture (RMC), relative to the nonradiative process, is sensitive to ${\mathit{g}}_{\mathit{p}}$, the induced pseudoscalar coupling constant of the weak hadronic current. The photon energy spectra from RMC on $^{27}\mathrm{Al}$, $^{28}\mathrm{Si}$, $^{40}\mathrm{Ca}$, $^{\mathrm{nat}}\mathrm{Mo}$, $^{\mathrm{nat}}\mathrm{Sn}$, and $^{\mathrm{nat}}\mathrm{Pb}$ have been measured using a high-acceptance pair spectrometer. The measured partial branching ratios, ${\mathit{R}}_{\ensuremath{\gamma}}$, for photons of ${\mathit{E}}_{\ensuremath{\gamma}}$>57 MeV are 1.43\ifmmode\pm\else\textpm\fi{}0.13, 1.93\ifmmode\pm\else\textpm\fi{}0.18, 2.09\ifmmode\pm\else\textpm\fi{}0.19, 1.11\ifmmode\pm\else\textpm\fi{}0.11, 0.98\ifmmode\pm\else\textpm\fi{}0.09, and 0.60\ifmmode\pm\else\textpm\fi{}0.07 respectively, in units of ${10}^{\mathrm{\ensuremath{-}}5}$. The results confirm the previously observed suppression of ${\mathit{R}}_{\ensuremath{\gamma}}$ with increasing Z for Z>20. For $^{40}\mathrm{Ca}$ the present result is in good agreement with previous measurements. For the heavier nuclei, the results are compared with two recent calculations performed in the Fermi-gas model. In one case the data indicate a complete quenching of ${\mathit{g}}_{\mathit{p}}$, but the more recent calculation does not reproduce the data for any value of ${\mathit{g}}_{\mathit{p}}$.