Charged-particle spectra from μ- capture on Al

Abstract

Published data on the emission of charged particles following nuclear muon capture are extremely limited. In addition to its interest as a probe of the nuclear response, these data are important for the design of some current searches for lepton flavor violation. This work presents momentum spectra of protons and deuterons following μ- capture in aluminum. It is the first measurement of a muon capture process performed with a tracking spectrometer. A precision of better than 10% over the momentum range of 100-190MeV/c for protons is obtained; for deuterons of 145-250MeV/c the precision is better than 20%. The observed partial yield of protons with emission momenta above 80MeV/c (kinetic energy 3.4MeV) is 0.0322±0.0007(stat)±0.0022(syst) per capture, and for deuterons above 130MeV/c (4.5MeV) it is 0.0122±0.0009(stat)±0.0006(syst). Extrapolating to total yields gives 0.045±0.001(stat)±0.003(syst)±0.001(extrapolation) per capture for protons and 0.018±0.001(stat)±0.001(syst)±0.002(extrapolation) for deuterons, which are the most precise measurements of these quantities to date.