Charged particle spectra from $\mu^{-}$ capture on Al

TL;DR

This paper reports the first measurement of charged particle spectra from muon capture on aluminum using a tracking spectrometer, achieving high precision and providing valuable data for nuclear response and lepton flavor violation searches.

Contribution

It presents the first tracking spectrometer measurement of muon capture on aluminum, with precise proton and deuteron spectra and total yields, improving upon previous limited data.

Findings

Protons emitted above 80 MeV/c have a yield of 0.0322 per capture.

Deuterons above 130 MeV/c have a yield of 0.0122 per capture.

Total yields are 0.045 for protons and 0.018 for deuterons per capture.

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 $\mu^{-}$ 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--190 MeV/c for protons is obtained; for deuterons of 145--250 MeV/c the precision is better than 20%. The observed partial yield of protons with emission momenta above 80 MeV/c (kinetic energy 3.4 MeV) is $0.0322\pm0.0007(\text{stat})\pm0.0022(\text{syst})$ per capture, and for deuterons above 130 MeV/c (4.5 MeV) it is $0.0122\pm0.0009(\text{stat})\pm0.0006(\text{syst})$. Extrapolating to total yields gives $0.045\pm0.001(\text{stat})\pm0.003(\text{syst}) \pm 0.001(\text{extrapolation})$ per capture for protons and $0.018\pm0.001(\text{stat})\pm0.001(\text{syst})\pm 0.002(\text{extrapolation})$ for deuterons, which are the most precise measurements of these quantities to date.