Measurement of proton, deuteron, triton, and α particle emission after nuclear muon capture on Al, Si, and Ti with the AlCap experiment

Abstract

Background: Heavy charged particles after nuclear muon capture are an important nuclear physics background to the muon-to-electron conversion experiments Mu2e and COMET, which will search for charged lepton flavor violation at an unprecedented level of sensitivity. Purpose: The AlCap experiment aimed to measure the yield and energy spectra of protons, deuterons, tritons, and α particles emitted after the nuclear capture of muons stopped in Al, Si, and Ti in the low-energy range relevant for the muon-to-electron conversion experiments. Methods: Individual charged particle types were identified in layered silicon detector packages and their initial energy distributions were unfolded from the observed energy spectra. Results: The proton yields per muon capture were determined as Y p (Al) = 26.64(28 stat.)(77 syst.) × 10 −3 and Y p (Ti) = 26.48(35)(80) × 10 −3 in the energy range 3.5-20.0 MeV, and as Y p (Si) = 52.5(6)(18) × 10 −3 in the energy range 4.0-20.0 MeV. Detailed information on yields and energy spectra for all observed nuclei are presented in the paper. Conclusions: The yields in the candidate muon stopping targets, Al and Ti, are approximately half of that in Si, which was used in the past to estimate this background. The reduced background allows for less shielding and a better energy resolution in these experiments. It is anticipated that the comprehensive information presented in this paper will stimulate modern theoretical calculations of the rare process of muon capture with charged particle emission and inform the design of future muon-to-electron conversion experiments.

Mu2e Run I Sensitivity Projections for the Neutrinoless μ− → e− Conversion Search in Aluminum

“…Simulation of protons and deuterons produced in nuclear muon captures relies on their inclusive yields in Al reported in Refs. [29,30]. As there are no published neutron spectra on Al, the simulation of neutrons relies on the neutron spectrum on Ca [31] and assumes 1.2 neutrons emitted per muon capture, in agreement with Ref.…”

Emissions of Hydrogen Isotopes from the Nuclear Muon Capture Reaction in natSi

“…4. Comparison of the measured spectra of proton, deuteron, and triton from the μNC reaction on silicon measured in the present and previous works [8,13]. The red, blue, and green symbols denote proton, deuteron, and triton spectra, respectively.…”

“…The measured spectra are compared to those in [8]. More recently, new experimental data have been published by Edmonds et al (AlCap Collaboration) in 2022 [13]. Figure 4 shows the energy spectra of protons, deuterons, and tritons measured in the present and previous works.…”

“…In Monte Carlo simulation with PHITS, a proton in the nucleus is randomly selected and changed to a neutron with the added excitation energy of the nucleus [13]. The time evolution of the highly-excited nucleons is simulated by the quantum molecular dynamics (QMD) model [15].…”

“…The closed circle represents the measured spectra in the present work. The open circle and square are the measured spectra in [13] and [8], respectively. results calculated by the QMD plus GEM and MQMD plus GEM are almost identical.…”