Measurements of Pion and Muon Nuclear Capture at Rest on Argon in the LArIAT Experiment

We report the measurement of the final-state products of negative pion and muon nuclear capture at rest on argon by the LArIAT experiment at the Fermilab Test Beam Facility. We measure a population of isolated MeV-scale energy depositions, or blips, in 296 LArIAT events containing tracks from stopping low-momentum pions and muons. The average numbers of visible blips are measured to be 0.74 \(\pm\) 0.19 and 1.86 \(\pm\) 0.17 near muon and pion track endpoints, respectively. The 3.6\(\sigma\) statistically significant difference in blip content between muons and pions provides the first demonstration of a new method of pion-muon discrimination in neutrino liquid argon time projection chamber experiments. LArIAT Monte Carlo simulations predict substantially higher average blip counts for negative muon (1.22 \(\pm\) 0.08) and pion (2.34 \(\pm\) 0.09) nuclear captures. We attribute this difference to Geant4's inaccurate simulation of the nuclear capture process.