Scintillation Light from Cosmic-Ray Muons in Liquid Argon

This paper reports the results of an experiment to directly measure the time-resolved scintillation signal from the passage of cosmic-ray muons through liquid argon. Scintillation light from these muons is of value to studies of weakly-interacting particles in neutrino experiments and dark matter searches. The experiment was carried out at the TallBo dewar facility at Fermilab using prototype light guide detectors and electronics developed for the Deep Underground Neutrino Experiment. Two models are presented for the time structure of the scintillation light, a phenomenological model and a composite model. Both models find $\tau_{\text{T}} = 1.52$ $\mu$s for the decay time constant of the Ar$_2^*$ triplet state. These models also show that the identification of the "early" light fraction in the phenomenological model, $F_{\text{E}}\approx 25\%$ of the signal, with the total light from singlet decays is an underestimate. The total fraction of singlet light is $F_{\text{S}} \approx 36\%$, where the increase over $F_{\text{E}}$ is from singlet light emitted by the wavelength shifter through processes with long decay constants. The models were further used to compute the experimental particle identification parameter $F_{\text{prompt}}$, the fraction of light coming in a short time window after the trigger compared with the light in the total recorded waveform. The models reproduce quite well the typical experimental value $\sim0.3$ found by dark matter and double $\beta$-decay experiments, which suggests this parameter provides a robust metric for discriminating electrons and muons from more heavily ionizing particles.