Pulseshape discrimination against low-energy Ar-39 beta decays in liquid argon with 4.5 tonne-years of DEAP-3600 data

TL;DR

This paper evaluates pulse-shape discrimination techniques in liquid argon to suppress Ar-39 beta decay backgrounds in the DEAP-3600 dark matter detector, demonstrating comparable performance of different algorithms and photon detection methods.

Contribution

It introduces and compares two PSD algorithms and two photon counting methods, providing insights into their effectiveness for background suppression in liquid argon detectors.

Findings

Prompt-fraction and log-likelihood-ratio PSD perform similarly under ideal conditions.

Photon detection timing biases affect PSD performance.

A model explains the information content of scintillation photons over time.

Abstract

The DEAP-3600 detector searches for the scintillation signal from dark matter particles scattering on a 3.3 tonne liquid argon target. The largest background comes from $^{39}$Ar beta decays and is suppressed using pulseshape discrimination (PSD). We use two types of PSD algorithm: the prompt-fraction, which considers the fraction of the scintillation signal in a narrow and a wide time window around the event peak, and the log-likelihood-ratio, which compares the observed photon arrival times to a signal and a background model. We furthermore use two algorithms to determine the number of photons detected at a given time: (1) simply dividing the charge of each PMT pulse by the charge of a single photoelectron, and (2) a likelihood analysis that considers the probability to detect a certain number of photons at a given time, based on a model for the scintillation pulseshape and for afterpulsing in the light detectors. The prompt-fraction performs approximately as well as the log-likelihood-ratio PSD algorithm if the photon detection times are not biased by detector effects. We explain this result using a model for the information carried by scintillation photons as a function of the time when they are detected.