Eur. Phys. J. C (2021) 81: 823
https://doi.org/10.1140/epjc/s10052-021-09514-w

Regular Article - Experimental Physics

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

¹ Department of Physics, University of Alberta, T6G 2R3, Edmonton, AB, Canada
² AstroCeNT, Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Rektorska 4, 00-614, Warsaw, Poland
³ Canadian Nuclear Laboratories Ltd, K0J 1J0, Chalk River, ON, Canada
⁴ Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, 28040, Madrid, Spain
⁵ Department of Physics, Carleton University, K1S 5B6, Ottawa, ON, Canada
⁶ Physics Department, Università degli Studi “Federico II” di Napoli, 80126, Naples, Italy
⁷ Physics Department, Università degli Studi di Cagliari, 09042, Cagliari, Italy
⁸ INFN Laboratori Nazionali del Gran Sasso, 67100, Assergi, AQ, Italy
⁹ Department of Physics and Astronomy, Laurentian University, P3E 2C6, Sudbury, ON, Canada
¹⁰ Instituto de Física, Universidad Nacional Autónoma de México, A. P. 20-364, 01000, Mexico, D.F., Mexico
¹¹ National Research Centre Kurchatov Institute, 123182, Moscow, Russia
¹² National Research Nuclear University MEPhI, 115409, Moscow, Russia
¹³ INFN Napoli, 80126, Naples, Italy
¹⁴ INFN Cagliari, 09042, Cagliari, Italy
¹⁵ PRISMA+ Cluster of Excellence and Institut für Kernphysik, Johannes Gutenberg-Universität Mainz, 55128, Mainz, Germany
¹⁶ Physics Department, Princeton University, 08544, Princeton, NJ, USA
¹⁷ Department of Physics, Engineering Physics and Astronomy, Queen’s University, K7L 3N6, Kingston, ON, Canada
¹⁸ Royal Holloway University London, Egham Hill, TW20 0EX, Egham, Surrey, UK
¹⁹ SNOLAB, P3Y 1M3, Lively, ON, Canada
²⁰ University of Sussex, Sussex House, BN1 9RH, Brighton, East Sussex, UK
²¹ TRIUMF, V6T 2A3, Vancouver, BC, Canada
²² Department of Physics, Technische Universität München, 80333, Munich, Germany
²³ Arthur B. McDonald Canadian Astroparticle Physics Research Institute, Queen’s University, K7L 3N6, Kingston, ON, Canada
²⁴ BP2, National Centre for Nuclear Research, ul. Pasteura 7, 02-093, Warsaw, Poland
²⁵ INAF-Astronomical Observatory of Capodimonte, Salita Moiariello 16, 80131, Naples, Italy
²⁶ Nuclear Science Division, Lawrence Berkeley National Laboratory, 94720, Berkeley, CA, USA
²⁷ Nikhef and the University of Amsterdam, Science Park, 1098 XG, Amsterdam, The Netherlands
²⁸ SNOLAB, P3Y 1M3, Lively, ON, Canada

^eq deap-papers@snolab.ca

Received: 24 March 2021
Accepted: 30 July 2021
Published online: 16 September 2021

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 beta decays and is suppressed using pulse-shape discrimination (PSD). We use two types of PSD estimator: 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 mean single-photoelectron charge, 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 pulse shape 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.