Pulse-shape discrimination against low-energy Ar-39 beta decays in liquid argon with 4.5 tonne-years of DEAP-3600 data
Prabal Adhikari, Rahaf Ajaj, M. Alpízar-Venegas, P. Amaudruz, D. J. Auty, M. Batygov, B. Beltrán, H. Benmansour, C. E. Bina, J. Bonatt, W. Bonivento, M. G. Boulay, B. Broerman, J. F. Bueno, P. M. Burghardt, A. Butcher, M. Cadeddu, B. Cai, M. Cárdenas-Montes, S. Cavuoti, M. Chen, Y. Chen, B. T. Cleveland, J. Corning, D. J. Cranshaw, S. J. Daugherty, P. DelGobbo, K. Dering, J. DiGioseffo, P. C. F. Di Stefano, L. Doria, F. A. Duncan, M. Dunford, E. Ellingwood, A. Erlandson, S. S. Farahani, N. Fatemighomi, G. Fiorillo, S. Florian, T. Flower, R. Ford, R. Gagnon, D. Gallacher, P. Garcı́a-Abia, S. Garg, P. Giampa, D. Goeldi, V. V. Golovko, P. Gorel, K. Graham, D. R. Grant, A. Grobov, A. L. Hallin, M. Hamstra, P. J. Harvey, C. Hearns, T. Hugues, A. Ilyasov, A. Joy, B. Jigmeddorj, C. J. Jillings, O. Kamaev, G. Kaur, A. Kemp, I. Kochanek, M. Kuźniak, M. Laí, S. Langrock, B. Lehnert, A. Leonhardt, N. Levashko, X. Li, J. Lidgard, T. Lindner, M. Lissia, James A. Lock, G. Longo, I. Machulin, A. B. McDonald, Thomas McElroy, T. McGinn, Joseph McLaughlin, R. Mehdiyev, C. Mielnichuk, J. Monroe, Philippe Nadeau, C. Nantais, C. Ng, A. J. Noble, E. O’Dwyer, G. Oliviéro, C. Ouellet, Sanjoy Kumar Pal, P. Pasuthip, S. J. M. Peeters, M. G. Perry, V. Pesudo, E. Picciau, M.-C. Piro, T. R. Pollmann
Abstract
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 $$^{39}\text{ Ar }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msup> <mml:mrow/> <mml:mn>39</mml:mn> </mml:msup> <mml:mspace/> <mml:mtext>Ar</mml:mtext> <mml:mspace/> </mml:mrow> </mml:math> 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.