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

The DEAP Collaboration;P. Adhikari;R. Ajaj;M. Alpízar-Venegas;P. -A. Amaudruz;D. J. Auty;M. Batygov;B. Beltran;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. M. Corning;D. Cranshaw;S. Daugherty;P. DelGobbo;K. Dering;J. DiGioseffo;P. 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. J. Ford;R. Gagnon;D. Gallacher;P. García Abia;S. Garg;P. Giampa;D. Goeldi;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. Lai;S. Langrock;B. Lehnert;A. Leonhardt;N. Levashko;X. Li;J. Lidgard;T. Lindner;M. Lissia;J. Lock;G. Longo;I. Machulin;A. B. McDonald;T. McElroy;T. McGinn;J. B. McLaughlin;R. Mehdiyev;C. Mielnichuk;J. Monroe;P. Nadeau;C. Nantais;C. Ng;A. J. Noble;E. O'Dwyer;G. Oliviéro;C. Ouellet;S. Pal;P. Pasuthip;S. J. M. Peeters;M. Perry;V. Pesudo;E. Picciau;M. -C. Piro;T. R. Pollmann;E. T. Rand;C. Rethmeier;F. Retière;I. Rodríguez-García;L. Roszkowski;J. B. Ruhland;E. Sánchez-García;R. Santorelli;D. Sinclair;P. Skensved;B. Smith;N. J. T. Smith;T. Sonley;J. Soukup;R. Stainforth;C. Stone;V. Strickland;M. Stringer;B. Sur;J. Tang;E. Vázquez-Jáuregui;S. Viel;J. Walding;M. Waqar;M. Ward;S. Westerdale;J. Willis;A. Zuñiga-Reyes

2021-01-01

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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 39Ar 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.