Pulseshape discrimination against low-energy Ar-39 beta decays in liquid argon with 4.5 tonne-years of DEAP-3600 data. (arXiv:2103.12202v2 [physics.ins-det] UPDATED)

Pulseshape discrimination against low-energy Ar-39 beta decays in liquid argon with 4.5 tonne-years of DEAP-3600 data. (arXiv:2103.12202v2 [physics.ins-det] UPDATED)
The <a href="http://arxiv.org/find/physics/1/au:+Collaboration_DEAP/0/1/0/all/0/1">DEAP Collaboration</a>: <a href="http://arxiv.org/find/physics/1/au:+Adhikari_P/0/1/0/all/0/1">P. Adhikari</a>, <a href="http://arxiv.org/find/physics/1/au:+Ajaj_R/0/1/0/all/0/1">R. Ajaj</a>, <a href="http://arxiv.org/find/physics/1/au:+Alpizar_Venegas_M/0/1/0/all/0/1">M. Alpízar-Venegas</a>, <a href="http://arxiv.org/find/physics/1/au:+Amaudruz_P/0/1/0/all/0/1">P.-A. Amaudruz</a>, <a href="http://arxiv.org/find/physics/1/au:+Auty_D/0/1/0/all/0/1">D. J. Auty</a>, <a href="http://arxiv.org/find/physics/1/au:+Batygov_M/0/1/0/all/0/1">M. Batygov</a>, <a href="http://arxiv.org/find/physics/1/au:+Beltran_B/0/1/0/all/0/1">B. Beltran</a>, <a href="http://arxiv.org/find/physics/1/au:+Benmansour_H/0/1/0/all/0/1">H. Benmansour</a>, <a href="http://arxiv.org/find/physics/1/au:+Bina_C/0/1/0/all/0/1">C. E. Bina</a>, <a href="http://arxiv.org/find/physics/1/au:+Bonatt_J/0/1/0/all/0/1">J. Bonatt</a>, <a href="http://arxiv.org/find/physics/1/au:+Bonivento_W/0/1/0/all/0/1">W. Bonivento</a>, <a href="http://arxiv.org/find/physics/1/au:+Boulay_M/0/1/0/all/0/1">M. G. Boulay</a>, <a href="http://arxiv.org/find/physics/1/au:+Broerman_B/0/1/0/all/0/1">B. Broerman</a>, <a href="http://arxiv.org/find/physics/1/au:+Bueno_J/0/1/0/all/0/1">J. F. Bueno</a>, <a href="http://arxiv.org/find/physics/1/au:+Burghardt_P/0/1/0/all/0/1">P. M. Burghardt</a>, <a href="http://arxiv.org/find/physics/1/au:+Butcher_A/0/1/0/all/0/1">A. Butcher</a>, <a href="http://arxiv.org/find/physics/1/au:+Cadeddu_M/0/1/0/all/0/1">M. Cadeddu</a>, <a href="http://arxiv.org/find/physics/1/au:+Cai_B/0/1/0/all/0/1">B. Cai</a>, <a href="http://arxiv.org/find/physics/1/au:+Cardenas_Montes_M/0/1/0/all/0/1">M. Cárdenas-Montes</a>, <a href="http://arxiv.org/find/physics/1/au:+Cavuoti_S/0/1/0/all/0/1">S. Cavuoti</a>, <a href="http://arxiv.org/find/physics/1/au:+Chen_M/0/1/0/all/0/1">M. Chen</a>, <a href="http://arxiv.org/find/physics/1/au:+Chen_Y/0/1/0/all/0/1">Y. Chen</a>, <a href="http://arxiv.org/find/physics/1/au:+Cleveland_B/0/1/0/all/0/1">B. T. Cleveland</a>, <a href="http://arxiv.org/find/physics/1/au:+Corning_J/0/1/0/all/0/1">J. M. Corning</a>, <a href="http://arxiv.org/find/physics/1/au:+Cranshaw_D/0/1/0/all/0/1">D. Cranshaw</a>, <a href="http://arxiv.org/find/physics/1/au:+Daugherty_S/0/1/0/all/0/1">S. Daugherty</a>, <a href="http://arxiv.org/find/physics/1/au:+DelGobbo_P/0/1/0/all/0/1">P. DelGobbo</a>, <a href="http://arxiv.org/find/physics/1/au:+Dering_K/0/1/0/all/0/1">K. Dering</a>, <a href="http://arxiv.org/find/physics/1/au:+DiGioseffo_J/0/1/0/all/0/1">J. DiGioseffo</a>, <a href="http://arxiv.org/find/physics/1/au:+Stefano_P/0/1/0/all/0/1">P. Di Stefano</a>, <a href="http://arxiv.org/find/physics/1/au:+Doria_L/0/1/0/all/0/1">L. Doria</a>, <a href="http://arxiv.org/find/physics/1/au:+Duncan_F/0/1/0/all/0/1">F. A. Duncan</a>, <a href="http://arxiv.org/find/physics/1/au:+Dunford_M/0/1/0/all/0/1">M. Dunford</a>, <a href="http://arxiv.org/find/physics/1/au:+Ellingwood_E/0/1/0/all/0/1">E. Ellingwood</a>, <a href="http://arxiv.org/find/physics/1/au:+Erlandson_A/0/1/0/all/0/1">A. Erlandson</a>, <a href="http://arxiv.org/find/physics/1/au:+Farahani_S/0/1/0/all/0/1">S. S. Farahani</a>, <a href="http://arxiv.org/find/physics/1/au:+Fatemighomi_N/0/1/0/all/0/1">N. Fatemighomi</a>, <a href="http://arxiv.org/find/physics/1/au:+Fiorillo_G/0/1/0/all/0/1">G. Fiorillo</a>, <a href="http://arxiv.org/find/physics/1/au:+Florian_S/0/1/0/all/0/1">S. Florian</a>, <a href="http://arxiv.org/find/physics/1/au:+Flower_T/0/1/0/all/0/1">T. Flower</a>, <a href="http://arxiv.org/find/physics/1/au:+Ford_R/0/1/0/all/0/1">R. J. Ford</a>, <a href="http://arxiv.org/find/physics/1/au:+Gagnon_R/0/1/0/all/0/1">R. Gagnon</a>, <a href="http://arxiv.org/find/physics/1/au:+Gallacher_D/0/1/0/all/0/1">D. Gallacher</a>, <a href="http://arxiv.org/find/physics/1/au:+Abia_P/0/1/0/all/0/1">P. García Abia</a>, <a href="http://arxiv.org/find/physics/1/au:+Garg_S/0/1/0/all/0/1">S. Garg</a>, <a href="http://arxiv.org/find/physics/1/au:+Giampa_P/0/1/0/all/0/1">P. Giampa</a>, <a href="http://arxiv.org/find/physics/1/au:+Goeldi_D/0/1/0/all/0/1">D. Goeldi</a>, <a href="http://arxiv.org/find/physics/1/au:+Golovko_V/0/1/0/all/0/1">V. Golovko</a>, <a href="http://arxiv.org/find/physics/1/au:+Gorel_P/0/1/0/all/0/1">P. Gorel</a>, <a href="http://arxiv.org/find/physics/1/au:+Graham_K/0/1/0/all/0/1">K. Graham</a>, <a href="http://arxiv.org/find/physics/1/au:+Grant_D/0/1/0/all/0/1">D. R. Grant</a>, <a href="http://arxiv.org/find/physics/1/au:+Grobov_A/0/1/0/all/0/1">A. Grobov</a>, <a href="http://arxiv.org/find/physics/1/au:+Hallin_A/0/1/0/all/0/1">A. L. Hallin</a>, <a href="http://arxiv.org/find/physics/1/au:+Hamstra_M/0/1/0/all/0/1">M. Hamstra</a>, <a href="http://arxiv.org/find/physics/1/au:+Harvey_P/0/1/0/all/0/1">P. J. Harvey</a>, <a href="http://arxiv.org/find/physics/1/au:+Hearns_C/0/1/0/all/0/1">C. Hearns</a>, <a href="http://arxiv.org/find/physics/1/au:+Hugues_T/0/1/0/all/0/1">T. Hugues</a>, <a href="http://arxiv.org/find/physics/1/au:+Ilyasov_A/0/1/0/all/0/1">A. Ilyasov</a>, <a href="http://arxiv.org/find/physics/1/au:+Joy_A/0/1/0/all/0/1">A. Joy</a>, <a href="http://arxiv.org/find/physics/1/au:+Jigmeddorj_B/0/1/0/all/0/1">B. Jigmeddorj</a>, et al. (68 additional authors not shown)

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.

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