Dark Matter Search with the DEAP-3600 Detector using the Profile Likelihood Ratio Method

Abstract

We present here a search for WIMP dark matter using 790.8 live-days of data collected with 3269 kg of liquid argon (1266 kg fiducial) by the DEAP-3600 detector at SNOLAB, using the Profile Likelihood Ratio method. The likelihood model is based on three parameters: estimated energy, pulse-shape discrimination parameter, and reconstructed position within the detector. Using this method, the expected signal sensitivity of DEAP-3600 benefits from an increased fiducial volume and improved event selection acceptance. Alpha-decays from a small number of dust particulates circulating within the liquid argon target are the dominant source of background events and limit the sensitivity of this search. This result provides improved exclusion upper limits on the WIMP-nucleon spin-independent cross section on liquid argon for WIMP masses between 20 GeV/$c^{2}$ and 100 GeV/$c^{2}$. At 100 GeV/$c^{2}$ the observed limit is 3.4 $\times$ 10$^{-45}$ cm$^2$ at 90% confidence level.

Figures (9)

• Figure 1: ROI contour in $F_{\text{prompt}}$ versus PE, illustrating the events passing all event selection criteria (red points) superimposed on: (top-left) the prior background model, (top-right) the WIMP model assuming a signal hypothesis of $M_{\chi}$ = 1 TeV/$c^{2}$, $\sigma_{\chi}$ = 10$^{-44}$ cm$^{2}$, (bottom-left) the best-fit background model under the background-only hypothesis, and (bottom-right) the background+signal best-fit model with $M_{\chi}$ = 1 TeV/$c^{2}$. Where applicable, the background model shown is the sum of radiogenic neutrons, surface $\alpha$-decays, shadowed $\alpha$-decays, and dust $\alpha$-decays. $^{39}$Ar $\beta$-decay events are seen outside the ROI to the bottom-left of each plot; this background is negligible within the ROI.
• Figure 2: Distributions in PE (top), $F_{\text{prompt}}$ (middle) and $R_{\text{rec}}$ (bottom) of the events observed in the WIMP ROI, superimposed with the prior background model with $\pm1\sigma$ systematic uncertainty bands (grey), the best-fit background model under the background only hypothesis (red), and the best-fit signal+background model under the $M_{\chi} = 1$ TeV/$c^{2}$ hypothesis (blue). Error bars shown on the data points are the 68% confidence intervals around the bin contents using the Pearson $\chi^{2}$ distribution.
• Figure 3: Observed 90% CL upper limit on the WIMP-nucleon spin-independent cross section as a function of WIMP mass (solid blue) power constrained to the 0.16 quantile (-1$\sigma$). Also shown are the median expected limit (solid black), the $\pm 1\sigma$ and $\pm 2\sigma$ quantile bands (green and yellow respectively), the observed 90% CL upper limit on the open dataset (dashed blue), the previous observed result from DEAP-3600 (solid red) and the result of repeating the previous analysis on the full dataset (dashed red). One additional constraint is applied: for each WIMP mass, the minimum value of the upper limit is never allowed go below 2.3 signal events excluded at 90% CL.
• Figure 4: Light yield in terms of PE from the Bayesian counting algorithm designed to remove PMT noise contributions such as afterpulsing, as a function of date across the dataset, determined from calibration fits to the $^{39}$Ar spectrum.
• Figure 5: Cumulative distribution functions (CDFs) of $N_{\rm PMT,FirstLAr}$ from simulated shadowed $\alpha$-decays from the neck and simulated WIMP-induced $^{40}$Ar NRs.