First Results from the DEAP-3600 Dark Matter Search with Argon at SNOLAB

This Letter reports the first results of a direct dark matter search with the DEAP-3600 single-phase liquid argon (LAr) detector. The experiment was performed 2 km underground at SNOLAB (Sudbury, Canada) utilizing a large target mass, with the LAr target contained in a spherical acrylic vessel of 3600 kg capacity. The LAr is viewed by an array of PMTs, which would register scintillation light produced by rare nuclear recoil signals induced by dark matter particle scattering. An analysis of 4.44 live days (fiducial exposure of 9.87 ton day) of data taken during the initial filling phase demonstrates the best electronic recoil rejection using pulse-shape discrimination in argon, with leakage $<1.2\times {10}^{-7}$ (90% C.L.) between 15 and $31{\mathrm{keV}}_{ee}$. No candidate signal events are observed, which results in the leading limit on weakly interacting massive particle (WIMP)-nucleon spin-independent cross section on argon, $<1.2\times {10}^{-44}{\mathrm{cm}}^2$ for a $100\mathrm{GeV}/c^2$ WIMP mass (90% C.L.).

Figure 1

(a) $F_{\text{prompt}}$ vs PE distribution slice at 80 PE, with and without the trigger efficiency correction, is shown together with the effective model fit (performed above the red dashed line, indicating the $F_{\text{prompt}}$ value below which the trigger efficiency is $<100\%$). The brown and orange lines correspond to 90% and 50% NRA. (b) Data and model for the 120–240 PE range with $1.87972\times {10}^7$ events, represented as leakage probability above given $F_{\text{prompt}}$. A conservative projection from DEAP-1 [5] is also shown with its NRA lines (dashed).

Figure 2

Measured, trigger-efficiency-corrected ${}^{39}\mathrm{Ar}$ $\beta$ spectrum from a subset of data and the fit function (red) based on simulation, with ${\chi }_{\mathrm{ndf}}^2=1.02$. The inset shows the energy response function, Eq. (1), from the ${}^{39}\mathrm{Ar}$ fit, and, as a cross-check, $\gamma$ lines from ${}^{40}\mathrm{K}$ and ${}^{208}\mathrm{Tl}$. ${}^{208}\mathrm{Tl}$ diverges from the function because of PMT and DAQ nonlinearity.

Figure 3

AmBe source data after cuts, with the WIMP search ROI (black box).

Figure 4

(a) The acceptance in the 80–240 PE window, with systematic errors (maximum variation about the weighted mean, run-by-run). Uncertainties on trigger acceptance and $F_{\text{prompt}}$ cut acceptance are discussed in the text. (b) $F_{\text{prompt}}$ vs PE for events passing cuts, with the WIMP search ROI (red).

Figure 5

Spin-independent WIMP-nucleon cross section 90% C.L. exclusion from 4.44 live days of DEAP-3600 data. Also shown are current results from other searches [23, 24, 25, 26, 27, 28], and projections for XENON1T and DEAP-3600 (a 3 ton year background-free exposure with a $15{\mathrm{keV}}_{ee}$ threshold).