Search for dark matter with a 231-day exposure of liquid argon using DEAP-3600 at SNOLAB

Abstract: DEAP-3600 is a single-phase liquid argon (LAr) direct-detection dark matter experiment, operating 2 km underground at SNOLAB (Sudbury, Canada). The detector consists of 3279 kg of LAr contained in a spherical acrylic vessel. This paper reports on the analysis of a 758 tonne\cdot day exposure taken over a period of 231 live-days during the first year of operation. No candidate signal events are observed in the WIMP-search region of interest, which results in the leading limit on the WIMP-nucleon spin-independent cross section on a LAr target of $3.9\times10^{-45}$ cm$^{2}$ ($1.5\times10^{-44}$ cm$^{2}$) for a 100 GeV/c$^{2}$ (1 TeV/c$^{2}$) WIMP mass at 90\% C. L. In addition to a detailed background model, this analysis demonstrates the best pulse-shape discrimination in LAr at threshold, employs a Bayesian photoelectron-counting technique to improve the energy resolution and discrimination efficiency, and utilizes two position reconstruction algorithms based on PMT charge and photon arrival times.

Analysis of DEAP-3600's Limit on Dark Matter Detection

The DEAP-3600 experiment, situated at SNOLAB, is a pivotal endeavor in the search for direct detection of dark matter. Using 3279 kg of liquid argon (LAr), the experiment focuses on identifying weakly interacting massive particles (WIMPs) by observing nuclear recoils resulting from the elastic scattering of WIMPs with LAr nuclei. The paper under discussion provides significant insights from a 231-day exposure, emphasizing the efficacy of liquid argon in dark matter detection through unparalleled pulse-shape discrimination.

Background and Methodology

The theoretical framework is anchored in astrophysical evidence positing that dark matter constitutes approximately 27% of the universe's energy density, yet this phenomenon remains elusive in direct detection experiments. Weakly interacting massive particles are prime candidates for this non-luminous matter. Despite extensive exploration, terrestrial experiments, including DEAP-3600, have not yet achieved direct detection.

DEAP-3600 utilizes LAr for its high scintillation yield and efficiency, paired with the capability of pulse-shape discrimination (PSD) to distinguish between nuclear and electronic recoils. This discrimination is pivotal in minimizing the electronic recoil background, exploiting the unique scintillation time profile of LAr to suppress it by a factor exceeding 2.7e-8 over the investigated energy range.

Results

The analysis of data spanning the first year of operation shows no candidate signal events in the specified WIMP search region. This result is crucial as it leads to setting an upper limit on the WIMP-nucleon spin-independent cross section at 3.9 × 10^-45 cm^2 for a WIMP mass of 100 GeV/c², establishing DEAP-3600 as the most sensitive dark matter search with an LAr target for WIMP masses above 30 GeV.

Experimental Techniques and Innovations

The DEAP-3600 experiment advances the field with several methodological innovations:
- Pulse Shape Discrimination: Significant improvements in PSD at threshold levels allow for exceptional background suppression, facilitating more precise discernment of potential WIMP signals.
- Bayesian Photoelectron Counting: This novel technique optimizes energy resolution and discrimination efficiency by refining pulse measurements to account for photomultiplier tube (PMT) effects such as afterpulsing.
- Position Reconstruction: Implementing two algorithms based on the charge and photon detection time distributions enhances event localization precision, crucial for isolating genuine signals from artifacts.

Implications and Future Directions

The findings underscore the utility of LAr coupled with sophisticated analytical techniques in elevating dark matter detection thresholds. With DEAP-3600 setting a new benchmark, future research directions may include further refinement of these techniques and their application in more extensive scale detectors. Expanded data collection promises to enhance sensitivity and potentially reveal low cross-section WIMP interactions. Moreover, the insights garnered here can inform the design of next-generation experimental setups, potentially paving the way towards breaking the detection barrier of the "neutrino floor"—the background limit of neutrino-induced events which pose a significant challenge to the field.

The paper concludes with the assurance that the DEAP-3600 experiment remains at the forefront of probing one of physics' most profound mysteries, offering vital contributions to the understanding of the universe's missing mass.