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 $758\text{ }\text{ }\mathrm{tonne}\ifmmode\cdot\else\textperiodcentered\fi{}\mathrm{day}$ 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 $3.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}45}\text{ }\text{ }{\mathrm{cm}}^{2}$ ($1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}44}\text{ }\text{ }{\mathrm{cm}}^{2}$) for a $100\text{ }\text{ }\mathrm{GeV}/{\mathrm{c}}^{2}$ ($1\text{ }\text{ }\mathrm{TeV}/{\mathrm{c}}^{2}$) 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 the charge and photon detection time distributions observed in each photomultiplier tube.