Backgrounds in the DEAP-3600 Dark Matter Experiment

The DEAP-3600 experiment, located at SNOLAB, is searching for dark matter with a single phase liquid argon (LAr) target. For a background-free exposure of 3000 kg$\cdot$yr, the projected sensitivity to the spin-independent WIMP-nucleon cross section at 100 GeV/c$^2$ WIMP mass is 10$^{-46}$ cm$^{2}$. The experimental signature of dark matter interactions is keV-scale argon recoils producing 128 nm LAr scintillation photons which are wavelength shifted and observed by 255 PMTs. To reach the large background-free exposure, a combination of careful material selection, passive shielding, active vetoes, fiducialization and pulse shape discrimination (PSD) is used. The main concept of the background rejection in DEAP-3600 is the powerful PSD, employing the large difference between fast and slow components of LAr scintillation light. The designed background level of DEAP-3600 is less than 0.6 events in a 3000 kg$\cdot$yr exposure. The experiment was filled in November 2016 and is currently taking dark matter search data.