DarkSide-50 WIMP search results with low radioactivity argon

Located at the Laboratori Nazionali del Gran Sasso, DarkSide-50 is the first physics detector of the DarkSide dark matter search program. The experiment features a dual-phase Time Projection Chamber as the WIMP detector, surrounded by an organic liquid-scintillator neutron veto and a water-Cherenkov muon detector. We report the results from the first use of low radioactivity argon extracted from underground sources in a dark matter search. We have determined that underground argon is depleted in $^{39}$Ar by a factor $(1.4\pm 0.2) \times10^3$ relative to atmospheric argon whose $^{39}$Ar activity is 1 bq/kg. The underground argon is also found to contain $(2.05 \pm 0.13)$ mBq/kg of $^{85}$Kr. We find no evidence for dark matter in the form of WIMPs in 70.9 live-days of data with a fiducial mass of $(36.9\pm 0.6)$ kg. When combined with our preceding search with an atmospheric argon target, we set a $90 \%$ C.L. upper limit on the WIMP-nucleon spin-independent cross section of $2.0\times10^{-44}$ cm$^2$ $(8.6\times10^{-44}$ {cm}$^2$, $8.0\times10^{-43}$ cm$^2$) for a WIMP mass of 100 GeV/c$^2$ (1 TeV/c$^2$, 10 TeV/c$^2$). DS-50 will continue dark matter search with the underground argon target for a total of 3 years. See also the DS-50 presentations by E. Edkins and G. Koh.