Liquid argon is a bright scintillator with potent particle identification properties, making it an attractive target for direct-detection dark matter searches. The DarkSide-50 dark matter search here reports the first WIMP search results obtained using a target of low-radioactivity argon. DarkSide-50 is a dark matter detector, using a two-phase liquid argon time projection chamber, located at the Laboratori Nazionali del Gran Sasso. The underground argon is shown to contain ${}^{39}\mathrm{Ar}$ at a level reduced by a factor $(1.4\pm 0.2)\times {10}^3$ relative to atmospheric argon. We report a background-free null result from $(2616\pm 43)\mathrm{kg}\mathrm{d}$ of data, accumulated over 70.9 live days. When combined with our previous search using an atmospheric argon, the 90% C.L. upper limit on the WIMP-nucleon spin-independent cross section, based on zero events found in the WIMP search regions, is $2.0\times {10}^{-44}{\mathrm{cm}}^2$ ($8.6\times {10}^{-44}{\mathrm{cm}}^2$, $8.0\times {10}^{-43}{\mathrm{cm}}^2$) for a WIMP mass of $100\mathrm{GeV}/c^2$ ($1\mathrm{TeV}/c^2$, $10\mathrm{TeV}/c^2$).

• Received 17 October 2015
• Corrected 16 February 2017

DOI:https://doi.org/10.1103/PhysRevD.93.081101