Results from the first use of low radioactivity argon in a dark matter search

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 Ar-39 at a level reduced by a factor (1.4 +/- 0.2) x 10(3) relative to atmospheric argon. We report a background-free null result from (2616 +/- 43) kg 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 x 10(-4)4 cm(2) (8.6 x 10(-44) cm(2), 8.0 x 10(-43) cm(2)) for a WIMP mass of 100 GeV/c(2) (1 TeV/c(2), 10 TeV/c(2)).