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 39 Ar at a level reduced by a factor ( 1.4 ± 0.2 ) × 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 × 10 − 44     cm 2 ( 8.6 × 10 − 44     cm 2 , 8.0 × 10 − 43     cm 2 ) for a WIMP mass of 100     GeV / c 2 ( 1     TeV / c 2 , 10     TeV / c 2 ).