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

Abstract

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 $(1.4\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}{10}^{3}$ relative to atmospheric argon. We report a background-free null result from $(2616\ifmmode\pm\else\textpm\fi{}43)\text{ }\text{ }\mathrm{kg}\text{ }\mathrm{d}$ 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 $2.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}44}\text{ }\text{ }{\mathrm{cm}}^{2}$ ($8.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}44}\text{ }\text{ }{\mathrm{cm}}^{2}$, $8.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}43}\text{ }\text{ }{\mathrm{cm}}^{2}$) for a WIMP mass of $100\text{ }\text{ }\mathrm{GeV}/{c}^{2}$ ($1\text{ }\text{ }\mathrm{TeV}/{c}^{2}$, $10\text{ }\text{ }\mathrm{TeV}/{c}^{2}$).