Background determination for the LUX-ZEPLIN dark matter experiment

Abstract

The LUX-ZEPLIN experiment recently reported limits on WIMP-nucleus interactions from its initial science run, down to $9.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}48}\text{ }\text{ }{\mathrm{cm}}^{2}$ for the spin-independent interaction of a $36\text{ }\text{ }\mathrm{GeV}/{\mathrm{c}}^{2}$ WIMP at 90% confidence level. In this paper, we present a comprehensive analysis of the backgrounds important for this result and for other upcoming physics analyses, including neutrinoless double-beta decay searches and effective field theory interpretations of LUX-ZEPLIN data. We confirm that the in-situ determinations of bulk and fixed radioactive backgrounds are consistent with expectations from the ex-situ assays. The observed background rate after WIMP search criteria were applied was $(6.3\ifmmode\pm\else\textpm\fi{}0.5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}\text{ }\text{ }\mathrm{events}/{\mathrm{keV}}_{ee}/\mathrm{kg}/\mathrm{day}$ in the low-energy region, approximately 60 times lower than the equivalent rate reported by the LUX experiment.