Background determination for the LUX-ZEPLIN dark matter experiment

The LUX-ZEPLIN experiment recently reported limits on WIMP-nucleus interactions from its initial science run, down to 9.2 X 10^-48 cm² for the spin-independent interaction of a 36 GeV/c² 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+0.5) X 10^-5 events/keV_ee/kg/day in the low-energy region, approximately 60 times lower than the equivalent rate reported by the LUX experiment.