Background Modeling for the First Science Run of the LUX-ZEPLIN Dark Matter Experiment

LUX-ZEPLIN (LZ) is a dark matter experiment located at the Sanford Underground Research Facility in South Dakota, USA employing a 7 tonne active volume of liquid xenon in a dual-phase time projection chamber (TPC). It’s surrounded by an instrumented xenon “skin” region and a gadolinium-loaded liquid scintillator outer detector all contained within an ultra-pure water tank, which primarily serve as active vetoes for gamma-ray and neutron backgrounds, respectively. A comprehensive material assay and selection campaign for detector components, along with a xenon purification campaign, have further ensured an ultra-low background environment. These mitigations have allowed LZ to achieve a background rate of (6.3 ± 0.5) x 10⁻⁵ events/kg/day/keVee in the low energy region (< 15 keVee), approximately 60 times lower than that of its predecessor LUX experiment. In this low background region, LZ has recently set new world-leading limits for the spin-independent elastic scattering of nuclear recoils of Weakly Interacting Massive Particles (WIMPs) with masses above 9 GeV/c² using an exposure of 60 live days and a fiducial mass of 5.5 tonnes. This talk will provide an overview of the backgrounds present in the detector with a specific emphasis on the background model used in LZ’s first results and how these backgrounds were constrained in situ.