The LZ cylindrical time-projection chamber contains ~7 active tonnes of purified liquid xenon in a ~1 kV/cm drift field below a thin gas-phase high-field scintillation region. Prompt scintillation light (S1) from a nuclear recoil event is observed by UV-sensitive photomultiplier tubes (PMTs) distributed above and below the xenon. The prompt primary signal is confirmed and localized in space by light (S2) generated in the gas phase by electrons which drift to the top of the liquid where they are efficiently extracted by the electric field. The ratio of the S1 and S2 signal intensities distinguishes electron recoils from nuclear recoils. The xenon cryostat is surrounded by an outer detector comprising gadolinium-loaded liquid scintillator and an outermost water shield instrumented with PMTs which serve to measure and veto cosmogenic and radiogenic background events, especially those producing neutrons. The LZ project concept was supported by the 2014 Particle Physics Project Prioritization Panel [P5] and LZ selected by the DOE Office of High Energy Physics as a 2nd generation direct dark matter search. The experiment will be located in the Sanford Underground Research Facility (SURF) with projected operation in 2019.
The LZ experiment is sensitive to keV-range nuclear recoil energies corresponding to WIMP masses of order 10 GeV/c2 with the expected galactic dark matter model velocity distribution. The projected reach in cross section sensitivity for spin-independent interactions is ~10-48 cm2/nucleon for 50 GeV/c2 WIMP mass. The isotope mix in naturally occurring xenon provides sensitivity to spin-dependent interactions at a reduced but also record level.