The LUX-ZEPLIN (LZ) Experiment is a next-generation dark matter direct detection experiment hoping to observe weakly interacting massive particles (WIMP) scatters on nuclei. It was formed in 2012 by combining the LUX and ZEPLIN groups. It is currently a collaboration of 30 institutes in the US, UK, Portugal and South Korea. The experiment is located at about 1,500 metres under the Sanford Underground Research Facility (SURF) in South Dakota, and is managed by the United States Department of Energy's (DOE) Lawrence Berkeley National Lab (Berkeley Lab). The experiment uses an ultra-sensitive detector made of 7 tonnes of liquid xenon to hunt for signals of WIMP-nucleus interactions. It is one of three such experiments which lead the search for direct detection of WIMPs above 10 GeV/c2, the other two being the XENON experiment and the PANDAX-4T experiment. In the spring of 2015, LZ passed the "Critical Decision Step 1" or CD-1 review, and became an official DOE project. U.S. Department of Energy officials on Sept. 21, 2020 formally signed off on project completion for LZ; DOE's project completion milestone is called Critical Decision 4, or CD-4. As of December 2025, results from LZ have found no evidence of WIMPs above a mass of 9 GeV/c2.
LZ as a low-background detector
To conclusively identify WIMP-nucleus scatters, LZ must be able to observe very small energy depositions in its active volume. However, it must also be able to differentiate true WIMP scatters from other interactions caused by bias. Examples of these known "backgrounds" are interactions from gamma rays produced by trace radioactivity in the environment, interactions from neutrons produced in the environment, and interactions from cosmic ray muons produced in the upper atmosphere. The two goals of a dark matter search are to minimize the number of these background interactions, and for those that do occur, to be able to identify that they are from background (as opposed to WIMPs). First, the innermost detector is composed of a dual-phase xenon time projection chamber (TPC). This detector is the target for WIMP-nucleus scatters. As discussed in the next section, this detector can perform a 3-D reconstruction of the position of an interaction in the xenon. This enables an identification and rejection of background interactions that happen near the periphery (sides, top, and bottom) of the detector. These peripheral interactions are overwhelmingly likely to be from external gamma rays or neutrons and radioactive decays of trace radionuclides in the detector components composing the TPC and cryostats. Moreover, the relatively large density of liquid xenon allows the TPC to "self-shield" to a degree: gamma rays (neutrons) entering the TPC can travel only approximately a few centimetres (10 centimetres) before scattering and being stopped. As a result, the innermost volume of the detector is largely free of many of these backgrounds. Because it is so quiet, this innermost, or "fiducial" volume is very sensitive to observing WIMP scatters above other backgrounds, and is the space in which LZ's WIMP searches are conducted. Next, the TPC is located inside several layers of active and passive shielding to reduce rates of external gamma rays and neutrons. The TPC is housed in an inner cryostat, which maintains the temperatures needed to keep the xenon in the liquid phase (approximately 178K). This inner cryostat is nested in a larger, outer cryostat, which helps limit heat transfer into the xenon. External to the outer cryostat is a set of acrylic tanks holding liquid scintillator. This scintillator is liquid-alkyl-benzene (LAB) loaded with gadolinium for more efficient neutron capture. If a gamma ray or neutron scatters once inside the TPC but then exits, it will likely also deposit energy in the scintillator. These energy deposits are accompanied by emission of optical photons, which can be detected by an array of photomultiplier tubes (PMTs) located outside of the acrylic tanks. By observing such a signal in coincidence with a scatter in the TPC, it becomes possible to reject backgrounds in the TPC that might otherwise look like WIMP scatters. This is particularly important for neutrons, which can penetrate farther than gamma rays and which scatter on the xenon nucleus in the same way that WIMPs are expected to (instead of on xenon's atomic electrons). The outer-detector PMT array is located in a larger water tank. Together, the water tank and liquid scintillator also provide significant passive shielding against external gamma rays and neutrons, stopping a vast majority of them before they have the chance to enter the TPC. The whole assembly is located approximately one mile underground, in the Davis Cavern at SURF. This underground location creates a rock overburden that significantly reduces the rate of cosmic ray muons entering the TPC relative to the rate at Earth's surface. All together these different strategies ensure that LZ is a detector capable of performing a very sensitive search for dark matter scatters on xenon nuclei.
LZ's Inner Detector: Dual Phase TPC
… excerpt ends here. Continue reading the full article.
