LUX-ZEPLIN (LZ)

LUX-ZEPLIN (LZ) is the world leading search for elusive dark matter particles. It's operated 1 mile under the Black Hills of South Dakota, in the Sanford Underground Research Facility (SURF).

Overview of the LZ experiment showing the primary components of the experiment. — *Overview of the* LZ experiment showing the primary components of the experiment.

LZ employs two-phase (liquid/gas) Time Projection Chamber (TPC) technology and contains seven active tonnes of liquid xenon. Collisions of dark matter particles with the xenon atoms would lead to light signals in the detector. Being sensitive to such rare interactions is only possible in a very quiet environment, where all backgrounds are thoroughly understood. To achieve this, the experiment is located 1 mile underground, and care has been taken in choosing radio-pure detector materials. Additionally, the central detector is surrounded by a liquid scintillator outer detector, used as a veto to improve the rejection of unwanted background events in the central region.

The fully assembled LZ TPC at the Surface Assembly Lab, SURF, in Aug 2019. (Photo credit, Matthew Kapust, SURF) — *The fully assembled* LZ TPC at the Surface Assembly Lab, *SURF, in Aug 2019. (Photo credit, Matthew Kapust, SURF)*

The LZ collaboration consists of 250 scientists in 37 US, UK, European, Australian, and Korean institutions. UCL co-led the crucial Backgrounds and Screening Work Package during the design and construction of the experiment. The results of the material screening are now used to construct the high-precision Background Model against which any possible signal will be evaluated. You can find more info about our cutting-edge radio-purity assay facilities used to screen potential construction materials here. UCL has also held significant leadership roles in constructing and commissioning the experiment.

Our 2024 WIMP search set the most stringent constraints on WIMP-nucleon scattering cross-sections and achieved 3σ WIMP discovery potential in previously untested parameter space. UCL was a locus of leadership for this analysis, with Prof. Chamkaur Ghag being the LZ Spokesperson and Dr. Amy Cottle being the working group lead for the WIMP search analysis. Also on our team for this analysis were the working group leads for statistical inference and wall background modelling. Our roster of PhD students contributed to a variety of critical analyses including: cuts and data quality, simulations, background modelling and background mitigation techniques.

Results of LZ's 2024 WIMP search analysis, setting world leading upper limits on spin-independent scattering cross-sections between nucleons and WIMPs with masses greater than 9 GeV/c2. In this analysis LZ was also able to achieve WIMP discovery potential at 3σ significance in previously untested parameter space.

The UCL group continues to lead in the areas of low-energy searches for low mass WIMPs and neutrino physics signals, as well as in the high energy regime with the search for 136Xe neutrinoless double-beta decay. We've also maintained our leadership in conducting the statistical inference for these analyses, and further developing machinery thereof for both LZ and XLZD—the next-generation dark matter search using liquid xenon TPC technology. Going forward, UCL is poised to keep being a central institution for LZ in its 1000 live day WIMP search, which promises to venture even further into uncharted territory in the search for galactic dark matter.