Projected WIMP sensitivity of the LUX-ZEPLIN (LZ) dark matter experiment (1802.06039v2)

Abstract: LUX-ZEPLIN (LZ) is a next generation dark matter direct detection experiment that will operate 4850 feet underground at the Sanford Underground Research Facility (SURF) in Lead, South Dakota, USA. Using a two-phase xenon detector with an active mass of 7~tonnes, LZ will search primarily for low-energy interactions with Weakly Interacting Massive Particles (WIMPs), which are hypothesized to make up the dark matter in our galactic halo. In this paper, the projected WIMP sensitivity of LZ is presented based on the latest background estimates and simulations of the detector. For a 1000~live day run using a 5.6~tonne fiducial mass, LZ is projected to exclude at 90\% confidence level spin-independent WIMP-nucleon cross sections above $1.4 \times 10^{-48}$~cm$^{2}$ for a 40~$\mathrm{GeV}/c^{2}$ mass WIMP. Additionally, a $5\sigma$ discovery potential is projected reaching cross sections below the exclusion limits of recent experiments. For spin-dependent WIMP-neutron(-proton) scattering, a sensitivity of $2.3 \times 10^{-43}$~cm$^{2}$ ($7.1 \times 10^{-42}$~cm$^{2}$) for a 40~$\mathrm{GeV}/c^{2}$ mass WIMP is expected. With underground installation well underway, LZ is on track for commissioning at SURF in 2020.

• The paper demonstrates that LZ achieves unprecedented sensitivity by targeting a WIMP-nucleon cross-section of 1.4×10⁻⁴⁸ cm² for 40 GeV/c² WIMPs.
• It employs a two-phase liquid/gas xenon TPC and robust veto systems to effectively reject over 99.5% of background events.
• The study’s comprehensive simulations and background analyses pave the way for refining dark matter models and experimental strategies.

Overview of the Projected WIMP Sensitivity of the LUX-ZEPLIN (LZ) Dark Matter Experiment

The search for Weakly Interacting Massive Particles (WIMPs), a primary dark matter candidate, continues to be a pivotal endeavor in particle astrophysics. The LUX-ZEPLIN (LZ) experiment represents a significant advancement in this domain, building upon the innovative foundation laid by its predecessors, the LUX and ZEPLIN collaborations. Situated deep underground at the Sanford Underground Research Facility (SURF), LZ is engineered to probe unexplored regions of WIMP parameter space with unprecedented sensitivity.

Experimental Design and Sensitivity

LZ employs a two-phase liquid/gas xenon time projection chamber (TPC) with a 7-tonne active target mass, optimized for detecting nuclear recoils that could signify WIMP interactions. Notably, LZ aims to achieve an exclusion sensitivity down to a WIMP-nucleon cross-section of $1.4 \times 10^{-48}$ cm$^{2}$ for a WIMP with a mass of 40 GeV/c$^2$. This sensitivity is supported by a robust set of simulations and background estimations, considering both electronic and nuclear recoil backgrounds.

Detector Configuration and Background Suppression

The LZ detector's architecture integrates an outer detector and xenon skin veto to tag and reject background events, particularly those arising from gamma rays and neutrons. These systems enhance the rejection capability for multi-site and asynchronous background events. The use of ultra-pure titanium for the cryostat, along with stringent cleanliness protocols, minimizes potential internal contaminant sources, such as radon emanation and surface radioactivity from plate-out processes.

Extensive simulations underscore the LZ's capability to distinguish potential WIMP signals from background events, leveraging the characteristic S1 (scintillation) and S2 (ionization) signal ratios and efficiencies. This ensures background rejection rates exceeding 99.5% for a 50% signal acceptance across multiple WIMP mass hypotheses.

Background Analysis and Management

Key background sources include intrinsic radioactivity within the detector materials, cosmic-ray-induced events, and irreducible neutrino interactions. Through meticulous radio-assay programs, LZ estimates that radon and other intrinsic contaminations will dominate the electronic recoil background. LZ has designed a proficient active veto system to handle neutron backgrounds, primarily utilizing gadolinium-loaded liquid scintillator, to detect and veto delayed neutron capture events.

Implications and Future Prospects

Anticipating commissioning in 2020, the LZ experiment is poised to significantly enhance detection capabilities for both spin-independent and spin-dependent WIMP interactions. Its sensitivity trajectory will impact dark matter model selection, particularly in scenarios where neutrino backgrounds become significant. Furthermore, LZ will contribute to broader astroparticle physics by offering potential insights into neutrino coherent scattering, a standard model process with observationalimplications beyond dark matter detection.

LZ's contribution to WIMP searches is not merely incremental but anticipates probing parameter spaces crucial for validating or refuting leading theoretical models within particle physics. Collaboration and integration with related experimental efforts are expected to sustain momentum in this domain, fostering progressive understanding of dark matter's role in the cosmic architecture.

In conclusion, the LZ experiment exemplifies advanced engineering, meticulous planning, and strategic placement, marking a sophisticated chapter in the pursuit of dark matter understanding, crucial for deciphering the cosmos's hidden mass framework.