LUX-ZEPLIN (LZ) Conceptual Design Report (1509.02910v2)

Abstract: The design and performance of the LUX-ZEPLIN (LZ) detector is described as of March 2015 in this Conceptual Design Report. LZ is a second-generation dark-matter detector with the potential for unprecedented sensitivity to weakly interacting massive particles (WIMPs) of masses from a few GeV/c2 to hundreds of TeV/c2. With total liquid xenon mass of about 10 tonnes, LZ will be the most sensitive experiment for WIMPs in this mass region by the end of the decade. This report describes in detail the design of the LZ technical systems. Expected backgrounds are quantified and the performance of the experiment is presented. The LZ detector will be located at the Sanford Underground Research Facility in South Dakota. The organization of the LZ Project and a summary of the expected cost and current schedule are given.

• The paper demonstrates a novel dark matter detector design employing a 7-tonne LXe TPC and achieving 99.5% electron recoil rejection.
• It details advanced background suppression techniques, including robust calibration and low-radioactivity PMTs, to enhance detection sensitivity.
• The design projects spin-independent WIMP cross section sensitivity down to 2×10^-48 cm², setting new benchmarks for dark matter research.

Review of the LUX-ZEPLIN (LZ) Conceptual Design for Dark Matter Detection

The LUX-ZEPLIN (LZ) Conceptual Design Report provides a comprehensive overview of the design and projected performance of the LZ detector—a second-generation dark matter detector aligned with the current cosmological understanding of the universe's composition. The gravitational effects attributed to dark matter necessitate advanced and sensitive detection methods, particularly for weakly interacting massive particles (WIMPs). This report presents LZ as the most sensitive experiment in its mass region, optimized for discerning dark matter interactions via terrestrial direct detection with unprecedented sensitivity levels.

Key Design Features and Performance Metrics

LZ aims to detect WIMPs through nuclear recoil (NR) interactions in a low-background environment by employing a liquid xenon (LXe) time projection chamber (TPC) containing 7 tonnes of active LXe. The detector's design focuses on several crucial performance drivers:

• Electron Recoil (ER) Discrimination: Achieving a 99.5% rejection rate of ER backgrounds at 50% NR acceptance is necessary for limiting misidentification of background events as WIMP interactions.
• Large Fiducial Mass: A 5.6-tonne fiducial mass within a total of 10 tonnes of LXe optimizes the statistical measurement capabilities while enhancing self-shielding from gamma and neutron backgrounds.
• Enhanced Veto System: Comprising both a liquid organic scintillator outer detector and an active xenon skin layer, this system rejects internally generated backgrounds with high efficiency, mitigating numerous false-positive signals from gamma and neutron sources potentially masquerading as WIMP interactions.
• High Voltage and Electric Field Uniformity: The substantial cathode potential of up to -100 kV facilitates the generation of a uniform electric field for effective charge drift, crucial for accurate interaction localization and efficient NR/ER discrimination.

Innovations and Technical Advances

Building on technologies developed within the LUX and ZEPLIN programs, LZ incorporates several innovative features:

• Thermal and Cryogenic Management: A robust liquid cryogenic system ensures operational stability, complemented by a xenon purification system capable of maintaining contamination levels below 0.1 ppb of electronegative impurities.
• Internal Calibration: The strategic use of internal sources like ^83mKr and tritiated methane allows in-situ calibration, which is vital for maintaining precision and accuracy in the data analysis process.
• Photomultiplier Tubes (PMTs): Employing low-radioactivity PMTs with high quantum efficiency reduces detector noise and enhances sensitivity to low-energy events, which is essential for detecting potential WIMP signals.

Background Rejection and Sensitivity Reach

The anticipated background levels from material radioactivity are rigorously minimized through material selection and screening strategies. Neutrino-induced ERs and beta-decay products from intrinsic LXe radioactive isotopes like ^136Xe are managed using enhanced discrimination techniques. The expected background for LZ is primarily due to solar neutrinos, which sets an eventual theoretical limitation for WIMP detection sensitivity.

The designed LZ sensitivity to WIMP-nucleon cross sections is projected to reach unmatched levels. For spin-independent interactions, it is predicted to be capable of excluding cross sections down to approximately 2 × 10^-48 cm^2. Furthermore, the "S2-only" analysis offers additional advantages in probing low-mass WIMPs, thus covering a broader parameter space.

Implications for Future Developments in Dark Matter Detection

The LZ project not only fortifies the potential for a significant leap in the direct detection of dark matter but also consolidates a framework for developing future, even more-sensitive detectors. Exploring the boundary where coherent neutrino scattering emerges as an irreducible background will refine theoretical and experimental approaches to dark matter searches. The ability to test prominent theoretical models, such as supersymmetry and extra-dimensional theories, positions LZ as a critical experiment in the landscape of dark matter research.

Conclusion

Overall, the LUX-ZEPLIN Conceptual Design Report delineates a meticulously crafted apparatus poised to explore dark matter with an unprecedented sensitivity. Through sophisticated design and comprehensive background suppression, LZ aspires to not only detect WIMPs but also open new avenues in particle astrophysics, potentially redefining fundamental physics paradigms concerning dark matter.