Session J09: Liquid Xenon Dark Matter Experiments

Signal Yields in Liquid Xenon with the LUX Experiment

Two-phase xenon time projection chambers are a leading strategy for dark matter direct detection experiments. Some of the leading limits in the field are from the Large Underground Xenon experiment (LUX), XENON1T, and PANDAX-II. These experiments operate by measuring the scintillation and ionization of xenon when an incident particle, potentially a WIMP, interacts in the detector medium. As a result, it is crucial to calibrate these detectors and understand the light and charge yields from nuclear recoils and electron recoils. This is complicated by the fact that when xenon atoms are ionized, the positive ions and liberated electrons can recombine to form more scintillation light, and the recombination fraction depends on factors such as energy deposited, electric field, and particle type. I will present work done on the second science search (WS2014-16) of LUX to explain how recombination is a function of both the electric drift field and the energy deposited by electron recoils. Specifically, I will focus on the isotopes Kr83m and Xe131m, as they provide excellent mono-energetic calibrations of our detector.   

Measurement of Charge and Light Yields of Liquid Xenon using Carbon-14 and Tritium Beta Decay Sources in the LUX Detector

Many of the most sensitive WIMP dark matter searches are based upon liquid xenon TPC technology. The Large Underground Xenon (LUX) experiment, which operated at the Sanford Underground Research Facility from 2013 to 2016, set stringent new constraints on the existence of WIMPs with masses above 6 GeV/c2. After the final LUX WIMP search run was completed (WS2014-16), Carbon-14 and tritium sources were dissolved into the detector to measure the charge and light yields of liquid xenon to beta decay events. We report on the results of these measurements as a function of energy and electric field. These results will be useful for future dark matter searches such as LZ, where beta decay events from radon daughter species are expected to be the dominant source of electron recoil backgrounds.   

Calibrating the XENON1T Dark Matter Detector

XENON1T is the world’s largest running direct dark matter detector. With its first results, it achieved the lowest background of any experiment of its kind, as well as the most stringent exclusion limits on the spin-independent WIMP-nucleon cross section. Since the experiment is searching for nuclear recoils of WIMPs in liquid xenon, and its background is dominated by electronic recoils from Rn222 contamination, it is crucial to understand both types of interactions through careful calibrations with various sources. This talk will review the acquisition and analysis of calibration data in the most recent run of XENON1T, and how this data was used to understand and model the production of observables in the detector.   

Ultra-low Background of XENON1T Dark Matter Search Experiment

The XENON1T is currently the most sensitive dark matter search experiment, giving a minimum limit of $7.7\times10^{-47}\ cm^2$ for $35-GeV/c^2$ WIMPs at 90\% confidence level from its 34.2 days of data in the first science run. The success of XENON1T experiment is benefiting from the largest target mass for dark matter search and an ultra-low background in the low energy region down to 0.2 events/ton/keVee. This is achieved by continuous online cryogenic distillation of krypton level in LXe down to sub-ppt level. Since then, Rn222 becomes the dominating background for electronic recoils with a concentration of 10uBq/kg and more than 230 days of dark matter search data was accumulated after the first science run. In this talk, I will talk about how to understand each background components in the detector and how they affect dark matter searches.   

Xenon circulation and liquid-level stability in the LUX-ZEPLIN dark matter experiment

Two-phase noble element time projection chambers are easily scaled to large target masses, and thus are used in dark matter direct detection experiments. In the liquid state, the xenon must be continuously purified, both to limit backgrounds and to provide a long electron lifetime. Maintaining a stable liquid level while simultaneously cycling the liquid into and out of the detector presents a challenge due to the size of the liquid surface and high xenon flow rates needed for a dark matter search. The height of the liquid surface partially determines the amount of light produced in the gas phase by extracted electrons (electroluminescence) and thus plays a role in both background discrimination and energy resolution. In this talk, I will discuss the efforts at the LUX-ZEPLIN (LZ) system test to study the relation between liquid level stability and xenon circulation geometry. I will also outline the near-term plans for LZ circulation tests at the Sanford Underground Research Facility and the circulation system for LZ.   

Development of the LZ High Voltage Grids

The LZ experiment’s hunt for dark matter relies on a set of electric fields to measure ionization signals from WIMP-xenon interactions, enabling precise 3D position reconstruction and nuclear/electron recoil discrimination. To establish these electric fields, LZ will construct four woven mesh high voltage grids and set them at different heights in the detector. Because of the large 1.5-meter diameter of these grids, the strong electric fields on the wire surfaces, and the importance of having spatial field uniformity in the detector, a considerable amount of R&D is required to ensure that these grids are built in a way that satisfies LZ’s physics goals. This talk will discuss the grid development process and highlight current R&D efforts for optimizing grid construction and treatment prior to installation into LZ. It will also present preliminary results of applying these development techniques to a full LZ-scale grid.   

Status of the LZ Cathode High Voltage Research and Design Project

LUX-ZEPLIN (LZ) is a 7-ton dual phase xenon time projection chamber (TPC) which will perform several rare event searches. While LZ is superficially similar to its predecessor, LUX, the immense size of LZ (1.456 meters drift length for LZ vs. 48.0 cm drift length in LUX) leads to many design challenges associated with the increased scale. The generation and maintenance of the drift field of 600.0 Volts / cm is one area that becomes particularly difficult at larger length scales. High voltage must be delivered to the cathode without producing any additional light, spoiling the xenon’s purity, or exceeding the allowed radioactive background. To ensure that these effects will not diminish LZ’s sensitivity, the cathode high voltage cable feedthrough and cathode connection design are being tested at Lawrence Berkeley National Lab under conditions which will mimic those in LZ. In this talk I present the current status of the LZ Cathode High Voltage research and design project.   

Recent results from the LZ System Test platform at SLAC

LZ is a next generation dark matter search experiment designed to significantly extend sensitivity to WIMP dark matter candidates. At the core of the LZ design is a dual-phase Xe time projection chamber (TPC) with a 7 ton active volume. A cryogenic test platform with ~100 kg of liquid Xe, including a 50 cm tall TPC, has been constructed at SLAC to test multiple subsystems at scales approaching or comparable to the LZ design. The platform focuses on testing the high voltage performance of the TPC and the Xe circulation and purification system, and also provides an opportunity to test the integration of other subsystems. Run 7 of this platform included several upgrades: a new high-speed triggerless DAQ, a new extraction region which is a cloned profile of LZ, increased light collection efficiency with a new 32 PMT array, additional data channels with an instrumented skin region, and suppression of liquid oscillations with a revised circulation path. A brief overview of the design and construction of the test platform will be given, followed by a discussion of current detector performance, with a close look at the high voltage performance of the TPC.