Low-energy (0.7-74 keV) nuclear recoil calibration of the LUX dark matter experiment using D-D neutron scattering kinematics

Abstract: The Large Underground Xenon (LUX) experiment is a dual-phase liquid xenon time projection chamber (TPC) operating at the Sanford Underground Research Facility in Lead, South Dakota. A calibration of nuclear recoils in liquid xenon was performed $\textit{in situ}$ in the LUX detector using a collimated beam of mono-energetic 2.45 MeV neutrons produced by a deuterium-deuterium (D-D) fusion source. The nuclear recoil energy from the first neutron scatter in the TPC was reconstructed using the measured scattering angle defined by double-scatter neutron events within the active xenon volume. We measured the absolute charge ($Q_{y}$) and light ($L_{y}$) yields at an average electric field of 180 V/cm for nuclear recoil energies spanning 0.7 to 74 keV and 1.1 to 74 keV, respectively. This calibration of the nuclear recoil signal yields will permit the further refinement of liquid xenon nuclear recoil signal models and, importantly for dark matter searches, clearly demonstrates measured ionization and scintillation signals in this medium at recoil energies down to $\mathcal{O}$(1 keV).

• The paper presents a D-D neutron scattering calibration method achieving precise low-energy (0.7-74 keV) nuclear recoil measurements in liquid xenon.
• It employs rigorous experimental design and Monte Carlo simulations to assess both ionization (0.7-24.2 keV) and scintillation (1.08-12.8 keV) signal yields.
• These findings enhance detection sensitivity for low-mass WIMPs and inform the design of future xenon-based dark matter detectors.

Low-energy (0.7-74 keV) Nuclear Recoil Calibration of the LUX Dark Matter Experiment

Introduction

The Large Underground Xenon (LUX) experiment employs a dual-phase liquid xenon time projection chamber (TPC) designed to detect WIMP dark matter. This experiment's calibration using nuclear recoils within liquid xenon is critical for refining detection methodologies, essential for enhancing dark matter detection sensitivity. This paper details the methods utilized for calibrating nuclear recoil using a collimated beam of mono-energetic 2.45 MeV neutrons, leveraging deuterium-deuterium (D-D) fusion to achieve precise nuclear recoil energy measurements. The calibration offers critical insights into signal yields within xenon, such as ionization and scintillation signals, extending detection sensitivity in the 0.7 to 74 keV range.

Figure 1: Conceptual diagram of the LUX D-D calibration experimental setup. The LUX TPC is centered, surrounded by a water shield, highlighting various calibration components.

Methodology

Experimental Setup

The experiment employed a carefully structured setup, anchored by the LUX TPC positioned centrally within an expansive water shield environment designed to minimize background noise (Figure 1). Utilizing a D-D neutron generator, neutrons are collimated through an air-filled conduit, ensuring precise entry into the xenon medium. This strategic design facilitates exact measurements necessary for effective calibration and subsequent dark matter searches. Notably, the setup ensures maximal purity and minimal scatter, attributes critical for achieving the desired low-energy calibration threshold.

Signal Yield Calibration

The LUX calibration explores both ionization and scintillation yields, essential for characterizing nuclear recoil responses. By implementing rigorous Monte Carlo simulations and leveraging sophisticated signal-processing models, the experiment delineates a robust light and charge yield profile across the 0.7-74 keV range.

Ionization Yield

Measured from 0.7 to 24.2 keV, the ionization yield provides insights into electron production during nuclear recoils. Utilizing double-scatter events, the energy deposition is scrutinized, enabling precise energy reconstructions critical to ensuring accurate calibration and alignment with theoretical predictions.

Figure 2: The measured ionization signal for each event, showcasing error margins and calculated ionization signals per nuclear recoil energy bin.

Scintillation Yield

Scintillation yield measurements extend from 1.08 to 12.8 keV, analyzing single-scatter events to enhance light yield calibrations (Figure 3). The collected data significantly align with state-of-the-art models, illustrating the potential for enhancing xenon-based detection capabilities.

Figure 3: Histogram representation of the scintillation yield distribution across nuclear recoil energy bins, aligned against a model fit.

Results and Implications

The calibration yields critical data, underscoring the LUX experiment’s capacity for fine-tuned dark matter searches. Highlighting a substantial advancement, the results unequivocally support an expanded accessible WIMP mass range, notably enhancing sensitivity to low-mass WIMPs—a crucial frontier in modern dark matter research.

Figure 4: Comparison of measured yields against model predictions, delineating a precise alignment across energy levels.

Future Developments

The insights garnered from LUX's calibration efforts lay foundational groundwork for future dark matter investigations. Enhancing sensitivity, particularly to lower WIMP masses, heralds significant breakthroughs in understanding dark matter properties. The methodologies and results from this study will likely inform the design and calibration techniques of future xenon-based detection systems.

Conclusion

The LUX experiment has successfully implemented a sophisticated calibration protocol using D-D neutron scatter kinematics, critically refining nuclear recoil signal detection capabilities. By precisely measuring signal yields at low-energy thresholds, this research paves the way for more sensitive and expansive dark matter explorations, enhancing our understanding of fundamental cosmic phenomena.