Session B11: Dark Matter Direct Detection I

The LUX-ZEPLIN (LZ) Experiment

LUX-ZEPLIN (LZ) is a direct detection dark matter experiment located at the Sanford Underground Research Facility in Lead, South Dakota. The experiment consists of three nested detectors; a dual phase xenon TPC, an actively instrumented liquid xenon skin, and an outer detector neutron veto formed by 10 acrylic tanks of gadolinium-loaded liquid scintillator. The active region of the xenon TPC contains 7 tonnes of liquid xenon with a 5.6 tonne fiducial volume, allowing us to reach a WIMP-nucleon spin-independent cross section sensitivity of 1.4 x 10^-48 cm^2 for a 40 GeV/c^2 mass in 1000 live days. This talk will provide an overview of the LZ experiment and report on its status.    

Backgrounds In The LZ Dark Matter Experiment

LUX-ZEPLIN (LZ) is a dark matter experiment employing a 7 tonnes active volume of liquid xenon in a dual-phase time projection chamber (TPC), surrounded by an instrumented xenon “skin” region and a gadolinium-loaded liquid scintillator outer detector that primarily serve as active vetoes for gamma-ray and neutron backgrounds, respectively. A comprehensive material assay and selection campaign for detector components, along with a xenon purification campaign, have further ensured an ultra-low background environment. These mitigations allow LZ to have a projected sensitivity to WIMP-nucleon spin-independent cross section of 1.4 x 10^-48 cm² for a 40 GeV/c² mass WIMP over a 1000 day livetime using a quiet 5.6 tonnes fiducial volume. In order to achieve such unprecedented sensitivity to dark matter, it is critical to understand the backgrounds present in the detector. This talk will provide an overview of backgrounds in the LZ detector with an emphasis on techniques to constrain these backgrounds in situ. The implications of a well-constrained background model on the LZ sensitivity will also be discussed.   

Bias mitigation in the LUX-ZEPLIN dark matter experiment

As LUX-ZEPLIN (LZ) prepares to push the limits of known physics and improve our understanding of the nature of dark matter, it is important to ensure that these gains are not mistakenly influenced by human biases towards achieving such results. Such biases often appear in the process of analysis when unconsciously or consciously expecting certain outcomes. Many techniques for avoiding these biases have been employed over the years including blinding and using hidden parameters. LZ will be using a method known as salting, in which fake signal events are injected into our data stream and removed only after analysis is complete. In this presentation I will explain the historical motivations for pursuing bias mitigation, the process through which LZ salts its data, and the status of salting in LZ.   

Event Reconstruction in LZ for Dark Matter Direct Detection

The LUX-ZEPLIN (LZ) Dark Matter experiment is a multi-tonne dual-phase liquid and gaseous xenon time projection chamber that aims to achieve world-leading sensitivity to dark matter. Energy depositions in the detector yield photons and electrons in the form of prompt scintillation light (S1) and delayed electroluminescent light (S2). In order to conduct a dark matter search, LZ must be able to accurately reconstruct events based on S1 and S2 light in the detector. Pulse finding is done using difference of Gaussians (DoG) filters, while the classification of found pulses is dependent on a logical decision tree of pulse parameters such as rise time, area, and PMT coincidence. Lastly, event classification into single scatter, multiple scatter, pile-up, and other scatters is done via a sorting algorithm that takes into account the relative timings of the different pulses in an event window. To ensure that potential dark matter single scatter events are accurately identified against the much larger background of other event morphologies, it is crucial that the event classification is as accurate and efficient as possible. In this talk I will present on the development and validation of the event reconstruction algorithms for LZ.   

Measuring Detector Gains with Electronic Recoils and a Doke Plot

Searches for dark matter in dual-phase time-projection chambers distinguish electronic recoils (ER) from nuclear recoils (NR) by comparing the strengths of prompt scintillation (S1) to delayed electroluminescence (S2). Utilizing both internal and external ER calibration sources, the LUX-ZEPLIN (LZ) experiment measures the S1 and S2 response to recoil events. Position dependent gain factors, g1 and g2, are defined that convert the S1 and S2 signals to the average number of photons and average number of electrons, respectively. The gain factor values may be measured in data using the Doke plot technique, in which the mean light yield is plotted against the mean charge yield. In this talk, I will present the status of this work and the values of g1 and g2 - critical indicators of an instrument's overall performance, thus determined.    

DD neutron generator calibration in LUX-ZEPLIN dark matter search experiment

Nuclear recoil (NR) calibration is critical to understanding the response of a detector searching for dark matter candidates through direct interaction with normal matter. The LUX-ZEPLIN (LZ) Experiment is one such direct detection experiment. Monoenergetic 2.45 MeV neutrons from Adelphi Technologies’ DD 109 neutron generator were used for absolute NR calibration. DD neutrons can also serve as a unique low monoenergetic source by reflecting generated neutrons off a deuterium- or hydrogen-loaded target, reducing the neutron energy to 350 keV, or to 10-100 keV, respectively. Here, scintillation light from the reflector allows for a direct measurement of the energy using the time of flight of tagged neutrons between the reflector and the target. DD neutrons can provide an in-situ measurement of LZ’s NR band, an independent light and charge yield calibration, and an understanding of other various aspects of detector behavior. In this talk, I will discuss NR calibration using both direct and reflected DD neutrons in addition to its physical implementation.   

The Migdal Effect in LZ

LZ is a two phase xenon time projection chamber with 7 tonnes active and 5.6 tonnes fiducial volume designed to achieve a spin-independent cross section sensitivity of 1.4 x 10^-48 cm² for a 40 GeV/c² WIMP mass. For lighter WIMP masses, below about 5 GeV/c², this sensitivity is diminished due to the small energy transfer in elastic collisions of dark matter and xenon nuclei. Sensitivity to WIMPs in this mass range can be extended by searching for the ionization electrons concurrently produced in nuclear collisions, in an inelastic process called the Migdal effect. These recoiling electrons can carry more experimentally detectable energy than corresponding nuclear recoils, providing sensitivity to dark matter particles that would otherwise produce sub-threshold recoils. I will discuss the expected signature of the Migdal effect in LZ and the sensitivity enhancement to low mass dark matter that can be expected from searching for these types of events.   

LZ Sensitivity to Effective Field Theory Couplings

LUX-ZEPLIN (LZ) is a dual-phase xenon dark matter detector at the Sanford Underground Research Facility designed with the primary objective to observe low-energy interactions with Weakly Interacting Massive Particles (WIMPs). We report the sensitivity of LZ to a model-agnostic set of non-relativistic operators that may elicit detector responses at higher energies. These operators are formulated in an Effective Field Theory (EFT) framework that describes several types of possible dark matter interactions with nucleons. In this talk we will describe the backgrounds relevant for this study, along with the techniques used to overcome challenges in their modeling and identification.