Session R13: Understanding Noble Element Detectors for Dark Matter and 0vBB

Live

Noble-element detectors are common in rare-event searches, such as those involving dark matter and neutrinos. The sensitivities of these searches require an accurate model to identify backgrounds and predict signal topologies. The Noble Event Simulation Technique (NEST) is a comprehensive, mostly-empirical package for complete and accurate simulation of noble element detector response. NEST’s capabilities include simulating the scintillation and ionization yields of various particle interactions with noble elements. I will present recent upgrades to the NESTv2.0 package concerning xenon response. NEST's use for xenon detectors is based heavily on experimental data from a number of solid, liquid, and gaseous detector experiments. Due to this abundance of data, most theoretical models in NEST for xenon have been replaced with simple, well-behaved, empirical formulas. Furthermore, NEST can be customized to a detector’s various experimental parameters such as electron lifetimes. NEST employs an empirical, non-binomial recombination model and simulates scintillation and ionization signals in dual-phase time-projection chambers with correct energy resolution. NESTv2.0 can be operated as a standalone command-line tool, used with GEANT4, or more recently with Python as nestpy.   

Live

Historically, dark matter direct detection experiments searching for dark matter in the form of Weakly Interacting Massive Particles (WIMPs) typically consider only two couplings between atomic nuclei and the WIMP: spin-independent and spin-dependent interactions. However, the lack of of an observed WIMP signal encourages consideration of more WIMP-nucleon interaction types, and recent theoretical work provides a basis composed of 14 independent effective field theory (EFT) operators. The inclusion of these additional operators leads to the possibility of WIMP interactions at higher energies than traditional WIMP analyses. In this presentation we will report on the search for WIMP-nucleon interactions at higher energies with data collected by the LUX detector during its tenure in the Davis Campus of the Sanford Underground Research Facility. Specifically, we will discuss background modeling for an EFT dark matter search, and we will present the results of the EFT analyses using LUX data.   

Live

The two-phase liquid xenon time projection chamber is one of the leading technologies used for dark matter direct detection. World-leading limits on dark matter interactions have been set by LUX and XENON1T, and the upcoming LZ and XENONnT experiments seek to push further. A crucial part of using this technology is being able to classify energy deposits as nuclear recoils (NR) or electron recoils (ER). In my talk, I will discuss how ER-NR discrimination can influence the performance of future detectors, informed by our analysis of LUX calibration data. I will focus on this via two paradigms: effects on discrimination from detector parameters like electric field and light collection, and from physical variables like pulse-shape and energy. I will also discuss the physical origins of fluctuations in electron recoil signals and how LUX data can inform our understanding of these effects.   

Live

The beta emitter Krypton-85 is a significant potential source of electron recoil backgrounds in liquid xenon dark matter experiments. The LUX-ZEPLIN (LZ) experiment uses a gas charcoal chromatography system to remove trace krypton from the xenon before it is deployed in the detector. This system, located at SLAC National Accelerator Laboratory, has successfully purified xenon to a total krypton concentration of less than 100 parts-per-quadrillion krypton, and will be used to process the full 10 tonnes of xenon for LZ. In this talk, I will present an overview of the system and report on the status of the krypton removal campaign.   

Live

The LUX-ZEPLIN (LZ) dark matter experiment will consist of 7 active tonnes of liquid xenon sensitive to the nuclear recoils induced by impinging weakly interacting massive particles (WIMPs). Backgrounds to a WIMP signal tend to populate the boundaries of the LZ sensitive volume, where gamma-rays and neutrons from nearby material can enter, scatter once, and exit. The Outer Detector (OD) of LZ consists of 17 tonnes of gadolinium-loaded liquid scintillator, surrounding the LZ liquid xenon, and is capable of efficiently tagging both gamma-rays and neutrons which have scattered in the liquid xenon. The OD provides a substantial increase in the background-free liquid xenon mass of LZ, expanding the volume available for WIMP search. We will report on the design studies, radioactive background requirements and results, and expected performance of the LZ Outer Detector.   

Argon Recoil Ionization and Scintillation from Electron Recoils

In 2018, the Argon Recoil Ionization and Scintillation (ARIS) experiment studied the response of liquid argon to monoenergetic neutrons. In a continuing effort to study the response of liquid argon at low energies, the proposed Argon Recoil Ionization and Scintillation from Electron Recoils (ARIS-ER) will measure the response of liquid argon to monoenergetic gammas which Compton scatter in the detector. The liquid argon Time Projection Chamber (TPC) will be exposed to 511 keV gammas produced by a Na-22 source. A Broad Energy Germanium (BEGe) detector will measure the energy of gammas which scatter in the TPC to determine the energy deposited. The ARIS-ER measurement will provide information on the scintillation and ionization energy scales, quenching factor, recombination probability, and time response of liquid argon to low-energy electron recoils.   

Purification of Xenon in the Liquid Phase in XENONnT

The observable ionization signal in liquid xenon (LXe) time-projection chambers is reduced if electronegative molecules attract and capture drifting electrons. Removing these impurities, which are outgassed from detector materials, is crucial to the performance of these detectors. The XENONnT experiment will be the first of its kind to benefit from the higher mass flow delivered by commercial cryogenic pumps, in contrast to high-temperature getters and gas pumps working with gaseous xenon. This requires a filter with a sufficiently high reaction rate and adsorption capacity at LXe temperatures, while also meeting the radio-purity requirements of these detectors as they search for rare signals. This talk will describe the design of the liquid xenon purification system built for the XENONnT dark matter experiment, preliminary results of its performance along with results from a dedicated test stand operated at Columbia University.   

Energy resolution of the XENON1T Dark Matter detector in the keV to MeV range

Xenon dual-phase time projection chambers designed to search for Weakly Interacting Massive Particles (WIMPs) have been characterized by deteriorating energy resolution for electronic recoil energies above $\sim$100 keV, due to their common emphasis on amplifying tiny signals for low energy threshold. In the XENON1T experiment, we have developed a signal correction method to rectify the saturation of the digitizer dynamic range and distortions due to the non-linear response of the photomultiplier tubes. Searches for signals of physics interest in the energy range from keV to MeV all benefit from this method. In particular, we demonstrate that at 2.46\, MeV, the expected energy of the neutrinoless double-beta decay signal of $^{136}$Xe, the relative energy resolution (at 1-$\sigma$) is as low as (0.81$\pm$0.02)\,\% for single-site interactions.   

Not Participating

The XENON1T experiment is the most sensitive direct detection experiment for WIMP dark matter with masses above 6 $GeV/c^2$. We used a dual-phase xenon time-projection chamber containing 2 metric tons of liquid xenon. An exposure of one tonne-year of science data was collected between October 2016 and February 2018. This talk will present the event selection method used in data analysis of XENON1T that led to the most stringent limits on various WIMP models. I will discuss the reconstruction and selection of the events used by that analysis, and their respective efficiencies. The goal of these selections were to isolate single-scatter events in the energy range of interest. These selections depends upon a series of reconstruction and correction steps on the data. Our analysis starts with understanding photosensor stability and performance, followed by how we reconstruct scintillation and ionization signals, before ending with how properties of interactions are determined. I will discuss these steps while also explaining certain classes of backgrounds that we remove.