Background Modeling and First Searches for Low Energy Signals in The LUX-ZEPLIN (LZ) Dark Matter Experiment PDF Download

Author: Daniel Kodroff
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Languages : en
Pages : 0

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Over the past half-century overwhelming evidence has mounted indicating the existence of a non-baryonic and enigmatic dark matter that constitutes approximately 85% of the total matter in the universe. Among the potential dark matter detection methods, dual- phase time projection chambers (TPCs) have emerged as the leading detector technology. LUX-ZEPLIN (LZ) is a direct detection dark matter experiment located at the 4850-ft depth level of the Sanford Underground Research Facility in South Dakota, USA, employing a 7 tonne active volume of liquid xenon in a dual-phase TPC. It's surrounded by an instrumented xenon "Skin" region and gadolinium-loaded liquid-scintillator outer detector, primarily serving as active vetoes for gamma-ray and neutron backgrounds, respectively, and contained within an ultra-pure water tank. The LZ detector began its first science run in December of 2021 and released its first results in the Summer of 2022. In order to ensure a low-background environment, a comprehensive material assay and selection campaign, for detector components, along with a xenon-purification campaign were pursued prior to and during construction. These mitigations have allowed LZ to achieve a background rate of 63.0 ± 4.5 x 10-6 events/keVee/kg/day in the low- energy region, approximately 60 times lower than that of its predecessor, the LUX experiment. LZ performed comprehensive measurements to constrain backgrounds in situ and construct a well-constrained time-dependent background model to use in searches for novel physics signals within this low-energy (

Author: Madan K. Sharma Timalsina
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Languages : en
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A hypothetical non-luminous existence of matter is known as dark matter, inferred by the convincing collection of astrophysical and cosmological indirect evidence. In spite of compelling indirect observations, the physical nature of dark matter remains one of the most profound questions in the field of modern physics. A Weakly Interacting Massive Particle (WIMP) is historically the most favored candidate particle for dark matter, which could nicely explain the observed indirect measurements with the direct detection of WIMPs for the first time. The new second-generation direct detection dark matter experiment LZ (LUXZEPLIN), designed for direct detection of WIMP dark matter, has performed the most sensitive search for spin-independent WIMP-nucleon interactions. LZ is located 4850 feet underground at the Sanford Underground Research Facility (SURF) in Lead, South Dakota, USA. LZ is employing a two-phase xenon detector with an active mass of 7 tonnes. With LZ we have recently managed in the summer of 2022 to provide the most rigorous exclusion limit for spin-independent WIMP-nucleon scattering with an upper limit on the cross-section of 6.5×10−48 cm2 (90 % confidence level) for a WIMP mass of 30 GeV/c2 . The first WIMP search result of LZ utilizes a fiducial mass of 5.5 tonnes of liquid xenon and an exposure time of 60 live days. WIMPs could interact in the cryogenic liquid xenon of the detector’s core by scattering off xenon nuclei, which would then recoil and produce both scintillation light and electric charge. The ratio of the immediately detected scintillation light (S1) and the delayed charge detection (S2) is characteristic for such a nuclear recoil (NR) from hypothesized dark matter, e.g. a WIMP, and differs significantly from an electron recoil (ER) produced by undesired background reactions. However, the precise knowledge of the energy-dependent ratio S1/S2, for which the ER-dominated regime transitions into the NR-dominated regime, is key hereby to separate WIMP dark matter signals from unwanted background signals. We performed calibrations with neutron sources to map out the NR signal region for the WIMP search. Instead, gamma- and beta-ray calibration sources were utilized to map out the ER region, characteristic for background signals to be discriminated against. In this thesis, the calibration data to map out the NR signal region has been extensively studied and compared to the results of a full LZ detector simulation. In addition, another crucial detector calibration, for which all LZ data has to be corrected, is the purity monitoring of the liquid xenon. The chemical purity determines the lifetime of signal electrons against the absorption on impurities during their drift within the liquid xenon time projection chamber of the LZ detector. This electron lifetime analysis has been performed on a daily basis within the framework of this thesis and results have been applied by every data evaluator within the large LZ collaboration and for obtaining the current world’s best exclusion limit on WIMP dark matter.

Author: Ryan Edward Linehan
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Languages : en
Pages : 0

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Approximately 15% of the matter density in the universe is composed of Standard Model particles, while the other 85% is composed of an enigmatic "dark" matter whose fundamental properties are unknown. In recent decades, there has been substantial interest in performing a direct detection of scattering between dark matter and Standard Model particles, but currently no such signature has been verifiably observed. The LUX-ZEPLIN experiment was built to perform such a direct detection of dark matter using a dual-phase xenon time projection chamber (TPC), which can observe both light and charge signals from an interaction in the xenon. The dual-phase xenon TPC technology is critically dependent on maintaining strong drift and extraction fields in order to observe the charge signal. These fields are established by a set of four stainless steel wire mesh high voltage electrode grids that span the full width of the TPC. During operation, these grids achieve wire surface fields well above 15 kV/cm. These high fields can produce spurious charge signals and signals from real radioactive decays with atypical light-to-charge ratios, both of which can lead to low-energy backgrounds in LZ science data. As a result, substantial effort in the design, construction, validation, and operation of these grids was required to ensure that the LZ detector could be used to pursue world-leading dark matter searches. This work presents a detailed characterization of these efforts, from grid design all the way through first science run data with LZ. This work also presents two analyses performed within the first six months of datataking: the first science run searching for WIMP dark matter, and an analysis of radiogenic backgrounds from the high voltage grids. While the former analysis represents an immediate world-leading sensitivity to WIMP dark matter between about 10 GeV/c^2 and 10 TeV/c^2, the latter analysis lays a new foundation for the background model needed to search for dark matter at much lower masses, down to approximately 1 GeV/c^2.

Author: Kelly M Stifter
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Languages : en
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Due to a compelling body of astrophysical and cosmological evidence, dark matter has come to be accepted as a crucial ingredient of modern cosmology, yet its physical nature remains one of the most pressing questions in the field of physics. One historically favored model of dark matter is weakly interacting massive particles, or WIMPs. LUX-ZEPLIN (LZ) is a next-generation dark matter detector designed to achieve field-leading sensitivity to much of the remaining accessible parameter space within the WIMP dark matter paradigm. To help realize the full-scale LZ detector, the System Test R&D platform was constructed at SLAC National Accelerator Laboratory to validate the performance of critical LZ subsystems at scales approaching or comparable to the LZ design. In this dissertation, I present results showing that the passivation of the high voltage electrodes in citric acid leads to a significant reduction in spontaneous emission of single electrons, potentially limiting a major instrumental background by up to several orders of magnitude and enabling a more sensitive dark matter search. The LZ detector has now been assembled at the Sanford Underground Research Facility (SURF) in Lead, South Dakota and is taking early data. I also give a first look at commissioning data that captured the first light from electrons in the LZ detector, and share methods to validate the in situ performance of the high voltage electrodes.

Author: Gus Eberlein
Publisher:
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Languages : en
Pages : 0

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Comprising 85% of the mass of the universe, dark matter is one of the most pressing outstanding questions of physics. When it comes to directly detecting dark matter, the LUX-ZEPLIN (LZ) experiment has unparalleled sensitivity. We examine the detector's capability to detect sub-GeV dark matter scattering off the electrons in xenon, LZ's scintillation medium. We develop a signal model by calculating the expected DM-electron event rates as a function of electron recoil energy and as a function of the number of freed electrons. Alongside an established backgrounds model, this signal model is used to simulate events in the LZ detector. With this simulated data and a cut-and-count analysis, we are able to estimate cross sections of dark matter-electron scattering down to which LZ can detect the signal over the background. We find that LUX-ZEPLIN will be able to detect certain light dark matter models at a much greater sensitivity than previous direct detection experiments.

Author: Elena Aprile
Publisher: John Wiley & Sons
ISBN: 3527609636
Category : Science
Languages : en
Pages : 362

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This book discusses the physical properties of noble fluids, operational principles of detectors based on these media, and the best technical solutions to the design of these detectors. Essential attention is given to detector technology: purification methods and monitoring of purity, information readout methods, electronics, detection of hard ultra-violet light emission, selection of materials, cryogenics etc. The book is mostly addressed to physicists and graduate students involved in the preparation of fundamental next generation experiments, nuclear engineers developing instrumentation for national nuclear security and for monitoring nuclear materials.

Author: Umit Utku
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Category :
Languages : en
Pages :

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Author: Jacob Edward Cutter
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Languages : en
Pages : 0

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One of the most important fundamental problems in physics today is to understand the nature of dark matter. The landscape of explanations for observed dark matter phenomena is vast and still expanding, and an impressive number of experiments have been built to probe the dark sector of the universe. A prominent class of detectors is aimed at discovering (or excluding) a particular kind of dark matter: the Weakly Interacting Massive Particle (WIMP). Searching for this popular dark matter candidate requires an ultra-sensitive, low-background target; xenon detectors serve as such a target for dark matter interactions. The Large Underground Xenon (LUX) detector is a dual-phase xenon time-projection chamber (TPC) which was operated underground at the Homestake Mine in Lead, South Dakota from 2013 to 2016, and was able to achieve the world's leading WIMP exclusion limit. However, successful reconstruction of WIMP-nucleus scatters in such detectors requires thorough understanding of the detection medium, which is made difficult by various confounding effects near the detector walls. Field-fringing is a major component of confusion in the periphery, and the large electric field non-uniformities in Run 4 of LUX provided a significant challenge in the dark matter analysis. Here is presented an algorithm to bijectively map between reconstructed event positions and true spatial coordinates, which serves as an important tool for studying field effects and fiducialization in LUX. Additionally, a successful dark matter search must model interfering background events in the WIMP search region which can't be directly vetoed. One class of unavoidable backgrounds comes from nuclear decay chain daughters in detector materials themselves, which may produce WIMP-like signals (an effect which is amplified due to various detector effects). The Davis Xenon (DAX) test bed system and a dual-phase TPC have been assembled and operated at UC Davis to characterize these common "wall backgrounds", as well as perform other R&D studies for the next-generation LUX-ZEPLIN (LZ) experiment. The DAX TPC specifically measures the xenon response to heavy nuclei produced by custom [alpha] decay sources created using novel chemical deposition procedures. In this thesis, results will be presented for the light and charge yields of immersed localized sources of 206Pb ions in liquid xenon, as well as a method for tagging such recoil events in situ by using PIN diodes as charged particle detectors to capture the correlated [alpha] particles. We also compare our isolated 206Pb events with previous WIMP search data from LUX, and discuss the significance of 206Pb as a WIMP background. Such information is most useful to future experiments if it can improve existing background models and simulations. The Noble Element Simulation Technique (NEST) is the ultimate software package for calculating expected signal yields in xenon detectors, but is an empirical framework that relies on experimental data to inform the models. We discuss the development of current NEST v2 models, specifically the heavy nuclear recoil models, as well as our current understanding of the xenon microphysics. We also show NEST predictions for mono-energetic 206Pb recoils, and discuss how our most recent DAX 206Pb measurements may inform NEST models in future work.

Author: Wei Ji
Publisher:
ISBN:
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Languages : en
Pages :

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Dark Matter is needed to explain many cosmological observations and therefore has been proposed for many decades, but it awaits direct detection. One of the most popular classes of dark matter candidates is Weakly Interacting Massive Particles (WIMPs), which have masses in the order of 100 GeV and couple to ordinary matter at weak scale. In WIMP direct detection experiments, we look for WIMPs being scattered by nuclei, a process which produces low energy (smaller than 100 keV) recoiling nuclei that can be observed. We are building LZ, a detector looking for WIMPs using liquid xenon in a dual-phase time projection chamber (TPC), at 4850 feet underground at the Sanford Underground Research Facility (SURF) in Lead, South Dakota, USA. LZ aims to achieve the world's highest sensitivity to find WIMPs via WIMP-nucleon interactions. After a brief discussion of dark matter and the LZ experiment, this dissertation presents the details of my study to solve the electron emission problem. The LZ TPC will consist of electrode grids and other metallic surfaces that can emit electrons when operated under voltage. Because the charge measurement in the LZ detector is sensitive to single electrons, electrons from the grids can be both a significant nuisance for data collection and a source of background at low-energies, limiting the sensitivity of the experiment for low-mass WIMPs. This has motivated us to develop a test detector to study how to reduce this background. The test detector consists of a pair of grids biased to high voltage and operated in xenon gas. The electric field between the grid causes the electrons to produce electroluminescence scintillation light that is measured by PMTs. This new technique is sensitive to single electrons emitted by the grids, allowing a measurement of emission currents as low as atto-amperes. We used this detector to study the properties of different grids and to determine what treatments can be done to reduce their electron emission. We found that passivation with citric acid reduces electron emission from stainless steel surfaces. This work was supervised by Professor Thomas Shutt and was completed in collaboration with members of the LZ collaboration and the SLAC LZ group.

Author: Jianting Gao
Publisher:
ISBN:
Category :
Languages : en
Pages :

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Galaxies and clusters of galaxies are believed to be dominated by non-luminous non-baryonic dark matter. A favored candidate is a new type of Weakly Interacting Massive Particle (WIMP) with a mass of order 100 GeV/c^2. The ZEPLIN II experiment is a WIMP search experiment that attempts to directly detect WIMP interactions using the two-phase xenon approach. The detector measures both scintillation and ionization generated by interactions in a 31 kg liquid xenon target. This approach provides a powerful discrimination between nuclear recoils, as expected from WIMPs, and background electron recoils. In this work, we develop a new [chi]^2 approach to determine the three dimensional event positions in an attempt to improve the background rejection. The optical properties of the PTFE reflectors and the grids of the detector were determined using the Geant4 simulation, and event positions were obtained by finding the best match to the amount of light in each photomultiplier. This was found to greatly improve the position resolution. The approach was then applied to the WIMP search data. It was found that one of the dominating background sources was events from the gas above the anode grid and not from the PTFE walls caused by the small signals as previously thought. WIMP search results were then obtained from the first 31 days of stable ZEPLIN II data using two methods. Although the [chi]^2 method greatly improved position resolution, the number of background events was not significantly altered and the new limit agreed well with the limit published by the collaboration.