Theses and Dissertations from UMD
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Item PANDAX-II DARK MATTER DETECTOR AND ITS FIRST RESULTS(2019) TAN, ANDI; Ji, Xiangdong; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The particle physics nature of dark matter (DM) is one of the most fundamental scientific questions nowadays. The leading candidates, weakly interacting massive particles (WIMPs), can be directly detected by looking for WIMP-nucleus scattering events in deep underground laboratories. Since the 1980s, physicists have improved the sensitivity of direct DM detection by about seven orders of magnitude. In the last decades, dual-phase xenon detectors exhibit their advantages in background rejection and scalability and lead the sensitivity in high mass WIMP direct searches. Experiments in XENON and LUX projects have been continuously pushing the exclusion limits of the elastic WIMP-nucleon scattering cross section into the parameter space predicted by various theoretical models. The Particle and astrophysical Xenon (PandaX) project is a series of xenon- based ultra-low background experiments in the China Jinping Underground Laboratory (CJPL) targeting the unknown physics of DM. The first stage of the project, the PandaX-I experiment, with a 120 kg sensitive liquid xenon (LXe) target, performed the WIMP search in 2014 with a 54×80.1 kg-day exposure. PandaX-I reported a strong limit on the WIMP-nucleon cross section for a WIMP mass of less than 10 GeV/c2, strongly disfavoring all positive claims from other experiments. The construction and installation of the second stage, PandaX-II experiment, with a half-ton scale LXe target, commenced after PandaX-I. In 2015, PandaX- II reported a WIMP search result with a 306×19.1 kg-day exposure from a short physics commissioning run with a notable 85Kr background. With 580 kg LXe in the sensitive region, PandaX-II was the largest running dual-phase xenon detector before the XENON1T detector in 2017. PandaX-II reported the most stringent limit on the WIMP-nucleon scattering cross section at 2.5×10−46 cm2 for the WIMP mass 40 GeV/c2 with a total exposure of 33 ton-day in 2016 and updated the limit to 8.6×10−47 in 2017 with a total exposure of 54 ton-day. In this dissertation, I will focus on the PandaX-II experiment, data analysis and its constraints on theoretical models. After a distillation campaign for krypton removal in 2017, the PandaX-II experiment achieved a background level of 0.8×10−3 event/kg/day/keV which was the lowest among similar detectors at the time. Compared to other dual-phase xenon detectors, we drift electrons by applying bias voltages on the electrodes which producing a stronger uniform electric field at a strength about 400 V/cm. After running for more than three years, more than 97% of 110 3′′ photomultiplier tubes (PMTs) perform stably. The analysis processes are continuously improved in various run periods in PandaX-II. The recorded waveforms were processed using a custom-developed software through several steps including hit finding and calculation, signal clustering, and so on to the final pairing analysis of scintillation and ionization signals. In this dissertation, I shall cover some topics in these steps as following. The amplification factors (gains) of the PMTs are calibrated using Light Emitting Diode (LED) light periodically for transforming the recorded waveform to the number of photon electrons (PE) and energy. An inefficiency raised from zero length encoding (ZLE), a data suppression firmware of the data acquisition system, is investigated in run periods with relatively low PMT gain. A data-driven algorithm is developed for the X-Y position reconstruction using the hit pattern of the proportional scintillation on the top PMT array. The mono-energetic events from xenon isotopes are studied to correct the non-uniformity of the detector response, and key parameters are extracted to reconstruct the deposited energy of events. The background analysis is critical for rare-event search experiments. I will present the study on the intrinsic electron recoil backgrounds from krypton, radon and xenon isotopes. The constraints of PandaX-II data on various theoretical models are investigated.Item Alphas and Surface Backgrounds in Liquid Argon Dark Matter Detectors(2017) Stanford, Chris; Meyers, Peter; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Current observations from astrophysics indicate the presence of dark matter, an invisible form of matter that makes up a large part of the mass of the universe. One of the leading theories for dark matter is that it is made up of Weakly Interacting Massive Particles (WIMPs). One of the ways we try to discover WIMPs is by directly detecting their interaction with regular matter. This can be done using a scintillator such as liquid argon, which gives off light when a particle interacts with it. Liquid argon (LAr) is a favorable means of detecting WIMPs because it has an inherent property that enables a technique called pulse-shape discrimination (PSD). PSD can distinguish a WIMP signal from the constant background of electromagnetic signals from other sources, like gamma rays. However, there are other background signals that PSD is not as capable of rejecting, such as those caused by alpha decays on the interior surfaces of the detector. Radioactive elements that undergo alpha decay are introduced to detector surfaces during construction by radon gas that is naturally present in the air, as well as other means. When these surface isotopes undergo alpha decay, they can produce WIMP-like signals in the detector. We present here two LAr experiments. The first (RaDOSE) discovered a property of an organic compound that led to a technique for rejecting surface alpha decays in LAr detectors with high efficiency. The second (DarkSide-50) is a dark matter experiment operated at LNGS in Italy and is the work of an international collaboration. A detailed look is given into alpha decays and surface backgrounds present in the detector, and projections are made of alpha-related backgrounds for 500 live days of data. The technique developed with RaDOSE is applied to DarkSide-50 to determine its effectiveness in practice. It is projected to suppress the surface background in DarkSide-50 by more than a factor of 1000.Item Signal Corrections and Calibrations in the LUX Dark Matter Detector(2016) Knoche, Richard; Hall, Carter; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Large Underground Xenon (LUX) Detector has recently finished a 332-day exposure and placed world-leading limits on the spin-independent WIMP-nucleon scattering cross section. In this work, we discuss the basic techniques to produce signal corrections, energy scale calibrations, and recoil band calibrations in a dark matter detector. We discuss a nonuniform electric field that was present during LUX's 332-day exposure, and detail how such a field complicates these calibration techniques. Finally, we expand on the techniques presented earlier, such that they account for all of the complication introduced by the nonuniform electric field, and allow a limit to be produced from the data.Item Measurement of the Electron Recoil Band of the LUX Dark Matter Detector With a Tritium Calibration Source(2014) Dobi, Attila; Hall, Carter; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Large Underground Xenon (LUX) experiment has recently placed the most stringent limit for the spin-independent WIMP-nucleon scattering cross-section. The WIMP search limit was aided by an internal tritium source resulting in an unprecedented calibration and understanding of the electronic recoil background. Here we discuss corrections to the signals in LUX, the energy scale calibration and present the methodology for extracting fundamental properties of electron recoils in liquid xenon. The tritium calibration is used to measure the ionization and scintillation yield of xenon down to 1 keV, the results is compared to other experiments. Recombination probability and its fluctuation is measured from 1 to 1000 keV, using betas from tritium and Compton scatters from an external 137-Cs source. Finally, the tritium source is described and the most recent results for ER discrimination in LUX is presented.Item Minimal Left-Right Symmetric Model, Neutron Electric Dipole Moment and Dark Matter(2011) An, Haipeng; Ji, Xiangdong; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In a class of other beyond-standard-model theories, CP-odd observables, such as the neutron electric dipole moment, receive significant contributions from flavorneutral P-odd and CP-odd four-quark operators. However, considerable uncertainties exist in the hadronic matrix elements of these operators strongly affecting the experimental constraints on CP-violating parameters in the theories. Here we study their hadronic matrix elements in combined chiral perturbation theory and nucleon models. We first classify the operators in chiral representations and present the leading-order QCD evolutions. We then match the four-quark operators to the corresponding ones in chiral hadronic theory, finding symmetry relations among the matrix elements. Although this makes lattice QCD calculations feasible, we choose to estimate the non-perturbative matching coefficients in simple quark models. We finally compare the results for the neutron electric dipole moment and P-odd and CP-odd pion-nucleon couplings with the previous studies using naive factorization and QCD sum rules. Our study shall provide valuable insights on the present hadronic physics uncertainties in these observables. Using an effective theory approach, the neutron electric dipole moment in the minimal left-right symmetric model with both explicit and spontaneous CP violations is recalculated systematically. Using the state-of-the-art hadronic matrix elements, nEDM as a function of right-handed W-boson mass and CP-violating parameters is obtained. The most stringent constraint yet on the left-right symmetric scale in the minimal version of left-right symmetric model is obtained to be MWR > (10 ± 3) TeV. Light WIMP (weakly interacting massive particle)-like signals were reported by dark matter direct detection experiments. WIMP candidates in this energy range can be constrained by various collider experiments. We show that colliders can impose strong constraints on models of low mass dark matter, in particular in the case that the direct detection interaction depends on the momentum of dark matter. We also find in the case of low mass dark matter, there are tensions between the observed relic abundance and collider constraints. Putting the constraints from collider physics, relic abundance and direct detection experiments, a large part of parameter space in different models can be ruled out.Item The Search for Neutralino Dark Matter with the AMANDA Neutrino Telescope(2008) Ehrlich, Ralf; Sullivan, Gregory; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)There is convincing indirect evidence based on cosmological data that approximately one quarter of the universe is made of dark matter. However, to this date there is no direct detection of the dark matter and its nature is unknown. Most theories suggest that this dark matter is made of Weakly Interacting Massive Particles (WIMPs), or more specifically: supersymmetric particles. The most promising candidate out of the supersymmetric particles is the lightest neutralino. These neutralinos can get trapped in the gravitational field of the Earth, where they accumulate and annihilate. The annihilation products decay and produce neutrinos (among other particles). These neutrinos (the focus is on muon-neutrinos here) can be detected with the AMANDA neutrino telescope located between one and two kilometers deep in the ice of the glacier near the South Pole. Neutrinos cannot be detected directly. However, there is a small possibility that they interact with nuclei of the ice and create charged leptons. These charged leptons continue to travel in the same direction as the neutrinos (accompanied by electromagnetic/hadronic cascades, and electrons). As long as their speed is higher than the speed of light of the ice, they emit Cherenkov radiation which can be captured by photomultipliers installed inside the ice. The signals collected by the photomultipliers can be used to reconstruct the track of the lepton. AMANDA - the Antarctic Muon and Neutrino Detector Array - makes use of the unique properties of the neutrino: Since neutrinos interact only weakly, they can travel through the Earth without being stopped. Therefore all detected particles which have been identified as upward going (i.e. through the Earth coming) must have been produced by charged leptons originating from neutrinos after they reacted with the nuclei of the ice. All other particles which do not come from below are rejected. If the neutrino flux coming from the neutralino annihilation inside Earth is strong enough to be detected with AMANDA, it should show up as an excess over the expected neutrino flux, which comes from the atmospheric neutrinos produced in the northern hemisphere. This analysis which used data from 2001 and 2002 showed that there is no significant excess, yielding an upper limit on the neutrino flux that could have come from WIMP annihilation.