CDMS Ph.D. Theses (listed in reverse chronological order)

N. Kurinsky, The Low-Mass Limit: Dark Matter Detectors With eV-Scale Energy Resolution, Ph.D. Thesis, Department of Physics, Stanford University
In this thesis, I cover three elements of the design of the CDMS HV detectors. I discuss the detector physics controlling how charges and phonons are generated in our detector crystals, com- paring theory to results of recent experiments carried out at Stanford. I move on to describe the operating principles of our phonon-mediated charge readout, as well as the design of the CDMS HV detector. I then describe the performance tests of early CDMS HV prototypes in conjunction with the SuperCDMS SNOLAB electronics, and discuss the path towards achieving single electron-hole pair resolving detectors at the kg-scale given the performance obtained thus far. As a result of these tests, we were able to refine our noise and sensor dynamics models, and develop new metrics for diagnosing non-ideal sources of noise to aid in reducing coupling of the external environment to our detectors. Read more here: pdf

D. Barker, SuperCDMS Background Models for Low-Mass Dark Matter Searches, Ph.D. Thesis, Department of Physics, University of Minnesota
The CDMSlite spectrum had a large contribution from electron recoil background events. From the information gained during the first two CDMSlite Runs, a background model was developed for the third and final CDMSlite Run. Analytical descriptions were identified for those backgrounds that were theoretically known, e.g. tritium β-spectrum, and Geant4 simulations were used to understand and predict the low-energy spectra from other sources, e.g. Compton scattering.

Multiple new models were developed for detectors operated in CDMSlite at Soudan. These include the analytical formula for Compton scattering, and empirical models for surface backgrounds from 210Pb contamination of the germanium crystals and detector housing. In order to accurately describe the surface events, a new detector response function was developed that included information about the electric field and energy resolution of the detector. These models were essential to the implementation of a profile likelihood analysis of the CDMSlite Run 3 data, which improved on the sensitivity to dark matter over the Run 2 optimum interval analysis for WIMP masses above 2.5 GeV/c2. This demonstrated a successful application of a likelihood analysis to the high-voltage operating mode, and the potential for these analyses in the future SuperCDMS SNOLAB experiment. Read more here: pdf

M. Daal, Kinetic Inductance Detectors for Dark Matter Searches, Ph.D. Thesis, Department of Physics, University of California, Berkeley
The detectors of the Cryogenic Dark Matter Search (CDMS) measure phonons and charge emanating from recoiling germanium nuclei. Its phonon sensors, though exquisitely sensitive, are relatively complicated to fabricate and deploy. Responding to the scientific imperative to increase dark matter sensitivity, we look for a simpler phonon sensor technology. Kinetic inductance based phonon sensors (KIPS) are superconducting thin film microwave frequency resonant circuits. Phonon energy impinging on the film alters its surface impedance, which alters the characteristics of the resonance. KIPS can be made with relatively large, millimeter sized features which makes them easier to fabricate and reduces variations between detectors.

This instrumentation focused thesis shows that KIPS can be used as a simple and sensitive phonon sensor for the CDMS detector. KIPS design aspects, competitiveness to the current transition edge phonon sensors, readout considerations and suggestions on how to instrument them in future dark matter experiments will be presented. The broader applicability of KIPS in nuclear non-proliferation and other physics investigations is also discussed. Read more here: pdf

D. Speller, A Background-Subtracted Search for Annual Modulation in CDMS II, Ph.D. Thesis, Department of Physics, University of California, Berkeley
The present work is a description of the search for a direct detection of super-symmetric dark matter via scattering from standard model particles. The Cryogenic Dark Matter Search (CDMS) Experiment uses ionization and athermal phonon sensor technologies to achieve event-by-event discrimination of electron and nuclear recoils in cryogenic germanium crystal detectors. At low energies, where the the ability to discriminate individ- ual nuclear recoil events from background is reduced, a periodic variation of the rate and crossover signatures in the energy spectrum can aid the identification of a WIMP signature in the presence of significant backgrounds. In general, the direct detection of dark matter is the first step toward the identification and classification of dark matter in the universe.

This work describes a background-subtracted search for annual modulation in the WIMP- search data acquired in the Cryogenic Dark Matter Search II (CDMS II) Experiment, which was the second implementation of the highly successful CDMS technology. We observe no significant modulation in the 2.7 keVnr to 11.9 keVnr (nuclear-recoil-equivalent) energy range selected for this analysis. These results are not compatible with a WIMP dark matter interpretation of the signals reported by the DAMA/LIBRA and CoGeNT experiments, and provide complementary support to earlier CDMS low-threshold germanium analyses. Read more here: pdf

A. Kennedy, Simulating The Supercdms Dark Matter Detector Response And Readout, Ph.D. Thesis, Department of Physics, University of Minnesota
The SuperCDMS test facility at University of Minnesota aids in the detector R&D and characterization of prototype detectors, as part of the scale-up effort for SuperCDMS SNOLAB. This thesis presents the first full ionization and phonon characterization study of a 100 mm diameter, 33 mm thick prototype Ge detector with interleaved phonon and ionization channels. Measurements include ionization collection efficiency, surface event rejection capabilities, and successful demonstration of nuclear recoil event discrimination. Results indicate that 100 mm diameter, interleaved Ge detectors show potential for use in SuperCDMS SNOLAB.

As part of detector R&D, the Minnesota test facility also looks beyond the next stage of SuperCDMS, investigating larger individual detectors as a means to easily scale up the sensitive mass of future searches. This thesis presents the design and initial testing results of a prototype 150 mm diameter, 33 mm thick silicon ionization detector, which is 5.2 times larger than those used in SuperCDMS at Soudan and 2.25 times larger than those planned for use at SuperCDMS SNOLAB. In addition, the detector was operated with contact-free ionization electrodes to minimize bias leakage currents, which can limit operation at high bias voltages. The results show promise for the operation of both large volume silicon detectors and contact-free ionization electrodes for scaling up detector mass and bias. Read more here: pdf

H. Qiu, A Search For Weakly Interacting Massive Particles Utilizing A Profile Likelihood Ratio Technique With The Supercdms Soudan Experiment, Ph.D. Thesis, Department of Physics, Southern Methodist University
A high mass WIMP search analysis is performed on the recent collected data sets at Soudan. It aims at exploring WIMPs with masses from the order of 10 GeV/c2 and above. With a raw exposure of 1657.54 kg-days, an exclusion limit is set on the spin-independent WIMP-nucleon cross section at 1.32×10−44 cm2 for a 75 GeV/c2 WIMP at a 90% confidence level by the profile likelihood ratio technique.

Neutrons are the most dangerous background in a direct dark matter search experiment. The nuclear recoil signal that a neutron produces in a Ge detector is indistinguishable from that a WIMP produces. Protection against them is one of the key aspects for the next generation of SuperCDMS experiment at SNOLAB. An active neutron veto system was proposed to be implemented in this future experiment to make it more robust from neutrons. The feasibility of both the plastic and liquid neutron veto systems was studied. Read more here: pdf

R. Agnese, SuperCDMS Prototype Detector Design and Testing, Ph.D. Thesis, Department of Physics, University of Florida
The detectors themselves, however, are quite complex; and a very detailed under- standing of the microscopic physics is helpful in analyzing the very rare events that occur within them. Furthermore, better understanding and modeling of the detectors can aid in the design and optimization of future iterations of the experiment.

This work describes the design and implementation of a low temperature condensed matter physics simulation library built on top of the popular Geant4 particle tracking framework. The library, named “Geant4 Condensed Matter Physics” or G4CMP, intro- duces several solid state concepts to the Geant4 framework such as crystal lattices, phonon quasiparticles, non-scalar effective masses, and implements several physics processes relevant to cryogenic temperature crystals.

In addition to the physics library, which is intended for general use, this work also describes a full Monte Carlo simulation package for the SuperCDMS iZIP detectors which utilizes G4CMP at its core and also fully simulates the detector readout sensors. Read more here: pdf

M. Pepin, Low-Mass Dark Matter Search Results and Radiogenic Backgrounds for the Cryogenic Dark Matter Search, Ph.D. Thesis, Department of Physics, University of Minnesota
Lower-mass dark matter O(10 GeV/c2) has become more prominent in the past few years. The CDMS detectors can be operated in an alternative, higher-biased, mode to decrease their energy thresholds and correspondingly increase their sensitivity to low-mass WIMPs. This is the CDMS low ionization threshold experiment (CDMSlite), which has pushed the frontier at lower WIMP masses. This dissertation describes the second run of CDMSlite at Soudan: its hardware, operations, analysis, and results. The results include new WIMP mass-cross section upper limits on the spin-independent and spin-dependent WIMP-nucleon interactions. Thanks to the lower background and threshold in this run compared to the first CDMSlite run, these limits are the most sensitive in the world below WIMP masses of ~4 GeV/c2. This demonstrates also the great promise and utility of the high-voltage operating mode in the SuperCDMS SNOLAB experiment. Read more here: pdf

Y. Chen, High-Energy Neutron Backgrounds For Underground Dark Matter Experiments, Ph.D. Thesis, Department of Physics, Syracuse University
... Neutrons are then the most crucial background for direct dark matter detection. It is important to understand and account for all sources of neutron backgrounds when claiming a discovery of dark matter detection or reporting limits on the WIMP-nucleon cross section. One type of neutron background that is not well understood is the cosmogenic neutrons from muons interacting with the underground cavern rock and materials surrounding a dark matter detector.

The Neutron Multiplicity Meter (NMM) is a water Cherenkov detector capable of measuring the cosmogenic neutron flux at the Soudan Underground Laboratory, which has an overburden of 2090 meters water equivalent. The NMM consists of two 2.2-tonne gadolinium-doped water tanks situated atop a 20-tonne lead target. It detects a high-energy (>~ 50MeV) neutron via moderation and capture of the multiple secondary neutrons released when the former interacts in the lead target. The multiplicity of secondary neutrons for the high-energy neutron provides a benchmark for comparison to the current Monte Carlo predictions. Combining with the Monte Carlo simulation, the muon-induced high-energy neutron flux above 50 MeV is measured to be (1.3 ± 0.2) × 10−9 cm−2s−1, in reasonable agreement with the model prediction. The measured multiplicity spectrum agrees well with that of Monte Carlo simulation for multiplicity below 10, but shows an excess of approximately a factor of three over Monte Carlo prediction for multiplicities ~10−20. Read more here: pdf

M. Penalver Martinez, Neutron background estimation for direct WIMP searches at SuperCDMS Soudan, M.Sc Thesis, Department of Physics, University of Durham
... different backgrounds that will affect these experiments are described, together with the various techniques employed to reject them. SuperCDMS Soudan is a direct detection experiment which uses Germanium semiconductor crystals detectors operating at mK temperatures. These detectors are equipped with phonon and charge sensors, enabling excellent rejection of electron recoil backgrounds. However, any irreducible neutron background from environmental radioactivity is still present in the experiment. The estimation of this background is presented with a detailed description of the process followed, in the context of the search for WIMPs with masses between 10-100 GeV/c2. Read more here: pdf

A. Phipps, Ionization Collection in Detectors of the Cryogenic Dark Matter Search, Ph.D. Thesis, Department of Physics, University of California, Berkeley
This dissertation focuses on ionization collection in these detectors under the sub-Kelvin, low electric field, and high crystal purity conditions unique to CDMS. The design and per- formance of a fully cryogenic HEMT-based amplifier capable of achieving the SuperCDMS SNOLAB ionization energy resolution goal of 100 eVee is presented. The experimental appa- ratus which has been used to record electron and hole properties under CDMS conditions is described. Measurements of charge transport, trapping, and impact ionization as a function of electric field in two CDMS detectors are shown, and the ionization collection efficiency is determined. The data is used to predict the error in the nuclear recoil energy scale under both CDMSlite and iZIP operating modes. A two species, two state model is developed to describe how ionization collection and space charge generation in CDMS detectors are controlled by the presence of “overcharged” D− donor and A+ acceptor impurity states. The thermal stability of these states is exclusive to sub-Kelvin operation, explaining why ioniza- tion collection in CDMS detectors differs from similar semiconductor detectors operating at higher temperature. This work represents a solid foundation for the understanding ionization collection in CDMS detectors. Read more here: pdf

R. Moffatt, Two-Dimensional Spatial Imaging of Charge Transport in Germanium Crystals at Cryogenic Temperatures, Ph.D. Thesis, Department of Physics, Stanford University
In this dissertation, I describe a novel apparatus for studying the transport of charge in semiconductors at cryogenic temperatures. The motivation to conduct this experiment originated from an asymmetry observed between the behavior of electrons and holes in the germanium detector crystals used by the Cryogenic Dark Matter Search (CDMS).

This asymmetry is a consequence of the anisotropic propagation of electrons in germanium at cryogenic temperatures. To better model our detectors, we incorporated this effect into our Monte Carlo simulations of charge transport. The purpose of the experiment described in this dissertation is to test those models in detail.

Our measurements have allowed us to discover a shortcoming in our most recent Monte Carlo simulations of electrons in germanium. This discovery would not have been possible without the measurement of the full, two-dimensional charge distribution, which our experimental apparatus has allowed for the first time at cryogenic temperatures. Read more here: pdf

B. Welliver, Dedicated Searches For Low And High Mass Wimps With The SuperCDMS Soudan iZIP Detectors, Ph.D. Thesis, Department of Physics, University of Florida
SuperCDMS Soudan operates specialized germanium detectors (iZIPs) that are cooled to milliKelvin temperatures deep underground in the Soudan Underground Laboratory with the hope of detecting a rare collision between dark matter and a nucleus. A search for low-mass dark matter comes with multiple unique challenges since the background discrimination abilities of these detectors becomes less powerful at the low energies needed to probe low-mass dark matter since the signal to noise ratio deteriorates. Using a sophisticated background model via a pulse rescaling technique, SuperCDMS Soudan was able to produce a world leading exclusion limit on low-mass dark matter.

Effort is to extend the analysis to higher masses require long running times during which many aspects of the detectors or the environment can change. Additional challenges are offered by the powerful background discrimination ability of the iZIP. The background distributions are well separated from the signal region, meaning most of the leakage arises from low-probability tails of the background distributions. In the absence of an enormous dataset, extrapolations from the bulk of the distribution are required. While attempting to obtain a model of gamma induced electron-recoils leaking into the signal region of the detector from high radius a curious asymmetry between the sides of the detectors was discovered potentially indicating an electronics or detector design problem. Read more here: pdf

A. Jastram, CDMS Detector Fabrication Improvements And Low Energy Nuclear Recoil Measurements In Germanium, Ph.D. Thesis, Department of Physics, Texas A&M University
As the CDMS (Cryogenic Dark Matter Search) experiment is scaled up to tackle new dark matter parameter spaces (lower masses and cross-sections), detector production efficiency and repeatability becomes ever more important. A dedicated facility has been commissioned for SuperCDMS detector fabrication at Texas A&M University (TAMU). The fabrication process has been carefully tuned using this facility and its equipment. Production of successfully tested detectors has been demonstrated. Significant improvements in detector performance have been made using new fabrication methods, equipment, and tuning of process parameters. This work has demonstrated the capability for production of next generation CDMS SNOLAB detectors.

Additionally, as the dark matter parameter space is probed further, careful calibrations of detector response to nuclear recoil interactions must be performed in order to extract useful information (in relation to dark matter particle characterizations) from experimental results. A neutron beam of tunable energy is used in conjunction with a commercial radiation detector to characterize ionization energy losses in germanium during nuclear recoil events. Data indicates agreement with values predicted by the Lindhard equation, providing a best-fit k-value of 0.146. Read more here: pdf

J. Yen, Phonon Sensor Dynamics For Cryogenic Dark Matter Search Experiment: A Study Of Quasiparticle Transport In Aluminum Coupled To Tungsten Transition Edge Sensors, Ph.D. Thesis, Department of Physics, Stanford University
Understanding the quasiparticle diffusion process inside sputtered aluminum (Al thin films ~0.1-1 μm) is critical for the Cryogenic Dark Matter Search (CDMS experiment to further optimize its detectors to directly search for dark matter. An initial study with Al films was undertaken by our group ~20 years ago, but some important questions were not answered at the time. This thesis can be considered a continuation of that critical study. Read more here: pdf

K. Schneck, Search For Low-Mass Dark Matter With Supercdms Soudan And Study Of Shorted Electric Field Configurations In CDMS Detectors, Ph.D. Thesis, Department of Physics, Stanford University
Analyzing SuperCDMS Soudan data to look for low-mass dark matter comes with particular challenges because of the low signal-to-noise very near threshold. However, with a detailed background model developed by scaling high-energy events down into the low-energy signal region, SuperCDMS Soudan produced world- leading limits on the existence of low-mass dark matter.

In addition, a few SuperCDMS Soudan detectors experienced cold hardware problems that can affect the data collected. Of particular interest is one detector considered for the low-mass WIMP search that has one of its charge electrodes shorted to chassis ground. Three events were observed in this detector upon unblinding the SuperCDMS Soudan low-energy data, even though <1 event was expected based on pre-unblinding calulations. However, the data collected by the shorted detector may have been compromised since an electrode shorted to ground will modify the electric field in the detector. The SuperCDMS Detector Monte Carlo (DMC) provides an excellent way to model the effects of the modified electric field, so a new model of the expected backgrounds in the low-mass WIMP search is developed using the DMC to try to explain how the short may have affected the data collected. Read more here: pdf

A.J. Anderson, A Search for Light Weakly-Interacting Massive Particles with SuperCDMS and Applications to Neutrino Physics, Ph.D. Thesis, Department of Physics, MIT
This thesis describes the results of such a search with the SuperCDMS experiment, which uses Ge detectors cooled to 50 mK to detect ionization and phonons produced by particle interactions. We perform a blind analysis of 577 kg d of exposure on 7 detectors targeting WIMPs with masses < 30GeV/c2, where anomalous results have been reported by previous experiments. No significant excess is observed and we set an upper limit on the spin-independent WIMP-nucleon cross section of 1.2×10-42 cm2 at 8 GeV/c2. We also set constraints on dark matter interactions independent of the dark matter halo physics, as well as on annual modulation of a dark matter signal.

Cryogenic detectors similar to SuperCDMS also have potential applications in neutrino physics. We study several configurations in which dark matter detectors could be used with an intense neutrino source to detect an unmeasured Standard Model process called coherent neutrino scattering. This process may be useful, for example, as a calibration for next-generation dark matter detectors, and for constraining eV-scale sterile neutrinos. In addition, small cryogenic X-ray detectors on sounding rockets with large fields-of-view have the unique ability to constrain sterile neutrino dark matter. We set limits on sterile neutrino dark matter using an observation by the XQC instrument, and discuss prospects for a future observation of the galactic center using the Micro-X instrument. Read more here: pdf

S.M. Fallows, Measurement of Nuclear Recoils in the CDMS II Dark Matter Search , Ph.D. Thesis, Department of Physics, University of Minnesota
The Cryogenic Dark Matter Search (CDMS) experiment is designed to directly detect elastic scatters of weakly-interacting massive dark matter particles (WIMPs), on target nuclei in semiconductor crystals composed of Si and Ge. These scatters would occur very rarely, in an overwhelming background composed primarily of electron recoils from photons and electrons, as well as a smaller but non-negligible background of WIMP-like nuclear recoils from neutrons. ...

Nuclear recoils have suppressed ionization signals relative to electron recoils of the same recoil energy, so the response of the detectors is calibrated differently for each recoil type. ...

I discuss systematic uncertainties affecting the reconstruction of this recoil energy, the primary analysis variable, and use several methods to constrain their magnitude. I present the resulting adjusted WIMP limits and discuss their impact in the context of current and projected constraints on the parameter space for WIMP interactions. Read more here: pdf

R. Basu Thakur, The Cryogenic Dark Matter Search Low Ionization-Threshold Experiment , Ph.D. Thesis, Department of Physics, University of Illinois at Urbana-Champaign
"CDMSlite" stands for CDMS - low ionization threshold experiment. Here we utilize a unique electron phonon coupling mechanism to measure ionization generated by scattering of light particles. Typically signals from such low energy recoils would be washed under instrumental noise. In CDMSlite via generation of Luke-Neganov phonons we can detect the small ionization energies, amplified in phonon modes during charge transport. This technology allows us to reach very low thresholds and reliably measure and investigate low energy recoils from light Dark Matter particles. This thesis describes the physics behind CDMSlite, the experimental design and the first science results from CDMSlite operated at the Soudan Underground Laboratory. Read more here: pdf

T. Hofer, Development of CDMS-II Surface Event Rejection Techniques and Their Extensions to Lower Energy Thresholds , Ph.D. Thesis, Department of Physics, University of Minnesota
The CDMS-II phase of the Cryogenic Dark Matter Search, a dark matter direct-detection experiment, was operated at the Soudan Underground Laboratory from 2003 to 2008. The full payload consisted of 30 ZIP detectors, totaling approximately 1.1 kg of Si and 4.8 kg of Ge, operated at temperatures of ~50 mK. ...

A full re-analysis of the CDMS-II data was motivated by an improvement in the event reconstruction algorithms which improved the resolution of ionization energy and timing information. The Ge data were re-analyzed using three distinct background-rejection techniques; the Si data from runs 125 - 128 were analyzed for the first time using the most successful of the techniques from the Ge re-analysis. The results of these analyses prompted a novel “mid-threshold” analysis, wherein energy thresholds were lowered but background rejection using phonon timing information was still maintained. This technique proved to have significant discrimination power, maintaining adequate signal acceptance and minimizing background leakage. Read more here: pdf

J. Zhang, A Dark Matter Search Using the Final CDMS-II Data and 100 mm SuperCDMS Germanium Detector Ionization Test, Ph.D. Thesis, Department of Physics, University of Minnesota
This thesis documents the test facility and the work involved in its development. In the test facility, we performed the first ionization collection efficiency measurements of the ionization test devices. The test devices are fabricated with detector-grade germanium crystals that are 100 mm in diameter, which is the largest available, and 33 mm in thickness. The measured efficiencies are consistent with the earlier measurements conducted with smaller Ge crystals, demonstrating that these 100 mm crystals can be used for development of the next generation dark matter detectors.

The data taken during the last four runs of CDMS II with total raw exposure 612 kg-day were reprocessed with improved ionization pulse reconstruction algorithm. We present the classic timing analysis with the reprocessed data in this thesis. For the four runs combined, this analysis resulted in a new WIMP-nucleon cross section 4.4×10-44 cm2 for a WIMP mass of 70 GeV/c2, which is a factor of 1.6 improvement compared to the original c58 classic timing analysis.
Read more here: pdf

B. Shank, Testing And Characterization Of Supercdms Dark Matter Detectors, Ph.D. Thesis, Department of Physics, Stanford University
The Cryogenic Dark Matter Search (SuperCDMS) relies on collection of phonons and charge carriers in semiconductors held at tens of milliKelvin as handles for detection of Weakly Interacting Massive Particles (WIMPs). This thesis begins with a brief overview of the direct dark matter search (Chapter 1) and SuperCDMS detectors (Chapter 2). In Chapter 3, a 3He evaporative refrigerator facility is described. Results from experiments performed in-house at Stanford to measure carrier transport in high-purity germanium (HPGe) crystals operated at sub-Kelvin temperatures are presented in Chapter 4. Finally, in Chapter 5 a new numerical model and a time-domain optimal filtering technique are presented, both developed for use with superconducting Transition Edge Sensors (TESs), that provide excellent event reconstruction for single particle interactions in detectors read out with superconducting W-TESs coupled to energy-collecting films of Al. Read more here: pdf

K. Prasad, Search for lightly ionizing particles using CDMS-II data and fabrication of CDMS detectors with improved homogeneity in properties, Ph.D. Thesis, Department of Physics, Texas A&M University
Fundamental particles are always observed to carry charges which are integral multiples of one-third charge of electron, e/3. While this is a well established experimental fact, the theoretical understanding for the charge quantization phenomenon is lacking. On the other hand, there exist numerous theoretical models that naturally allow for existence of particles with fractional electromagnetic charge. These particles, if existing, hint towards existence of physics beyond the standard model. Multiple high energy, optical, cosmological and astrophysical considerations restrict the allowable mass-charge parameter space for these fractional charges. Still, a huge unexplored region remains.

The Cryogenic Dark Matter Search (CDMS-II), located at Soudan mines in northern Minnesota, employs germanium and silicon crystals to perform direct searches for a leading candidate to dark matter called Weakly Interacting Massive Particles (WIMPs). Alternately, the low detection threshold allows search for fractional electromagnetic-charged particles, or Lightly Ionizing Particles (LIPs), moving at relativistic speed. Background rejection is obtained by requiring that the magnitude and location of energy deposited in each detector be consistent with corresponding "signatures" resulting from the passage of a fractionally charged particle. In this dissertation, the CDMS-II data is analyzed to search for LIPs, with an expected background of 0.078±0.078 events. No candidate events are observed, allowing exclusion of new parameter space for charges between e/6 and e/200. ...
Read more here: pdf

K. Page, A modified detector concept for SuperCDMS: The HiZIP and its charge performance, M.Sc Thesis, Department of Physics, Queen's University Kingston ON, Canada
SuperCDMS (Super Cryogenic Dark Matter Search) is a leading direct dark matter search experiment which uses solid state detectors (Ge crystals) at milliKelvin temperatures to look for nuclear recoils caused by dark matter interactions in the detector. `Weakly Interacting Massive Particles' (WIMPs) are the most favoured dark matter candidate particles. SuperCDMS, like many other direct dark matter search experiments, primarily looks for WIMPs. The measurement of both the ionization and the lattice vibration (phonon) signals from an interaction in the detector allow it to discriminate against electron recoils which are the main source of background for WIMP detection.

SuperCDMS currently operates about 9 kg of Ge detectors at the Soudan underground lab in northern Minnesota. In its next phase, SuperCDMS SNOLAB plans to use 100-200 kg of target mass (Ge) which would allow it to probe more of the interesting and as of yet unexplored parameter space for WIMPs predicted by theoretical models. The SuperCDMS Queen's Test Facility is a detector test facility which is intended to serve for detector testing and detector research and development purposes for the SuperCDMS experiment.
A modied detector called the `HiZIP' (Half-iZIP), which is reduced in complexity in comparison to the currently used iZIP (interleaved Z-sensitive Ionization and Phonon mediated) detectors, is studied in this thesis. ... Read more here: pdf

K. McCarthy, Detector Simulation and WIMP Search Analysis for the Cryogenic Dark Matter Search Experiment, Ph.D. Thesis, Department of Physics, Massachusetts Institute of Technology
Astrophysical and cosmological measurements on the scales of galaxies, galaxy clusters, and the universe indicate that ~85% of the matter in the universe is composed of dark matter, made up of non-baryonic particles that interact with cross-sections on the weak scale or lower. Hypothetical Weakly Interacting Massive Particles, or WIMPs, represent a potential solution to the dark matter problem, and naturally arise in certain Standard Model extensions.

The Cryogenic Dark Matter Search (CDMS) collaboration aims to detect the scattering of WIMP particles from nuclei in terrestrial detectors. Germanium and silicon particle detectors are deployed in the Soudan Underground Laboratory in Minnesota. These detectors are instrumented with phonon and ionization sensors, which allows for discrimination against electromagnetic backgrounds, which strike the detector at rates orders of magnitude higher than the expected WIMP signal.

This dissertation presents the development of numerical models of the physics of the CDMS detectors, implemented in a computational package collectively known as the CDMS Detector Monte Carlo (DMC). After substantial validation of the models against data, the DMC is used to investigate potential backgrounds to the next iteration of the CDMS experiment, known as SuperCDMS. Finally, an investigation of using the DMC in a reverse Monte Carlo analysis of WIMP search data is presented. ... Read more here: pdf

J. Kiveni, A Search for WIMP Dark Matter using an Optimized Chi-square Technique on the Final Data from the Cryogenic Dark Matter Search Experiment (CDMS II) , Ph.D. Thesis, Department of Physics, Syracuse University
This dissertation describes the results of a WIMP search using CDMS II data sets accumulated at the Soudan Underground Laboratory in Minnesota. Results from the original analysis of these data were published in 2009; two events were observed in the signal region with an expected leakage of 0.9 events. Further investigation revealed an issue with the ionization-pulse reconstruction algorithm leading to a software upgrade and a subsequent reanalysis of the data. As part of the reanalysis, I performed an advanced discrimination technique to better distinguish (potential) signal events from backgrounds using a 5-dimensional chi-square method. This data analysis technique combines the event information recorded for each WIMP-search event to derive a background-discrimination parameter capable of reducing the expected background to less than one event, while maintaining high efficiency for signal events. Furthermore, optimizing the cut positions of this 5-dimensional chi-square parameter for the 14 viable germanium detectors yields an improved expected sensitivity to WIMP interactions relative to previous CDMS results. This dissertation describes my improved (and optimized) discrimination technique and the results obtained from a blind application to the reanalyzed CDMS II WIMP-search data.

This analysis achieved the best expected sensitivity of the three techniques developed for the reanalysis and so was chosen as the primary timing analysis whose limit will be quoted in a on-going publication paper which is currently in preparation. For this analysis, a total raw exposure of 612.17 kg-days are analyzed for this work. No candidate events was observed, and a corresponding upper limit on the WIMP-nucleon scattering cross section as a function of WIMP mass is defined. These data set a 90% upper limit on spin-independent WIMP-nucleon elastic-scattering cross section of 3.19×10-44 cm2 for a WIMP mass of 60 GeV/c2. Combining this result with all previous CDMS II data gives an upper limit of 1.96×10-44 cm2 for a WIMP of mass 60 GeV/c2 (a factor of 2 better than the original analysis). ...
Read more here: pdf

S. Hertel, Advancing the Search for Dark Matter: from CDMS II to SuperCDMS, Ph.D. Thesis, Department of Physics, Massachusetts Institute of Technology
An overwhelming proportion of the universe (83% by mass) is composed of particles we know next to nothing about. Detecting these dark matter particles directly, through hypothesized weak-force-mediated recoils with nuclear targets here on earth, could shed light on what these particles are, how they relate to the standard model, and how the standard model ts within a more fundamental understanding.

This thesis describes two such experimental efforts: CDMS II (2007-2009) and SuperCDMS Soudan (ongoing). The general abilities and sensitivities of both experiments are laid out, placing a special emphasis on the detector technology, and how this technology has evolved from the first to the second experiment. Some topics on which I spent significant efforts are described here only in overview (in particular the details of the CDMS II analysis, which has been laid out many times before), and some topics which are not described elsewhere are given a somewhat deeper treatment.

In particular, this thesis is hopefully a good reference for those interested in the annual modulation limits placed on the low-energy portion of the CDMS II exposure, the design of the detectors for SuperCDMS Soudan, and an overview of the extremely informative data these detectors produce. It is an exciting time. The technology I've had the honor to work on the past few years provides a wealth of information about each event, more so than any other direct detection experiment, and we are still learning how to optimally use all this information. Initial tests from the surface and now underground suggest this technology has the background rejection abilities necessary for a planned 200kg experiment or even ton-scale experiment, putting us on the threshold of probing parameter space orders of magnitude from where the field currently stands. Read more here: pdf

D. Moore, A Search for Low-Mass Dark Matter with the Cryogenic Dark Matter Search and the Development of Highly Multiplexed Phonon-Mediated Particle Detectors, Ph.D. Thesis, Division of Physics, Mathematics & Astronomy, California Institute of Technology
A wide variety of astrophysical observations indicate that approximately 85% of the matter in the universe is nonbaryonic and nonluminous. Understanding the nature of this "dark matter" is one of the most important outstanding questions in cosmology. Weakly Interacting Massive Particles (WIMPs) are a leading candidate for dark matter since they would be thermally produced in the early universe in the correct abundance to account for the observed relic density of dark matter. If WIMPs account for the dark matter, then rare interactions from relic WIMPs should be observable in terrestrial detectors. Recently, unexplained excess events in the DAMA/LIBRA, CoGeNT, and CRESST-II experiments have been interpreted as evidence of scattering from WIMPs with masses ~10 GeV and spin-independent scattering cross sections of 10-41-10-40 cm2.

The Cryogenic Dark Matter Search (CDMS II) attempts to identify WIMP interactions using an array of cryogenic germanium and silicon particle detectors located at the Soudan Underground Laboratory in northern Minnesota. In this dissertation, data taken by CDMS II are reanalyzed using a 2 keV recoil energy threshold to increase the sensitivity to WIMPs with masses ∼10 GeV. These data disfavor an explanation for the DAMA/LIBRA, CoGeNT, and CRESST-II results in terms of spin-independent elastic scattering of WIMPs with masses ~12 GeV, under standard assumptions. At the time of publication, they provided the strongest constraints on spin-independent elastic scattering from 5-9 GeV, ruling out previously unexplored parameter space. ... Read more here:pdf

M. Pyle, Optimizing the Design and Analysis of Cryogenic Semiconductor Dark Matter Detectors for Maximum Sensitivity, Ph.D. Thesis, Department of Physics, Stanford University
For the past 15 years, the Cryogenic Dark Matter Search or CDMS has searched for Weekly Interacting Massive Particle dark matter (WIMPs) using Ge and Si semiconductor crystals instrumented with both ionization and athermal phonon sensors so that the much more common electron recoil leakage caused by photons and Βs from naturally present radioactive elements can be easily distinguished from elastic WIMP nucleon interactions by looking at the fraction of total recoil energy which ends up as potential energy of e-/h+ pairs.

Due to electronic carrier trapping at the surface of our semiconductor crystals, electron recoils which occur near the surface have suppressed ionization measurements and can not be distinguished from WIMP induced nuclear recoils and thus sensitivity to the WIMP nucleon interaction cross section was driven in CDMS II by our ability to define a full 3D fiducial volume in which all events had full collection. To remain background free and maximally sensitive to the WIMP-nucleus interaction cross section, we must improve our 3D fiducial volume definition at the same rate as we scale the mass of the detector, and thus proposed next generation experiments with an order of magnitude increase in active mass were unfortunately not possible with our previous CDMS II detector design, and a new design with significantly improved fiducialization performance is required. Read more here: pdf

K.M. Sundqvist, Carrier Transport and Related Effects in Detectors of the Cryogenic Dark Matter Search, Ph.D. Thesis, Department of Physics, University of California, Berkeley
The Cryogenic Dark Matter Search (CDMS) is searching for weakly-interacting massive particles (WIMPS), which could explain the dark matter problem in cosmology and particle physics.

By simultaneously measuring signals from deposited charge and the energy in nonequilibrium phonons created by particle interactions in intrinsic germanium crystals at a temperature of 40 mK, a signature response for each event is produced. This response, combined with phonon pulse-shape information, allows CDMS to actively discriminate candidate WIMP interactions with nuclei from electromagnetic radioactive background which interacts with electrons. Read more here: pdf

Z. Ahmed, A Dark-Matter Search Using the Final CDMS II Dataset and a Novel Detector of Surface Radiocontamination, Ph.D. Thesis, Division of Physics, Mathematics & Astronomy, California Institute of Technology
Substantial evidence from galaxies, galaxy clusters, and cosmological scales suggests that ~ 85% of the matter of our universe is invisible. The missing matter, or "dark matter", is likely composed of non-relativistic, non-baryonic particles, which have very rare interactions with baryonic matter and with one another. Among dark matter candidates, Weakly Interacting Massive Particles (WIMPs) are particularly well motivated. In the early universe, thermally produced particles with weak-scale mass and interactions would `freeze out at the correct density to be dark matter today. Extensions to the Standard Model of particle physics, such as Supersymmetry, which solve gauge hierarchy and coupling unification problems, naturally provide such particles.

Interactions of WIMPs with baryons are expected to be rare, but might be detectable in low-noise detectors. The Cryogenic Dark Matter Search (CDMS) experiment uses ionizationand phononsensitive germanium particle detectors to search for such interactions. CDMS detectors are operated at the Soudan Underground Laboratory in Minnesota, within a shielded environment to lower cosmogenic and radioactive background. The combination of phonon and ionization signatures from the detectors provides excellent residual-background rejection. Read more here: pdf

R. Bunker, A Low-threshold Analysis of Data from the Cryogenic Dark Matter Search Experiment, Ph.D. Thesis, Department of Physics, University of California Santa Barbara
Although dark matter appears to constitute over 80% of the matter in the Universe, its composition is a mystery. Astrophysical observations suggest that the luminous portions of the Galaxy are embedded in a halo of darkmatter particles. Weakly Interacting Massive Particles (WIMPs) are the most studied class of dark-matter candidates and arise naturally within the context of many weak-scale supersymmetric theories. Direct-detection experiments like the Cryogenic Dark Matter Search (CDMS) strive to discern the kinetic energy of recoiling nuclei resulting from WIMP interactions with terrestrial matter. This is a considerable challenge in which the low (expected) rate of WIMP interactions must be distinguished from an overwhelming rate due to known types of radiation.

An incontrovertible positive detection has remained elusive. However, a few experiments have recorded data that appear consistent with a low-mass WIMP. This thesis describes an attempt to probe the favored parameter space. To increase sensitivity to low-mass WIMPs, a low-threshold technique with improved sensitivity to small energy depositions is applied to CDMS shallow site data. Four germanium and two silicon detectors were operated between December 2001 and June 2002, yielding 118 days of exposure. By sacrificing some of the CDMS detectors’ ability to discriminate signal from background, energy thresholds of ~1 and ~2 keV were achieved for three of the germanium and both silicon detectors, respectively. A large number of WIMP candidate events are observed, most of which can be accounted for by misidentification of background sources. No conclusive evidence for a low-mass WIMP signal is found. The observed event rates are used to set upper limits on the WIMP-nucleon scattering cross section as a function of WIMP mass. Interesting parameter space is excluded for WIMPs with masses below ~9GeV/c2. Under standard assumptions, the parameter space favored by interpretations of other experiments’ data as low-mass WIMP signals is partially excluded, and new parameter space is excluded for WIMP masses between 3 and 4 GeV/c2. Read more here: pdf

M. Fritts, Background Characterization and Discrimination in the Final Analysis of the CDMS II Phase of the Cryogenic Dark Matter Search, Ph.D. Thesis, School of Physics and Astronomy, University of Minnesota
The Cryogenic Dark Matter Search (CDMS) is designed to detect Weakly-Interacting Massive Particles (WIMPs) in the Milky Way halo. The phase known as CDMS II was performed in the Soudan Underground Laboratory. The final set of CDMS II data, collected in 2007-8 and referred to as Runs 125-8, represents the largest exposure to date for the experiment.

We seek collisions between WIMPs and atomic nuclei in disk-shaped germanium and silicon detectors. A key design feature is to keep the rate of collisions from known particles producing WIMP-like signals very small. The largest category of such background is interactions with electrons in the detectors that occur very close to one of the faces of the detector. The next largest category is collisions between energetic neutrons that bypass the experimental shielding and nuclei in the detectors. Analytical efforts to discriminate these backgrounds and to estimate the rate at which such discrimination fails have been refined and improved throughout each phase of CDMS.

Next-generation detectors for future phases of CDMS require testing at cryogenic test facilities. One such facility was developed at the University of Minnesota in 2007 and has been used continuously since then to test detectors for the next phase of the experiment, known as SuperCDMS. Read more here: pdf

T. Bruch, A Search for Weakly Interacting Particles with the Cryogenic Dark Matter Search Experiment, Ph.D. Thesis, Department of Physics, University of Zurich
Cosmological observations in the last decades have led to a concordance model of the universe, where ~85% of matter is non-baryonic, non-luminous and non-relativistic at the time of structure formation. Theories of physics beyond the Standard Model of particle physics propose a wide array of candidates for the nature of this unseen dark matter. Weakly interacting massive particles (WIMPs) are a class of candidates which is well motivated by thermal production models for dark matter in the early universe. WIMPs (or any other dark matter candidate), distributed in a spherical isothermal halo surrounding our galaxy (the standard halo model; SHM), could be detected in terrestrial detectors.

In the standard model of disc galaxy formation, a dark matter disc forms as massive satellites are preferentially dragged into the disc-plane and dissolve. The low velocity of the dark matter particles in the dark disc with respect to the Earth enhances detection rates at low recoil energy in direct detection experiments. For WIMP masses &50GeV/c2, the detection rates increase by up to a factor of 3 in the 5-20 keV recoil energy range. Comparing this with rates at higher energies may be sensitive to the WIMP mass, providing stronger mass constraints particularly for masses ~100GeV/c2. The annual modulation signal is significantly boosted and the modulation phase is shifted by ~3 weeks relative to the dark halo. The variation of the observed phase with recoil energy determines the particle's mass, once the dark disc properties are fixed by future astronomical surveys. ... Read more here: pdf

X. Qiu, Advanced Analysis and Background Techniques for the Cryogenic Dark Matter Search, Ph.D. Thesis, Department of Physics, University of Minnesota
Read more here: pdf

C.N. Bailey, The Cryogenic Dark Matter Search: First 5-Tower Data and Improved Understanding of Ionization Collection, Ph.D. Thesis, Department of Physics, Case Western Reserve University
The Cryogenic Dark Matter Search (CDMS) is searching for Weakly Interacting Massive Particles (WIMPs) with cryogenic particle detectors. These detectors have the ability to discriminate between nuclear recoil candidate and electron recoil background events by collecting both phonon and ionization energy from recoils in the detector crystals. The CDMS-II experiment has completed analysis of the first data runs with 30 semiconductor detectors at the Soudan Underground Laboratory, resulting in a world leading WIMP-nucleon spin-independent cross section limit for WIMP masses above 44 GeV/c2.

As CDMS aims to achieve greater WIMP sensitivity, it is necessary to increase the detector mass and discrimination between signal and background events. Incomplete ionization collection results in the largest background in the CDMS detectors as this causes electron recoil background interactions to appear as false candidate events. Two primary causes of incomplete ionization collection are surface and bulk trapping.

Recent work has been focused on reducing surface trapping through the modification of fabrication methods for future detectors. Analyzing data taken with test devices has shown that hydrogen passivation of the amorphous silicon blocking layer worsens surface trapping. Additional data has shown that the iron-ion implantation used to lower the critical temperature of the tungsten transition-edge sensors causes a degradation of the ionization collection. Using selective implantation on future detectors may improve ionization collection for events near the phonon side detector surface. ... Read more here: pdf

J.P. Filippini, A Search for WIMP Dark Matter Using the First Five-Tower Run of the Cryogenic Dark Matter Search, Ph.D. Thesis, Department of Physics, University of California, Berkeley
In recent decades astronomers and physicists have accumulated a vast array of evidence that the bulk of the universe's matter is in some non-baryonic form that remains undetected by electromagnetic means. This "dark matter" resides in diffuse halos surrounding galaxies and other cosmic structures. Particle theorists have proposed a wide array of candidates for its nature. One particularly promising class of candidates are Weakly Interacting Massive Particles (WIMPs): quanta with masses of order 100 GeV/c2 and interactions characteristic of the weak nuclear force.

The Cryogenic Dark Matter Search (CDMS) experiment seeks to directly detect the rare elastic interactions of galactic WIMPs with terrestrial nuclei. To this end, CDMS operates an array of crystalline Ge and Si particle detectors in Soudan Underground Laboratory in northern Minnesota. These crystals are operated at millikelvin temperatures and instrumented to measure the ionization and athermal phonons generated by each particle interaction. This combination provides a powerful two-fold discrimination against the interactions of particles generated by radioactive decay and cosmogenic showers.

This dissertation describes the commissioning, analysis, and results of the first WIMP-search data runs of the CDMS experiment with its full complement of 5 "Towers" of detectors. These data represent a substantial increase in target mass and exposure over previous CDMS results. The results of this work place the most stringent limits yet set upon the WIMP-nucleon spin-independent cross section for WIMP masses above ~ 44 GeV/c2, as well as setting competitive limits on spin-dependent WIMP-nucleon interactions. This work also outlines the larger context of this and other probes of the WIMP theory of dark matter, as well as some current development efforts toward a larger cryogenic experiment. Read more here: pdf

R. Hennings-Yeomans, First 5 Tower WIMP-Search Results from the Cryogenic Dark Matter Search with Improved Understanding of Neutron Backgrounds and Benchmarking, Ph.D. Thesis, Department of Physics, Case Western Reserve University
Non-baryonic dark matter makes one quarter of the energy density of the Universe and is concentrated in the halos of galaxies, including the Milky Way. The Weakly Interacting Massive Particle (WIMP) is a dark matter candidate with a scattering cross section with an atomic nucleus of the order of the weak interaction and a mass comparable to that of an atomic nucleus. The Cryogenic Dark Matter Search (CDMS-II) experiment, using Ge and Si cryogenic particle detectors at the Soudan Underground Laboratory, aims to directly detect nuclear recoils from WIMP interactions.

This thesis presents the first 5 tower WIMP-search results from CDMS-II, an estimate of the cosmogenic neutron backgrounds expected at the Soudan Underground Laboratory, and a proposal for a new measurement of high-energy neutrons underground to benchmark the Monte Carlo simulations.

Based on the non-observation of WIMPs and using standard assumptions about the galactic halo [68], the 90% C.L. upper limit of the spin-independent WIMPnucleon cross section for the first 5 tower run is 6.6 x 10^-44cm2 for a 60 GeV/c2 WIMP mass. Read more here: pdf

R.W. Ogburn, A search for particle dark matter using cryogenic germanium and silicon detectors in the one- and two-tower runs of CDMS-II at Soudan, Ph.D. Thesis, Department of Physics, Stanford University
Images of the Bullet Cluster of galaxies in visible light, X-rays, and through gravitational lensing confirm that most of the matter in the universe is not composed of any known form of matter. The combined evidence from the dynamics of galaxies and clusters of galaxies, the cosmic microwave background, big bang nucleosynthesis, and other observations indicates that 80% of the universe's matter is dark, nearly collisionless, and cold. The identity of the dark matter remains unknown, but weakly interacting massive particles (WIMPs) are a very good candidate. They are a natural part of many supersymmetric extensions to the standard model, and could be produced as a nonrelativistic, thermal relic in the early universe with about the right density to account for the missing mass. The dark matter of a galaxy should exist as a spherical or ellipsoidal cloud, called a "halo" because it extends well past the edge of the visible galaxy.

The Cryogenic Dark Matter Search (CDMS) seeks to directly detect interactions between WIMPs in the Milky Way's galactic dark matter halo using crystals of germanium and silicon. Our Z-sensitive ionization and phonon ("ZIP") detectors simultaneously measure both phonons and ionization produced by particle interactions. In order to find very rare, low-energy WIMP interactions, we must identify and reject background events caused by environmental radioactivity, radioactive contaminants on the detectors, and cosmic rays. In particular, sophisticated analysis of the timing of phonon signals is needed to eliminate signals caused by beta decays at the detector surfaces. ... Read more here: pdf

J. Sander, Results from the Crogenic Dark Matter Search Using a Chi Squared Analysis, Ph.D. Thesis, Department of Physics, University of California at Santa Barbara
Most of the mass-energy density of the universe remains undetected and is only understood through its affects on visible, baryonic matter. The visible, baryonic matter accounts for only about half of a percent of the universe's total mass-energy budget, while the remainder of the mass-energy of the universe remains dark or undetected. About a quarter of the dark mass-energy density of the universe is comprised of massive particles that do not interact via the strong or electromagnetic forces. If these particles interact via the weak force, they are termed weakly interacting massive particles or WIMPs, and their interactions with baryonic matter could be detectable.

The CDMS II experiment attempts to detect WIMP interactions in the Soudan Underground Laboratory using germanium detectors and silicon detectors. A WIMP can interact a with detector nuclei causing the nuclei to recoil. A nuclear recoil is distinguished from background electron recoils by comparing the deposited ionization and phonon energies. Electron recoils occurring near detector surfaces are more difficult to reject.

This thesis describes the results of a x^2 analysis designed to reject events occurring near detector surfaces. Because no WIMP signal was observed, separate limits using the germanium and silicon detectors are set on the WIMP cross section under standard astrophysical assumptions. Read more here: pdf or ps

M.J. Attisha, Cryogenic Dark Matter Search (CDMS-II) - Application of Neural Networks and Wavelets to Event Analysis, Ph.D. Thesis, Department of Physics, Brown University
The Cryogenic Dark Matter Search (CDMS) experiment is designed to search for dark matter in the form of Weakly Interacting Massive Particles (WIMPs) via their elastic scattering interactions with nuclei. This dissertation presents the CDMS detector technology and the commissioning of two towers of detectors at the deep underground site in Soudan, Minnesota. CDMS detectors comprise crystals of Ge and Si at temperatures of 20 mK which provide ~keV energy resolution and the ability to perform particle identification on an event by event basis. Event identification is performed via a two-fold interaction signature; an ionization response and an athermal phonon response. Phonons and charged particles result in electron recoils in the crystal, while neutrons and WIMPs result in nuclear recoils. Since the ionization response is quenched by a factor ~ 3(2) in Ge(Si) for nuclear recoils compared to electron recoils, the relative amplitude of the two detector responses allows discrimination between recoil types. The primary source of background events in CDMS arises from electron recoils in the outer 50 µm of the detector surface which have a reduced ionization response. We develop a quantitative model of this dead layer effect and successfully apply the model to Monte Carlo simulation of CDMS calibration data. Analysis of data from the two tower run March-August 2004 is performed, resulting in the world's most sensitive limits on the spin-independent WIMP-nucleon cross-section, with a 90% C.L. upper limit of 1.6 x 10^-43 cm2 on Ge for a 60 GeV WIMP. An approach to performing surface event discrimination using neural networks and wavelets is developed. A Bayesian methodology to classifying surface events using neural networks is found to provide an optimized method based on minimization of the expected dark matter limit. The discrete wavelet analysis of CDMS phonon pulses improves surface event discrimination in conjunction with the neural network analysis, giving a 20% improvement to the expected and final WIMP-nucleon cross-section upper limits. Read more here: pdf

S. W. Leman, Development Of Phonon-Mediated Transition-Edge-Sensor X-Ray Detectors For Use In Astronomy, Ph.D. Thesis, Department of Physics, Stanford University
In this thesis I discuss the development of a novel phonon-mediated distributed transition-edge-sensor X-ray detector which would be useful for astrophysical studies such as magnetic recombination in the solar corona, the warm-hot intergalactic medium and surveys of clusters and groups of galaxies.

The detector uses a large semiconductor absorber and Transition-Edge-Sensors (TESs) to readout the absorbed energy. Calorimetry is performed on individual photons and a partitioning of the energy between various TESs allows for position determination. Hence time varying astronomical sources can be spectroscopically studied and imaged. Read more here: pdf

A.J. Reisetter, Results from the Two-Tower Run of the Cryogenic Dark Matter Search, Ph.D. Thesis, Department of Physics, University of Minnesota
The Cryogenic Dark Matter Search has completed two runs at the Soudan Underground Laboratory In the second, two towers of detectors were operated from March to August 2004. CDMS used Ge and Si ZIP (Z-sensitive, Ionization, and Phonon) detectors, operated at 50mK, to look for Weakly Interacting Massive Particles (WIMPs) which may make up most of the dark matter in our universe. These detectors are surrounded by lead and polyethylene shielding as well as an active muon veto. These shields, as well as the overburden of Soudan rock, provide a low background environment for the detectors.

The ZIP detectors record the ratio of ionization signal to phonon signal to discriminate between nuclear recoils, characteristic of WIMPs and neutrons, and electron recoils, characteristic of gamma and beta backgrounds. They also provide timing information from the four phonon channels that is used to reject surface events, for which ionization collection is poor. A blind analysis, defined using calibration data taken in situ throughout the run, provides a definition of the WIMP signal region by rejecting backgrounds. This analysis applied to the WIMP search data gives a limit on the spin independent WIMP-nucleon cross-section that is an order of magnitude lower than any other experiment has published. Read more here: pdf

G. Wang, The Cryogenic Dark Matter Search and Background Rejection with Event Position Information, Ph.D. Thesis, Department of Physics, Case Western Reserve University
Evidence from observational cosmology and astrophysics indicates that about one third of the universe is matter, but that the known baryonic matter only contributes to the universe at 4%. A large fraction of the universe is cold and non-baryonic matter, which has important role in the universe structure formation and its evolution. The leading candidate for the non-baryonic dark matter is Weakly Interacting Massive Particles (WIMPs), which naturally occurs in the supersymmetry theory in particle physics.

The Cryogenic Dark Matter Search (CDMS) experiment is searching for evidence of a WIMP interaction off an atomic nucleus in crystals of Ge and Si by measuring simultaneously the phonon energy and ionization energy of the interaction in the CDMS detectors. The WIMP interaction energy is from a few keV to tens of keV with a rate less than 0.1 events/kg/day. To reach the goal of WIMP detection, the CDMS experiment has been conducted in the Soudan mine with an active muon veto and multistage passive background shields.

The CDMS detectors have a low energy threshold and background rejection capabilities based on ionization yield. However, betas from contamination and other radioactive sources produce surface interactions, which have low ionization yield, comparable to that of bulk nuclear interactions. The low-ionization surface electron recoils must be removed in the WIMP search data analysis. An emphasis of this thesis is on developing the method of the surface-interaction rejection using location information of the interactions, phonon energy distributions and phonon timing parameters. The results of the CDMS Soudan run118 92.3 live day WIMP search data analysis is presented, and represents the most sensitive search yet performed. Read more here: pdf

S. Kamat, Extending the Sensitivity to the Detection of WIMP Dark Matter with an Improved Understanding of the Limiting Neutron Backgrounds, Ph.D. Thesis, Department of Physics, Case Western Reserve University
The Cryogenic Dark Matter Search (CDMS) uses position-sensitive Germanium and Silicon crystals in the direct detection of Weakly Interacting Massive Particles (WIMPs) believed to constitute most of the dark matter in the Universe. WIMP interactions with matter being rare, identifying and eliminating known backgrounds is critical for detection. Event-by-event discrimination by the detectors rejects the predominant gamma and beta backgrounds while Monte Carlo simulations help estimate, and subtract, the contribution from the neutrons.

This thesis describes the effort to understand neutron backgrounds as seen in the two stages of the CDMS search for WIMPs. The first stage of the experiment was at a shallow site at the Stanford Underground Facility where the limiting background came from high-energy neutrons produced by cosmic-ray muon interactions in the rock surrounding the cavern.

Simulations of this background helped inform the analysis of data from an experimental run at this site and served as input for the background reduction techniques necessary to set new exclusion limits on the WIMP-nucleon cross-section, excluding new parameter space for WIMPs of masses 8-20 GeV/c2. Read more here: pdf

C.L. Chang, The Cryogenic Dark Matter Search (CDMS-II) Experiment - First Results from the Soudan Mine, Ph.D. Thesis, Stanford University
There is an abundance of evidence that the majority of the mass of the universe is in the form of non-baryonic non-luminous matter that was non-relativistic at the time when matter began to dominate the energy density. Weakly Interacting Massive Particles, or WIMPs, are attractive cold dark matter candidates because they would have a relic abundance today of ~0.1 which is consistent with precision cosmological measurements. WIMPs are also well motivated theoretically. Many minimal supersymmetric extensions of the Standard Model have WIMPs in the form of the lightest supersymmetric partner, typically taken to be the neutralino.

The CDMS II experiment searches for WIMPs via their elastic scattering off of nuclei. The experiment uses Ge and Si ZIP detectors, operated at <50 mK, which simultaneously measure the ionization and athermal phonons produced by the scattering of an external particle. The dominant background for the experiment comes from electromagnetic interactions taking place very close to the detector surface. Analysis of the phonon signal from these interactions makes it possible to discriminate them from interactions caused by WIMPs. This thesis presents the details of an important aspect of the phonon pulse shape analysis known as the Lookup Table Correction. The Lookup Table Correction is a position dependent calibration of the ZIP phonon response which improves the rejection of events scattering near the detector surface. ... Read more here: pdf

V. Mandic, First Results from the Cryogenic Dark Matter Search Experiment at the Deep Site, Ph.D. Thesis, Department of Physics, University of California, Berkeley
The Cryogenic Dark Matter Search (CDMS) experiment is designed to search for dark matter in the form of the Weakly Interacting Massive Particles (WIMPs). For this purpose, CDMS uses detectors based on crystals of Ge and Si, operated at the temperature of 20 mK, and providing a two-fold signature of an interaction: the ionization and the athermal phonon signals. The two signals, along with the passive and active shielding of the experimental setup, and with the underground experimental sites, allow very effective suppression and rejection of different types of backgrounds.

This dissertation presents the commissioning and the results of the first WIMP- search run performed by the CDMS collaboration at the deep underground site at the Soudan mine in Minnesota. We develop different methods of suppressing the dominant background due to the electron-recoil events taking place at the detector surface and we apply these algorithms to the data set. These results place the world's most sensitive limits on the WIMP-nucleon spin-independent elastic-scattering cross-section. Finally, we examine the compatibility of the supersymmetric WIMP-models with the direct-detection experiments (such as CDMS) and discuss the implications of the new CDMS result on these models. Read more here: pdf

D.D. Driscoll, Development and Performance of Detectors for the Cryogenic Dark Matter Search Experiment with an Increased Sensitivity Based on a Maximum Likelihood Analysis of Beta Contamination, Ph.D. Thesis, Department of Physics, Case Western Reserve University
The Cryogenic Dark Matter Search (CDMS) uses cryogenically-cooled detectors made of germanium and silicon in an attempt to detect dark matter in the form of Weakly- Interacting Massive Particles (WIMPs). The expected interaction rate of these particles is on the order of 1/kg/day, far below the 200/kg/day expected rate of background interactions after passive shielding and an active cosmic ray muon veto. Our detectors are instrumented to make a simultaneous measurement of both the ionization energy and thermal energy deposited by the interaction of a particle with the crystal substrate. A comparison of these two quantities allows for the rejection of a background of electromagnetically-interacting particles at a level of better than 99.9%. The dominant remaining background at a depth of ~ 11 m below the surface comes from fast neutrons produced by cosmic ray muons interacting in the rock surrounding the experiment.

Contamination of our detectors by a beta emitter can add an unknown source of unrejected background. In the energy range of interest for a WIMP study, electrons will have a short penetration depth and preferentially interact near the surface. Some of the ionization signal can be lost to the charge contacts there and a decreased ionization signal relative to the thermal signal will cause a background event which interacts at the surface to be misidentified as a signal event. We can use information about the shape of the thermal signal pulse to discriminate against these surface events. Using a subset of our calibration set which contains a large fraction of electron events, we can characterize the expected behavior of surface events and construct a cut to remove them from our candidate signal events. ... Read more here: pdf

T. Saab, Search for Weakly Interacting Massive Particles with the Cryogenic Dark Matter Search Experiment, Ph.D. Thesis, Stanford University
From individual galaxies, to clusters of galaxies, to in between the cushions of your sofa, Dark Matter appears to be pervasive on every scale. With increasing accuracy, recent astrophysical measurements, from a variety of experiments, are arriving at the following cosmological model : a flat cosmology ( omega k = 0) with matter and energy densities contributing roughly 1/3 and 2/3 ( omega m = 0.35, omega lambda = 0.65). Of the matter contribution, it appears that only ~10% ( omega b ~ 0.04) is attributable to baryons. Astrophysical measurements constrain the remaining matter to be non-realtivistic, interacting primarily gravitationally. Various theoretical models for such Dark Matter exist. A leading candidate for the non-baryonic matter are Weakly Interacting Massive Particles (dubbed WIMPS). These particles, and their relic density may be naturally explained within the framework of Super-Symmetry theories. Super- Symmetry also offers predictions as to the scattering rates of WIMPs with baryonic matter allowing for the design and tailoring of experiments that search specifically for the WIMPs. The Cryogenic Dark Matter Search experiment is searching for evidence of WIMP interactions in crystals of Ge and Si. Using cryogenic detector technology to measure both the phonon and ionization response to a particle recoil the CDMS detectors are able to discriminate between electron and nuclear recoils, thus reducing the large rates of electron recoil backgrounds to levels with which a Dark Matter search is not only feasible, but far-reaching. This thesis will describe in some detail the physical principles behind the CDMS detector technology, highlighting the final step in the evolution of the detector design and characterization techniques. In addition, data from a 100 day long exposure of the current run at the Stanford Underground Facility will be presented, with focus given to detector performance as well as to the implications on allowable WIMP mass - cross-section parameter space. Read more here: pdf

T.A. Perera, The Limiting Background in a Dark Matter Search at Shallow Depth, Ph.D. Thesis, Department of Physics, Case Western Reserve University
A convincing body of evidence from observational and theoretical astrophysics suggests that matter in the universe is dominated by a non-luminous, non-baryonic, non-relativistic component. Weakly Interacting Massive Particles (WIMPs) are a proposed particle candidate that satisfy all of the above criteria. They are a front-runner among dark matter candidates because their predicted contribution to matter in the universe is cosmologically significant and because they may arise naturally from supersymmetric (SUSY) models of particle physics. The Cryogenic Dark Matter Search (CDMS) employs advanced detectors sensitive to nuclear recoils caused by WIMP scatters and capable of rejecting ionizing backgrounds.

The first phase of the experiment, conducted at a shallow site, is limited by a background of neutrons which are indistinguishable from WIMPs in terms of the acquired data. By accounting for and statistically subtracting these neutrons, CDMS I provides the best dark matter limits to date over a wide range of WIMP masses above 10 GeV/c2. These results also exclude the signal region claimed by the DAMA annual modulation search at a >71% confidence level.

The second phase of the experiment, located at a deep site, is scheduled to begin data acquisition in 2002. Due to longer exposures, larger detector mass, and low background rates at this site, data from CDMS II are expected to improve on present WIMP sensitivity by about two orders of magnitude.... Read more here: pdf

S. Golwala, Exclusion Limits on WIMP-Nucleon Elastic Scattering Cross-Section from the Cryogenic Dark Matter Search, Ph.D. Thesis, Department of Physics, University of California, Berkeley
Extensive evidence indicates that a large fraction of the matter in the universe is nonluminous, nonbaryonic, and cold: nonrelativistic at the time matter began to dominate the energy density of the universe. Weakly Interacting Massive Particles (WIMPs) are an excellent candidate for nonbaryonic, cold dark matter. Minimal supersymmetry provides a natural WIMP candidate in the form of the lightest superpartner, with a typical mass Md ~ 100 GeV c-2. WIMPs are expected to have collapsed into a roughly isothermal, spherical halo within which the visible portion of our galaxy resides. They would scatter off nuclei via the weak interaction, potentially allowingth eir direct detection.

The Cryogenic Dark Matter Search (CDMS) employs Ge and Si detectors to search for WIMPs via their elastic-scatteringin teractions with nuclei while discriminating against interactions of background particles. The former yield nuclear recoils while the latter produce electron recoils. The ionization yield (the ratio of ionization production to recoil energy in a semiconductor) of a particle interaction differs greatly for nuclear and electron recoils. CDMS detectors measure phonon and electron-hole-pair production to determine recoil energy and ionization yield for each event and thereby discriminate nuclear recoils from electron recoils.

This dissertation reports new limits on the spin-independentWIMP-nucleon elastic-scattering cross section that exclude unexplored parameter space above 10 GeV c-2 WIMP mass and, at > 75% CL, the entire 3s allowed region for the WIMP signal reported by the DAMA experiment. The experimental apparatus, detector performance, and data analysis are fully described. Read more here: pdf

A.H. Sonnenschein, A Search for Weakly Interacting Dark Matter Particles with Low Temperature Detectors Capable of Simultaneously Measuring Ionization and Heat, Ph.D. Thesis, Department of Physics, University of California at Santa Barbara
Lots of gravitating material that doesn't emit or absorb light seems to be required in all sensible accounts of the dynamics of large-scale structures in the universe. The nature and extent of this mysterious "dark matter" has been one of the central puzzles in cosmology over the last decade. This dissertation describes an experiment that tests one possibility, that the dark matter is in the form of undiscovered Weakly Interacting Massive Particles (WIMPs) produced as a thermal relic of the big bang. In this chapter, we will review the most important observations that suggest the dark matter must exist and discuss the forms it could take. Read more here: pdf

R. Clarke, An Athermal Phonon Mediated Dark Matter Detector With Surface Event Discrimination, Ph.D. Thesis, Department of Physics, Stanford University
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S. Nam, Development of Phonon-Mediated Cryogenic Particle Detectors with Electron and Nuclear Recoil Discrimination, Ph.D. Thesis, Department of Physics, Stanford University
Observations have shown that galaxies, including our own, are surrounded by halos of "dark matter". One possibility is that this may be an undiscovered form of matter, weakly interacting massive particls (WIMPs).

This thesis describes the development of silicon based cryogenic particle detectors designed to directly detect interactions with these WIMPs. These detectors are part of a new class of detectors which are able to reject background events by simultaneously measuring energy deposited into phonons versus electron hole pairs. By using the phonon sensors with the ionization sensors to compare the partitioning of energy between phonons and ionizations we can discriminate betweeen electron recoil events (background radiation) and nuclear recoil events (dark matter events). These detectors with built-in background rejection are a major advance in background rejection over previous searches.

Much of this thesis will describe work in scaling the detectors from 1/4 g prototype devices to a fully functional prototype 100 g dark matter detector. In particular, many sensors were fabricated and tested to understand the behavior of our phonon sensors, Quasipartice trapping assisted Electrothermal feedback Transition edge sensors (QETs). The QET sensors utilize aluminum quasiparticle traps attached to tungsten superconducting transition edge sensors patterned on a silicon substrate. The tungsten lines are voltage biased and self-regulate in the transition region. Phonons from particle interations within the silicon propogate to the surface where they are absorbed by the aluminum generating quasiparticles in the aluminum. The quasiparticles diffuse into the tungsten and couple energy into the tungsten electron system. Consequently, the tungsten increases in resistance and causes a current pulse which is measured with a high bandwidth SQUID system.... Read more here: pdf

P. D. Barnes, Jr., The Cryogenic Dark Matter Search (CDMS), Ph.D. Thesis, Department of Physics, University of California, Berkeley
A substantial body of observational evidence indicates that the universe contains much more material than we observe directly via photons of any wavelength. The existence of this "missing" mass or "dark" matter is inferred by its gravitational effects on the luminous material. Accepting the existence of dark matter has profoundly shaken our understanding in most areas of cosmology. If it exists at the lowest densities measured it is hard to understand in detail the creation of the elements in the early universe. If moderate density values are correct, then we have trouble understanding how the universe came to have so much structure on large scales. If the largest densities are correct, then dark matter is not ordinary matter, but must be something exotic like a new fundamental particle.

We would like to measure the properties of the dark matter directly. Supposing that the dark matter consists of a new fundamental particle, a WIMP, that was in thermal equilibrium in the early universe, we have built an experiment to detect dark matter directly by elastic scattering with germanium or silicon nuclei. Our detectors are large (~ 200 g) calorimeters that can discriminate between interactions with the electrons, due to background photons and beta particles, and interactions with the nuclei, due to WIMPs and background neutrons. The detectors operate at low temperatures (~ 20 mK) in a specially constructed cryostat. To reduce the rate of background events to a manageable level, the detectors and cryostat have been constructed out of selected materials and properly shielded. This dissertation discusses the properties of the hypothetical WIMPs, the detectors, cryostat, and shielding system, and finally, the analysis methods. Read more here: pdf

A.J. Da Silva, Development of a Low Background Environment for the Cryogenic Dark Matter Search, Ph.D. Thesis, Department of Physics, University British Columbia
A major problem currently facing astrophysics and cosmology is the question of dark matter. Although there is little doubt about the existence of dark matter, there is considerable uncertainty about the abundance and nature of this matter. One possibility is that dark matter consists of weakly interacting massive particles (WIMPs), such as the lightest stable particle in supersymmetry models.

Direct detection experiments look for nuclear recoils from WIMPs scattering in a detector. The first generation of direct detection experiments were ultimately limited by radioactive backgrounds. The Cryogenic Dark Matter Search (CDMS) is a direct detection experiment based on novel particle detectors operated at millikelvin temperatures that provide intrinsic background rejection. This capability, however, is not 100% effective. Therefore a low background environment is essential to the experiment.

To create such an environment, all possible background sources have been extensively studied both by measuring the background contribution from muons, phonons and neutrons and by performing detailed Monte Carlo simulations of the photon and neutron backgrounds. The results of this investigation, as discussed in this thesis, have influenced all aspects of the CDMS experiment.... Read more here: pdf

W.K. Stockwell, "A Cryogenic Search for WIMP Dark Matter," Ph.D. Thesis, Department of Physics, University of California, Berkeley
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K.D. Irwin, Phonon-Mediated Particle Detection Using Superconducting Tungsten Transition-Edge Sensors, Ph.D. Thesis, Department of Physics, Stanford University
This thesis describes the development of several superconducting tungsten thin film based particle detector technologies. The initial motivation for this work was the construction of detectors sensitive to dark matter and neutrino scattering events. These technologies also show promise in other applications, including high resolution x-ray spectroscopy.

The detectors described here consist of a tungsten thin film deposited on a silicon substrate. When an incident particle scatters in the silicon crystal, it deposits energy in the form of phonons which propagate to the surface of the crystal where they are absorbed in the tungsten thin film. The superconducting film is biased at or near its transition temperature. Changes in the resistance of the film are measured.

The superconducting titanium transition-edge sensors previously developed by our group exhibit a threshold phonon energy density below which no signal is detectable. This threshold density poses severe restrictions on resolution, energy threshold, and absorber mass. In order to overcome these limitations, several new technologies were developed. In each case, a superconducting film with a sharp transition well below that of titanium (~ 380 mK) is necessary. To this end superconducting W films were developed with ~ 1 mK wide transitions at 70 mK. Before this work W thin films always exhibited transition temperatures > 600 mK.... Read more here: pdf

M. Penn, Nuclear-Recoil/Electron-Recoil Discrimination In Cryogenic Silicon Detectors For Use In Dark Matter Searches, Ph.D. Thesis, Department of Physics, Stanford University
We have configured a double-sided Silicon Crystal Acoustic Detector (SiCAD) for simultaneous measurement of both phonons and ionization. This detector operates at ~ 370 mK and consists of a Ti Transition Edge Sensor (TES), which is the phonon detector, on one side, and a similar pattern of metal, acting as an electrode for the ionization measurement, on the other side of a 300 μm thick high-purity, monocrystalline Si wafer. The phonon sensor is also position sensitive: it measures the distance between the event location and the detecting surface and can be used to reject background events occurring near the surface. In addition, a coupling between the phonon sensor and the ionization sensor allows the determination of event position in one dimension in the plane of the detecting surface. We present the results of experiments which demonstrate the discrimination capability and position sensitivity of the detector for energy depositions above ~ 3 keV.

The physics of charge measurement, necessary for the background rejection technique, in silicon at low temperature (T < 0.5 K) and low applied electric field (E = 0.1 - 100 V/cm) has been examined in a variety of high purity, p-type silicon samples with room temperature resistivity in the range 2 - 40 kΩ-cm. The samples varied in thickness from 300 μm to nearly 5 mm. Charge loss at low electric field due to trapping during charge drift is present but the data suggest that another charge-loss mechanism is also important. We present results which indicate that a significant fraction of the total charge loss (compared to full collection) occurs in the initial charge cloud near the event location. A simple model of charge trapping both in the initial cloud and along the electric field induced drift to the electrodes is developed and satisfactory comparison to the data is found. In addition, measurements of the lateral size, transverse to the applied electric field, of the initial electron-hole cloud indicate large transverse diffusion lengths. At the lowest fields a lateral diameter on the order of 1 mm is found in samples ~ 5 mm thick. Read more here: pdf

T. A. Shutt, A Dark Matter Detector Based on the Simultaneous Measurement of Phonons and Ionization at 20 mK, Ph.D. Thesis, Department of Physics, University of California, Berkeley
One of the most important issues in astrophysics and cosmology is understanding the nature of dark matter. One possibility is that it is made of weakly interacting subatomic particles created in the big bang, such as the lightest particle in supersymmetry models. These particles should scatter elastically of nuclei in a detector on earth at a rate of ~ events/kg/week, and will deposit energies of a few keV. Current attempts to detect these interactions are limited by a radioactive background of photons and beta particles which scatter on electrons.

We have developed a novel particle detector to look for dark matter based on the simultaneous measurement of ionization and phonons in a 60 g crystal of high purity germanium at a temperature of 20 mK. Background events can be distinguished by our detector because they produce more ionization per unit phonon energy than dark matter interactions.

The phonon energy is measured as a temperature change in the detector by means of neutron transmutation doped germanium thermistors attached to they crystal. The ionization measurement is accomplished by applying a bias to implanted contacts on the faces of the disk. Charge collection differs from the normal situation at 77 K in efficiency is good with an electric field of only ~0.2 V/cm after the charged impurities in the crystal have been neutralized by free charge created by particle interactions from a radioactive source. For fields below this charge collection is poor, and affects the amount of phonon energy measured. We have modeled this in terms of charge trapping. Read more here: pdf

N. Wang, "A Cryogenic Phonon Detector to Search for Dark Matter Particles," Ph.D. Thesis, Department of Physics, University of California, Berkeley (not available in electronic format)

This work is supported by the National Science Foundation and the Department of Energy