Dissertations

Link: PDF

Abstract: In recent years, the dark matter direct detection community has become increasingly interested in dark matter below the mass scale of the WIMP. Often called `low mass dark matter' or `light dark matter', this refers to a collection of models for fermionic dark matter with masses typically in the range eV-GeV. Due to the slow relative velocity of the dark matter in the local halo, the dark matter is poorly kinematically matched to typical detector targets, leaving a detectable amount of energy that is orders of magnitude below the current state-of-the-art detector technology.

In this thesis, I will discuss the details of the athermal phonon sensor mediated detector technology used by the SuperCDMS and SPICE/HeRALD collaborations. I will motivate how this technology can be used to reach detector baseline energy thresholds of O(meV). I then use these concepts in the design of the SPICE MELANGE detectors - the initial prototype dark matter detectors for the SPICE/HeRALD collaboration, with baseline energy resolutions expected to be sub-100 meV. I present the characterization from the testing of the initial fabrication on Si substrates. These Si versions are expected to be able to explore nuclear recoil dark matter parameter space for masses of O(MeV-GeV). A future fabrication on Sapphire is planned, which will extend this mass range down easily into the keV range.

A major hurdle in the realization of ultra-low noise detector technology is the fact that the sensitivity to noise and backgrounds also increases. As such, much of this thesis is also dedicated to the characterization of ultra-sensitive cryogenic calorimeters and analysis of noise - from both intrinsic and environmental sources.

Link: PDF

Abstract: In recent years, theoretical and experimental interest in dark matter (DM) candidates have shifted focus from primarily Weakly-Interacting Massive Particles (WIMPs) to an entire suite of candidates with masses from the zeV-scale to the PeV-scale to 30 solar masses. One particular recent development has been searches for light dark matter (LDM), which is typically defined as candidates with masses in the range of keV to GeV. In searches for LDM, eV-scale and below detector thresholds are needed to detect the small amount of kinetic energy that is imparted to nuclei in a recoil. One such detector technology that can be applied to LDM searches is that of Transition-Edge Sensors (TESs). Operated at cryogenic temperatures, these sensors can achieve the required thresholds, depending on the optimization of the design.

In this thesis, I will motivate the evidence for DM and the various DM candidates beyond the WIMP. I will then detail the basics of TES characterization, expand and apply the concepts to an athermal phonon sensor–based Cryogenic PhotoDetector (CPD), and use this detector to carry out a search for LDM at the surface. The resulting exclusion analysis provides the most stringent limits in DM-nucleon scattering cross section (comparing to contemporary searches) for a cryogenic detector for masses from 93 to 140 MeV, showing the promise of athermal phonon sensors in future LDM searches. Furthermore, unknown excess background signals are observed in this LDM search, for which I rule out various possible sources and motivate stress-related microfractures as an intriguing explanation. Finally, I will shortly discuss the outlook of future searches for LDM for various detection channels beyond nuclear recoils.