Strategic Planning and Sensitivity-Enhancing Tactics for Detecting Low-Mass Particle Dark Matter with Phonon-Mediated Detectors

Citation

Wen, Osmond (2025) Strategic Planning and Sensitivity-Enhancing Tactics for Detecting Low-Mass Particle Dark Matter with Phonon-Mediated Detectors. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/g23k-c067. https://resolver.caltech.edu/CaltechTHESIS:12182024-174539067

Abstract

A non-baryonic matter beyond the framework of the Standard Model is required to explain a vast set of astrophysical and cosmological phenomena in our universe; it is referred to as dark matter and comprises 85% of all matter. Previously centered on particle candidates in the 1 GeV to 10 TeV mass range, dark matter model building has expanded to masses well beyond that range, with an emphasis toward low-mass particles below 1 GeV. Low-mass dark matter models have invoked new and creative mechanisms for producing the relic abundance of dark matter and in doing so have provided a variety of new laboratory-testable hypotheses about the early universe.

Direct detection experiments seek to directly measure a dark matter particle interaction from the Milk Way dark matter halo with ultra-sensitive detector technologies in low-background environments. As the paradigm has shifted toward lower-mass particle candidates, detector technologies have followed suit: single-charge-sensitive detectors and low-threshold, purely phonon-mediated detectors are among the best detector architectures for probing the most immediately accessible theoretical models of low-mass dark matter. The Super Cryogenic Dark Matter Search (SuperCDMS) has used and developed detector technologies on both of these fronts. On the axis of single-charge-sensitive detectors, the High Voltage (HV) detector program of the SuperCDMS Collaboration has demonstrated gram-scale, single-charge sensitive detectors known as HVeV detectors. Recent advances in the collaboration’s understanding of single-charge backgrounds have enabled much improved sensitivity to low-mass dark matter parameter space with even these gram-scale detectors. HVeV detectors are a prototype version of the HV kg-scale detectors to be deployed at the flagship SuperCDMS experiment in SNOLAB. HV detectors are projected to test vast regions of unconstrained parameter space for both electron- and nuclear-recoiling dark matter, as shown in Chapter 4 of this thesis among many other SuperCDMS sensitivity projections.

A potentially limiting background for SuperCDMS detectors at SNOLAB is the zero-charge low energy excess, which is characterized by an exponentially rising spectrum of background phonon events below about 100 eV to 1 keV recoil energy. Chapter 5 of this thesis presents a data-driven technique to subtract the zero-charge low energy excess (0QLEE) as observed in HVeV detectors. A search for charge-producing, nuclear-recoiling dark matter is performed with this background-subtraction technique. The resultant exposure-limited constraint on the nucleon-dark-matter cross section is nearly a factor of 10× stronger than the background-limited constraint and is within tens of percent from unconstrained parameter space. The two dominant sources of systematic uncertainties for this search are (1) the uncertainty on the total rate and spectral shape of zero-charge low energy excess events and (2) the completely unknown behavior of the ionization yield function in silicon for nuclear recoils below 100 eV.

On the axis of low-threshold phonon-mediated detectors, the SuperCDMS Collaboration must improve phonon energy thresholds to below 1 eV in order to attain sensitivity to sub-GeV nucleon-coupled dark matter. Presently, SuperCDMS has achieved detector phonon thresholds in the range from 10 eV to 200 eV depending on detector size. In Chapters 6, 7, and 8, we present a radically different phonon sensor architecture that may provide long-term gains in sensitivity: the kinetic inductance detector.

There are two main quantities that constrain the capacity of kinetic inductance detectors to be effective phonon sensors: the detector readout noise and the phonon collection efficiency. Chapter 7 explores the former, detailing the variety of noise sources in kinetic inductance detectors and how they may impact a sensor’s energy resolution using both theoretical calculations and experimental measurements. In general, resolution on energy absorbed in the sensor is presently limited to a range from 1 eV to 5 eV. Chapter 8 then reports on the overall detector energy performance of three different KID-based phonon-mediated (KIPM) detectors, each of which suffers from percent-scale phonon collection efficiencies. An empirical model is then built to parametrize and understand the reasons for the poor phonon collection efficiencies, thereby outlining a path forward to lowering energy thresholds in KIPM detectors.

Thesis Files