Direct Detection of Light Dark Matter with Electrons, Phonons, and Magnons

Citation

Trickle, Tanner David (2022) Direct Detection of Light Dark Matter with Electrons, Phonons, and Magnons. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/n4j3-1b24. https://resolver.caltech.edu/CaltechTHESIS:05242022-185317418

Abstract

Discovering the nature of dark matter (DM) remains one of the most important outstanding questions in particle physics. While the astrophysical evidence for its existence continues to accumulate, we know very little about its fundamental constituents, and how it connects to the Standard Model. Terrestrial direct detection experiments offer a unique experimental perspective. Detection of a signal would be the first evidence for non-gravitational interactions between DM and ordinary matter, a crucial clue in understanding the particle nature of DM. Moreover detection in a laboratory is less susceptible to astrophysical uncertainties which accompany indirect detection strategies, and offer a wider vantage than colliders with the ability to swap target materials and search for modulation effects. In this dissertation I will discuss searching for DM with the current state-of-the-art direct detection experiments based on electronic excitations, as well as a potential direction for future experiments based on phonon and magnon excitations.

Previous generations of direct detection experiments utilized nuclear recoil to search for DM particles. While this process is well suited for DM candidates with masses above typical nuclear masses, ℴ(GeV), sensitivity drops precipitously for lighter DM. New physical processes must be utilized to facilitate the search for well-motivated light, sub-GeV, DM candidates. Electronic excitations are one such process which can probe DM candidates which have enough energy to excite electrons across the band gap. I will discuss many aspects of this DM-induced excitation rate which were developed in this work: the most advanced first-principles calculations of DM scattering and absorption signals using density functional theory (DFT) input, daily modulation effects in anisotropic crystal targets, comparisons of a wide variety of potential detector targets, theoretical development of a non-relativistic effective field theory (NR EFT) to aid in the calculation of DM absorption rates, as well as a study of interactions in cutting edge small gap, spin-orbit coupled targets.

While direct detection experiments using electronic excitations are currently underway, to reach even lower DM masses, which do not carry enough energy to excite states across the band gap, new ideas must be explored. Collective excitations, such as phonons and magnons, exist below the electronic band gap and offer an exciting future for direct detection experiments. Detectors searching for single phonon excitations are currently in development, whereas those based on magnons are in their infancy. Similar to the electronic excitations we will discuss a myriad of topics involving single phonon and magnon excitations: an EFT of DM-collective excitation scattering for general UV theories, potential uses for detection of axion DM, as well as advanced first-principles calculations, detailed study of the directional detectability in anisotropic crystal targets, and comparisons across a variety of candidate target materials.

Thesis Files