Particle physics in the sub-keV energy regime

Coherent elastic neutrino-nucleus scattering (CEνNS) and other rare-event physics searches, like dark matter detection, have been especially furthered by increasing sensitivity to low-energy particle interactions. Experiments using multiple detector technologies have sought CEνNS at the most intense terrestrial sources of neutrinos: spallation facilities and nuclear reactors. Here I report on the feasibility of using cryogenic pure CsI as an improved next-generation CEνNS target at the up-and-coming European Spallation Source. The calibrations and simulations presented predict an increase by a factor of at least ∼ 33 in the rate of observable neutrino-induced events per unit mass, compared to past use of room-temperature CsI[Na]. I also report on the first measurement of CEνNS from antineutrinos at the Dresden Generating Station, a power nuclear reactor, employing a large-mass semi-conducting germanium diode dubbed NCC-1701, and its next steps. Alongside each step in detecting these neutrino couplings, the importance of understanding device response to low-energy nuclear recoils is highlighted. Finally, finding synergy for tools developed to extricate sub-keV CEνNS signals, a search for the exotic mode of muon decay μ⁺ → e^-X⁰ was performed. New sensitivity limits in previously untouched parameter space for a massive boson dark matter candidate of cosmological interest are presented.