Sources of Low-Energy Events in Low-Threshold Dark-Matter and Neutrino Detectors

Detecting dark-matter particles in the laboratory is crucial to reveal its elusive nature. In recent years, tremendous progress has been made in the search for dark-matter particles with masses less than that of a proton. However, the first generation of experiments see many background events of unexplained origin, among which dark-matter signals may be hiding. To fully realize the potential of experiments and clear the path toward the direct detection of dark matter, it is crucial to identify the origin of these events. Here, we identify three unexplored processes that surreptitiously mimic dark-matter signals at laboratories.Specifically, we explore three processes that arise when high-energy particles interact with materials in dark-matter detectors: Cherenkov radiation, transition radiation, and luminescence or phonons from electron-hole recombination. We demonstrate that the event rates of these processes at several experiments are so large that they not only explain a large fraction of the observed event rates but also significantly impede the discovery of many dark-matter candidates. To solve this issue, we provide clear methodologies to calculate the effect of these processes and propose several important design strategies to mitigate them.The tools and ideas that we provide will have profound consequences for the development of improved data analyses and for the design of future dark-matter experiments. Moreover, our findings could also have an important impact on the development of quantum computers and neutrino detectors, whose performance can be affected by the same processes.