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

We discuss several low-energy backgrounds to sub-GeV dark-matter searches, which arise from high-energy particles of cosmic or radioactive origin that interact with detector materials. We focus, in particular, on Cherenkov radiation, transition radiation, and luminescence or phonons from electron-hole pair recombination and show that these processes are an important source of backgrounds at both current and planned detectors. We perform detailed analyses of these backgrounds at several existing and proposed experiments based on a wide variety of detection strategies and levels of shielding. We find that a large fraction of the observed single-electron events in the SENSEI 2020 run originate from Cherenkov photons generated by high-energy events in the Skipper charge coupled device and from recombination photons generated in a phosphorus-doped layer of the same instrument. In a SuperCDMS HVeV 2020 run, Cherenkov photons produced in printed-circuit boards located near the sensor likely explain the origin of most of the events containing 2-6 electrons. At SuperCDMS SNOLAB, radioactive contaminants inside the Cirlex located inside or on the copper side walls of their detectors produce many Cherenkov photons, which could dominate the low-energy backgrounds. For the EDELWEISS experiment, Cherenkov or luminescence backgrounds are subdominant to their observed event rate but could still limit the sensitivity of their future searches. We also point out that Cherenkov radiation, transition radiation, and recombination could be a significant source of backgrounds at future experiments aiming to detect dark matter via scintillation or phonon signals. We also discuss the implications of our results for the development of superconducting qubits and low-threshold searches for coherent neutrino scattering. Fortunately, several design strategies to mitigate these backgrounds can be implemented, such as minimizing nonconductive materials near the target, implementing active and passive shielding, and using multiple nearby detectors.