Direct detection of fast-moving low-mass dark matter

We examine the signals produced by dark-matter interactions with electrons, which play a crucial role in direct detection experiments employing heavy target materials, particularly in many well-motivated sub-GeV dark-matter scenarios. When the momentum transfer to target electrons is comparable to or exceeds their binding energy, atomic effects related to electron ionization become essential for accurately determining signal rates—especially in the case of fast-moving dark matter. In this paper, we revisit and extend the atomic ionization formalism, systematically comparing different approaches used to formulate the ionization form factor and identifying their respective domains of validity. As practical applications, we explore detection prospects in xenon target experiments. To illustrate our findings, we consider a specific scenario involving boosted dark matter, which often leads to high-momentum electron recoils. Our analysis demonstrates that the choice of formalism can significantly influence the interpretation of experimental data, depending on the regions of parameter space.