Modulation Effects in Dark Matter-Electron Scattering Experiments
  One of the next frontiers in dark-matter direct-detection experiments is to
explore the MeV to GeV mass regime. Such light dark matter does not carry
enough kinetic energy to produce an observable nuclear recoil, but it can
scatter off electrons, leading to a measurable signal. We introduce a
semi-analytic approach to characterize the resulting electron-scattering events
in atomic and semiconductor targets, improving on previous analytic proposals
that underestimate the signal at high recoil energies. We then use this
procedure to study the time-dependent properties of the electron-scattering
signal, including the modulation fraction, higher-harmonic modes and modulation
phase. The time dependence can be distinct in a non-trivial way from the
nuclear scattering case. Additionally, we show that dark-matter interactions
inside the Earth can significantly distort the lab-frame phase-space
distribution of sub-GeV dark matter.
