Electron-interacting dark matter: Implications from DAMA/LIBRA-phase2
  and prospects for liquid xenon detectors and NaI detectors
  We investigate the possibility for the direct detection of low mass (GeV
scale) WIMP dark matter in scintillation experiments. Such WIMPs are typically
too light to leave appreciable nuclear recoils, but may be detected via their
scattering off atomic electrons. In particular, the DAMA Collaboration [R.
Bernabei et al., Nucl. Phys. At. Energy 19, 307 (2018)] has recently presented
strong evidence of an annual modulation in the scintillation rate observed at
energies as low as 1 keV. Despite a strong enhancement in the calculated event
rate at low energies, we find that an interpretation in terms of
electron-interacting WIMPs cannot be consistent with existing constraints. We
also demonstrate the importance of correct treatment of the atomic
wavefunctions, and show the resulting event rate is very sensitive to the
low-energy performance of the detectors, meaning it is crucial that the
detector uncertainties be taken into account. Finally, we demonstrate that the
potential scintillation event rate can be much larger than may otherwise be
expected, meaning that competitive searches can be performed for GeV scale
WIMPs using the conventional prompt S1 scintillation signals. This is important
given the recent and upcoming very large liquid xenon detectors.
