Direct Detection of sub-GeV Dark Matter with Semiconductor Targets
  Dark matter in the sub-GeV mass range is a theoretically motivated but
largely unexplored paradigm. Such light masses are out of reach for
conventional nuclear recoil direct detection experiments, but may be detected
through the small ionization signals caused by dark matter-electron scattering.
Semiconductors are well-studied and are particularly promising target materials
because their ${\cal O}(1~\rm{eV})$ band gaps allow for ionization signals from
dark matter as light as a few hundred keV. Current direct detection
technologies are being adapted for dark matter-electron scattering. In this
paper, we provide the theoretical calculations for dark matter-electron
scattering rate in semiconductors, overcoming several complications that stem
from the many-body nature of the problem. We use density functional theory to
numerically calculate the rates for dark matter-electron scattering in silicon
and germanium, and estimate the sensitivity for upcoming experiments such as
DAMIC and SuperCDMS. We find that the reach for these upcoming experiments has
the potential to be orders of magnitude beyond current direct detection
constraints and that sub-GeV dark matter has a sizable modulation signal. We
also give the first direct detection limits on sub-GeV dark matter from its
scattering off electrons in a semiconductor target (silicon) based on published
results from DAMIC. We make available publicly our code, QEdark, with which we
calculate our results. Our results can be used by experimental collaborations
to calculate their own sensitivities based on their specific setup. The
searches we propose will probe vast new regions of unexplored dark matter model
and parameter space.
