Directional Detection of Dark Matter with Two-Dimensional Targets
  We propose two-dimensional materials as targets for direct detection of dark
matter. Using graphene as an example, we focus on the case where dark matter
scattering deposits sufficient energy on a valence-band electron to eject it
from the target. We show that the sensitivity of graphene to dark matter of MeV
to GeV mass can be comparable, for similar exposure and background levels, to
that of semiconductor targets such as silicon and germanium. Moreover, a
two-dimensional target is an excellent directional detector, as the ejected
electron retains information about the angular dependence of the incident dark
matter particle. This proposal can be implemented by the PTOLEMY experiment,
presenting for the first time an opportunity for directional detection of
sub-GeV dark matter.
