Fuelling the search for light dark matter-electron scattering
  Dark matter (DM) detectors employing a Spherical Proportional Counter (SPC)
have demonstrated a single-electron detection threshold and are projected to
have small background rates. We explore the sensitivity to DM-electron
scattering with SPC detectors in the context of DarkSphere, a proposal for a
300 cm diameter fully-electroformed SPC. SPCs can run with different gases, so
we investigate the sensitivity for five targets: helium, neon, xenon, methane,
and isobutane. We use tools from quantum chemistry to model the atomic and
molecular systems, and calculate the expected DM induced event rates. We find
that DarkSphere has the potential to improve current exclusion limits on DM
masses above 4 MeV by up to five orders of magnitude. Neon is the best
all-round gas target but using gas mixtures, where methane and isobutane
constitute 10% of the gas, can improve the sensitivity, especially when
combined with helium. Our study highlights the currently untapped potential of
SPCs to search for DM-electron scattering in the MeV-to-GeV DM mass range.
