First Dark Matter Constraints from a SuperCDMS Single-Charge Sensitive
  Detector
  We present the first limits on inelastic electron-scattering dark matter and
dark photon absorption using a prototype SuperCDMS detector having a charge
resolution of 0.1 electron-hole pairs (CDMS HVeV, a 0.93 gram CDMS HV device).
These electron-recoil limits significantly improve experimental constraints on
dark matter particles with masses as low as 1 MeV/$\mathrm{c^2}$. We
demonstrate a sensitivity to dark photons competitive with other leading
approaches but using substantially less exposure (0.49 gram days). These
results demonstrate the scientific potential of phonon-mediated semiconductor
detectors that are sensitive to single electronic excitations.
