Constraints on low-mass, relic dark matter candidates from a surface-operated SuperCDMS single-charge sensitive detector

This article presents an analysis and the resulting limits on light dark matter inelastically scattering off of electrons, and on dark photon and axionlike particle absorption, using a second-generation SuperCDMS high-voltage eV-resolution detector. The 0.93 g Si detector achieved a 3 eV phonon energy resolution; for a detector bias of 100 V, this corresponds to a charge resolution of 3% of a single electron-hole pair. The energy spectrum is reported from a blind analysis with 1.2 g-days of exposure acquired in an above-ground laboratory. With charge carrier trapping and impact ionization effects incorporated into the dark matter signal models, the dark matter-electron cross section (sigma) over bar (e) is constrained for dark matter masses from 0.5 to 10(4) MeV/c(2); in the mass range from 1.2 to 50 eV/c(2) the dark photon kinetic mixing parameter e and the axioelectric coupling constant gae are constrained. The minimum 90% confidence-level upper limits within the above-mentioned mass ranges are (sigma) over bar (e) = 8.7 x 10(-34) cm(2), epsilon = 3.3 x 10(-14), and g(ae) = 1.0 x 10(-9).