Investigating the sources of low-energy events in a SuperCDMS-HVeV
  detector
  Recent experiments searching for sub-GeV/$c^2$ dark matter have observed
event excesses close to their respective energy thresholds. Although specific
to the individual technologies, the measured excess event rates have been
consistently reported at or below event energies of a few-hundred eV, or with
charges of a few electron-hole pairs. In the present work, we operated a 1-gram
silicon SuperCDMS-HVeV detector at three voltages across the crystal (0 V, 60 V
and 100 V). The 0 V data show an excess of events in the tens of eV region.
Despite this event excess, we demonstrate the ability to set a competitive
exclusion limit on the spin-independent dark matter--nucleon elastic scattering
cross section for dark matter masses of $\mathcal{O}(100)$ MeV/$c^2$, enabled
by operation of the detector at 0 V potential and achievement of a very low
$\mathcal{O}(10)$ eV threshold for nuclear recoils. Comparing the data acquired
at 0 V, 60 V and 100 V potentials across the crystal, we investigated possible
sources of the unexpected events observed at low energy. The data indicate that
the dominant contribution to the excess is consistent with a hypothesized
luminescence from the printed circuit boards used in the detector holder.
