Title: Low-Mass Dark Matter Search with CDMSlite,
Abstract: The SuperCDMS experiment is designed to directly detect weakly interacting
massive particles (WIMPs) that may constitute the dark matter in our Galaxy.
During its operation at the Soudan Underground Laboratory, germanium detectors
were run in the CDMSlite mode to gather data sets with sensitivity specifically
for WIMPs with masses ${<}$10 GeV/$c^2$. In this mode, a higher detector-bias
voltage is applied to amplify the phonon signals produced by drifting charges.
This paper presents studies of the experimental noise and its effect on the
achievable energy threshold, which is demonstrated to be as low as 56
eV$_{\text{ee}}$ (electron equivalent energy). The detector-biasing
configuration is described in detail, with analysis corrections for voltage
variations to the level of a few percent. Detailed studies of the
electric-field geometry, and the resulting successful development of a fiducial
parameter, eliminate poorly measured events, yielding an energy resolution
ranging from ${\sim}$9 eV$_{\text{ee}}$ at 0 keV to 101 eV$_{\text{ee}}$ at
${\sim}$10 eV$_{\text{ee}}$. New results are derived for astrophysical
uncertainties relevant to the WIMP-search limits, specifically examining how
they are affected by variations in the most probable WIMP velocity and the
Galactic escape velocity. These variations become more important for WIMP
masses below 10 GeV/$c^2$. Finally, new limits on spin-dependent low-mass
WIMP-nucleon interactions are derived, with new parameter space excluded for
WIMP masses $\lesssim$3 GeV/$c^2$