SENSEI: Direct-Detection Constraints on Sub-GeV Dark Matter from a
  Shallow Underground Run Using a Prototype Skipper-CCD
  We present new direct-detection constraints on eV-to-GeV dark matter
interacting with electrons using a prototype detector of the Sub-Electron-Noise
Skipper-CCD Experimental Instrument. The results are based on data taken in the
MINOS cavern at the Fermi National Accelerator Laboratory. We focus on data
obtained with two distinct readout strategies. For the first strategy, we read
out the Skipper-CCD continuously, accumulating an exposure of 0.177 gram-days.
While we observe no events containing three or more electrons, we find a large
one- and two-electron background event rate, which we attribute to spurious
events induced by the amplifier in the Skipper-CCD readout stage. For the
second strategy, we take five sets of data in which we switch off all
amplifiers while exposing the Skipper-CCD for 120k seconds, and then read out
the data through the best prototype amplifier. We find a one-electron event
rate of (3.51 +- 0.10) x 10^(-3) events/pixel/day, which is almost two orders
of magnitude lower than the one-electron event rate observed in the
continuous-readout data, and a two-electron event rate of (3.18 +0.86 -0.55) x
10^(-5) events/pixel/day. We again observe no events containing three or more
electrons, for an exposure of 0.069 gram-days. We use these data to derive
world-leading constraints on dark matter-electron scattering for masses between
500 keV to 5 MeV, and on dark-photon dark matter being absorbed by electrons
for a range of masses below 12.4 eV.
