  The Sub-Electron-Noise Skipper CCD Experimental Instrument (SENSEI) uses the
recently developed Skipper-CCD technology to search for electron recoils from
the interaction of sub-GeV dark matter particles with electrons in silicon. We
report first results from a prototype SENSEI detector, which collected 0.019
gram-days of commissioning data above ground at Fermi National Accelerator
Laboratory. These commissioning data are sufficient to set new direct-detection
constraints for dark matter particles with masses between ~500 keV and 4 MeV.
Moreover, since these data were taken on the surface, they disfavor previously
allowed strongly interacting dark matter particles with masses between ~500 keV
and a few hundred MeV. We discuss the implications of these data for several
dark matter candidates, including one model proposed to explain the anomalously
large 21-cm signal observed by the EDGES Collaboration. SENSEI is the first
experiment dedicated to the search for electron recoils from dark matter, and
these results demonstrate the power of the Skipper-CCD technology for dark
matter searches.
