This Letter reports the event-by-event observation of Cherenkov light from sub-MeV electrons in a high scintillation light-yield liquid argon detector by the coherent CAPTAIN-Mills (CCM) experiment. The CCM200 detector, located at Los Alamos National Laboratory, instruments seven tons (fiducial volume) of liquid argon with 200 eight-inch photomultiplier tubes, 80% of which are coated in a wavelength-shifting material and the remaining 20% are uncoated. In the prompt time region of an event, defined as $-6\le t<0\mathrm{ns}$ relative to the event start time $t=0$, the uncoated photomultiplier tubes are primarily sensitive to visible Cherenkov photons. Using gamma rays from a ${}^{22}\mathrm{Na}$ source for production of sub-MeV electrons, we isolated prompt Cherenkov light with $>5\sigma$ confidence and developed a selection to obtain a low-background electromagnetic sample. This is the first event-by-event observation of Cherenkov photons from sub-MeV electrons in a high-yield scintillator detector and represents a milestone in low-energy particle detector development.

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• Particle detection signatures
• Neutrinos
• Cherenkov detectors
• Multi-purpose particle detectors
• Neutrino detection
• Scintillators