First Event-by-Event Identification of Cherenkov Radiation from Sub-MeV Particles in Liquid Argon
A. A. Aguilar-Arevalo, S. Biedron, J. Boissevain, M. Borrego, L. Bugel, M. Chavez-Estrada, J. M. Conrad, R. L. Cooper, J. R. Distel, J. C. D'Olivo, E. Dunton, B. Dutta, D. E. Fields, M. Gold, E. Guardincerri, E. C. Huang, N. Kamp, D. Kim, K. Knickerbocker, W. C. Louis, C. F. Macias-Acevedo, R. Mahapatra, J. Mezzetti, J. Mirabal, M. J. Mocko, D. A. Newmark, P. deNiverville, C. O'Connor, V. Pandey, D. Poulson, H. Ray, E. Renner, T. J. Schaub, A. Schneider, M. H. Shaevitz, D. Smith, W. Sondheim, A. M. Szelc, C. Taylor, A. Thompson, W. H. Thompson, M. Tripathi, R. T. Thornton, R. Van Berg, R. G. Van de Water
Published: 2025/7/10
Abstract
This Letter reports the event-by-event observation of Cherenkov light from sub-MeV electrons in a high scintillation light-yield liquid argon (LAr) detector by the Coherent CAPTAIN-Mills (CCM) experiment. The CCM200 detector, located at Los Alamos National Laboratory, instruments 7 tons (fiducial volume) of LAr with 200 8-inch photomultiplier tubes (PMTs), 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 $-6 \leq t < 0$ ns relative to the event start time $t=0$, the uncoated PMTs are primarily sensitive to visible Cherenkov photons. Using gamma-rays from a $^{22}$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.