First Event-by-Event Identification of Cherenkov Radiation from Sub-MeV Particles in Liquid Argon

TL;DR

This paper reports the first event-by-event detection of Cherenkov radiation from sub-MeV electrons in a liquid argon detector, demonstrating a new capability for low-energy particle detection with high confidence.

Contribution

It introduces a method to observe Cherenkov light from sub-MeV electrons event-by-event in a high-yield liquid argon detector, a novel achievement in the field.

Findings

Cherenkov light from sub-MeV electrons was isolated with >5σ confidence.

First event-by-event observation of Cherenkov photons in a high-yield scintillator detector.

Developed a selection method for low-background electromagnetic samples.

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.