A Demonstration of Light-Augmented Calorimetry for Low-Energy Electrons in Liquid Argon

The ability to resolve electrons with energies as low as 5 MeV in liquid argon (LAr) is important for future studies of astrophysical neutrinos in liquid argon time projection chambers (LArTPCs). Existing methods for measuring the energy of electromagnetic showers in LArTPCs rely solely on charge-based reconstruction which requires correcting for assumed electron-ion recombination. However, LAr scintillation light has the potential to improve energy reconstruction over charge-based measurements alone. Here we present a demonstration of a light-augmented calorimetric technique for low-energy electrons in LAr that foregoes the need to correct for recombination. A sample of Michel electrons from stopping cosmic muons is collected in LArIAT (Liquid Argon In A Testbeam), a single-phase LArTPC at Fermilab's Test Beam Facility. Michel electron energy spectra are reconstructed using both a traditional charge-based approach as well as a more holistic approach that incorporates both charge and light. A maximum-likelihood fitter, using LArIAT's well-tuned simulation, is developed for combining these quantities to achieve optimal energy resolution. A sample of isolated electrons with $E_e < 60$~MeV is then simulated to better determine the energy resolution of this topology. Due to LArIAT's low wire noise, the addition of light is not found to significantly improve upon the achieved charged-based resolution of $\sigma/E \simeq 9\%/\sqrt{E} \oplus 1\%$. However, additional samples are generated with varying wire noise levels and light yields to gauge the impact of light-augmented calorimetry in large LArTPCs such as DUNE and SBND. At a charge-readout signal-to-noise of $\approx$10, the energy resolution for low-energy electrons is improved by $\sim$15\%, $\sim$25\%, and $\sim$45\% over charge-only calorimetry for average light yields of 10, 20, and 100 pe/MeV, respectively.