Using Simulation to Interpret the Liquid Argon Scattering Experiment's Results*

The Deep Underground Neutrino Experiment’s (DUNE) primary physics goals include determining the neutrino mass hierarchy and measuring the CP violating phase. Both of these are imperative to understanding the origin of matter and progressing toward a grand unification of forces. DUNE must possess unprecedented experimental capabilities in order to have the ability to span these areas of study within neutrino physics. This includes the ability to detect any trace an interacting neutrino can leave, including photon emissions. Presently, scintillation light analysis in liquid argon based neutrino detectors is restrained in capability due to uncertainty in fundamental constants critical to the analysis process. One such property is the Rayleigh scattering length of liquid argon. 

In the fall of 2023, the TallBo cryostat, located at Fermilab, was used to study the scattering length of liquid argon in the Liquid Argon Scattering (LArS) experiment. Due to systematic errors unknown during measurement analysis, LArS’s measurements were quite uncertain. By simulating the LArS experiment using Geant4, we found that the downturn in detector count rate as a function of liquid argon height at low heights was caused by a misplaced detector. To account for this discrepancy, we performed a likelihood ratio test to pinpoint the best simulation conditions for reproducing the experimental data. With this information, we can extract correction factors that allow us to deduce a tangible scattering length measurement from the experimental data. 

With concentrated effort on the analysis frontier, LArS has great promise of delivering a concrete experimental measurement of the Rayleigh scattering length of liquid argon. This measurement is critical to scintillation light analysis, which will be foundational to event reconstruction methods used by the large liquid argon time projection chambers of DUNE.