Complementary Time- and Distance-Based Methods for Cosmogenic $^{9}$Li/$^{8}$He Background Estimation

Abstract

Cosmogenic $^{9}$Li and $^{8}$He isotopes constitute an important correlated background in low-energy neutrino experiments because their $β$-delayed neutron decay signatures can mimic inverse beta decay signals. Conventional estimates based on the time since the last muon become challenging at high muon rates, while muon-related vetoes further reduce the residual isotope statistics. We extend the conventional time fit to an event-level muon-categorized joint time (J-MuCAT) fit, which uses the time to the most recent preceding muon in each energy-loss category. A complementary estimate is obtained from the candidate-to-muon-track distance distribution (TraDiTS). The muon-uncorrelated component is determined from the far-distance region and subtracted. This estimate is then used to constrain the J-MuCAT fit, defining the distance-constrained J-MuCAT (DCJ-MuCAT) fit. In detector-level simulation with successive cosmogenic-background vetoes, DCJ-MuCAT reduces the statistical uncertainty by more than $40\%$ relative to J-MuCAT and by more than $10\%$ relative to the TraDiTS. The fitted results remain consistent with the simulation truth. Applicability studies further show good performance over a broad range of muon rates and isotope fractions. The proposed framework provides a practical approach for estimating residual $^{9}$Li/$^{8}$He backgrounds in large neutrino detectors.