Simulation Study on the Discrimination of $0\hat{I}½{\hat{I}}^2{\hat{I}}^2$ Events from Single-Electron Events Using Orthogonal-Strip HPGe Detectors

Abstract

Neutrinoless double beta decay ($0\hat{I}½{\hat{I}}^2{\hat{I}}^2$) offers a sensitive probe of neutrino mass and its Majorana nature. Orthogonal-strip high-purity germanium (HPGe) detectors with high spatial resolution provide a promising approach for distinguishing $0\hat{I}½{\hat{I}}^2{\hat{I}}^2$ events from single-electron backgrounds. In this work, a simulation framework was developed to evaluate the discrimination performance of these detectors. The framework combined Geant4 simulations with a hybrid numerical-analytical approach to model charge cloud dynamics. A dual-branch convolutional neural network (CNN) was implemented to extract topological features for event classification. The impact of detector geometry on discrimination performance was quantitatively assessed. For a fixed crystal thickness of 15 mm, the background rejection efficiency decreased from 79.5\% to 59.0\% as the strip pitch increased from 0.1 mm to 0.5 mm. For a strip pitch of 0.25 mm, a crystal thickness of 20 mm was found to be optimal, balancing full-energy peak (FEP) efficiency with discrimination capability. These results demonstrate that orthogonal-strip HPGe detectors can effectively suppress single-electron backgrounds, and provide quantitative guidance for detector design in ${}^{76}$Ge $0\hat{I}½{\hat{I}}^2{\hat{I}}^2$ decay searches.