Speaker
Description
Neutrinoless double beta decay (0νββ) is a lepton-number–violating process whose observation would establish the Majorana nature of neutrinos. KamLAND-Zen 800 currently sets the most stringent lower limit on the 0νββ half-life of $^{136}$Xe at $3.8 \times 10^{26}$ years at 90% C.L. A precise understanding of two-neutrino double beta decay (2νββ) is essential for future 0νββ searches. In addition to constituting an irreducible background, 2νββ measurements provide important constraints on nuclear matrix element calculations. While ground-state transitions have been measured in several isotopes, decays to excited states of the daughter nucleus are rare and remain largely unexplored. We present the status and analysis strategy for a search for 2νββ decay of $^{136}$Xe to excited states of $^{136}$Ba in KamLAND-Zen 800. The analysis framework includes dedicated event selection, spectral modeling of de-excitation γ signatures, and a background rejection study using KamNet, a spherical convolutional neural network developed for KamLAND-Zen. We evaluate KamNet’s performance in suppressing dominant internal and external backgrounds while preserving signal efficiency, and estimate the impact on the sensitivity of this search.