Enhancing the sensitivity of neutrinoless double beta decays via combined multi-transition analysis

Abstract: Next-generation neutrinoless double-beta ($0\nu\beta\beta$) decay experiments, with projected half-life sensitivities approaching $10^{28}$ years, aim to fully probe the parameter space associated with the inverted neutrino mass ordering. However, this discovery potential remains uncertain, as it depends sensitively on the nuclear matrix element, which exhibits significant model dependence. In this work, we propose a novel strategy to enhance experimental sensitivity by performing a combined analysis of $0\nu\beta\beta$ decay to both the ground state and the first excited $0^+$ state of the daughter nucleus. This approach is particularly promising for large liquid xenon detectors, such as the proposed PandaX-xT and XLZD experiments, which are capable of identifying decays of $^{136}$Xe to excited states with high efficiency. Our analysis demonstrates that such a combined multi-transition analysis can improve the sensitivity to $|m_{\beta\beta}|$ by more than a factor of two for a nominal xenon detector setup, and by up to an order of magnitude in an ideal scenario, potentially accelerating access to the entire inverted ordering regime. These findings underscore the importance of probing multiple decay channels simultaneously in future $0\nu\beta\beta$ decay searches to maximize discovery potential.