Sensitivity of neutrinoless double beta decays from a combined analysis of ground and excited states

Next-generation neutrinoless double-beta ($0νββ$) decay experiments, with projected half-life sensitivities approaching $10^{28}$ years, are expected to probe the entire parameter space of the inverted neutrino mass ordering. However, this discovery reach remains limited by the substantial model dependence of the nuclear matrix elements (NMEs). In this work, we propose a strategy based on a combined analysis of $0νββ$ decays to both the ground state and the first excited $0^+$ state of the daughter nucleus. We show that such a multi-channel approach can significantly enhance experimental sensitivity, depending on the underlying NME predictions. This method is particularly well suited for large liquid xenon detectors, such as the proposed PandaX-xT and XLZD experiments, which can efficiently identify transitions of \nuclide[136]{Xe} to excited states. Our results highlight the importance of exploiting multiple decay channels in future $0νββ$ searches to maximize their discovery potential.