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

Next-generation neutrinoless double-beta ($0\nu \beta \beta$) decay experiments, with projected half-life sensitivities approaching ${10}^{28}$ years, aim to probe the entire parameter space of the inverted neutrino mass ordering in the light-neutrino-exchange scenario. However, this reach remains uncertain by the substantial model dependence of the nuclear matrix elements (NMEs). In this work, we propose a strategy based on a combined analysis of $0\nu \beta \beta$ 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 ${}^{136}$Xe to excited states. Our results highlight the importance of exploiting multiple decay channels in future $0\nu \beta \beta$ searches to maximize their discovery potential.