Ba 138 (d,α) Study of States in Cs 136: Implications for New Physics Searches with Xenon Detectors

We used the Ba138(d,α) reaction to carry out an in-depth study of states in Cs136, up to around 2.5 MeV. In this Letter, we place emphasis on hitherto unobserved states below the first 1+ level, which are important in the context of solar neutrino and fermionic dark matter (FDM) detection in large-scale xenon-based experiments. We identify for the first time candidate metastable states in Cs136, which would allow a real-time detection of solar neutrino and FDM events in xenon detectors, with high background suppression. Our results are also compared with shell-model calculations performed with three Hamiltonians that were previously used to evaluate the nuclear matrix element (NME) for Xe136 neutrinoless double beta decay. We find that one of these Hamiltonians, which also systematically underestimates the NME compared with the others, dramatically fails to describe the observed low-energy Cs136 spectrum, while the other two show reasonably good agreement.