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

We used the 138Ba(d, alpha) reaction to carry out an in-depth study of states in Cs-136, 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 Cs-136, which would allow a realtime 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 Xe-136 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 Cs-136 spectrum, while the other two show reasonably good agreement.