${}^{138}\mathrm{Ba}(d,\hat{I}\pm )$ study of states in ${}^{136}\mathrm{Cs}$: Implications for new physics searches with xenon detectors

Abstract

We used the ${}^{138}$Ba$(d,\hat{I}\pm )$ reaction to carry out an in-depth study of states in ${}^{136}$Cs, up to around 2.5~MeV. In this work, 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 experiments. We identify for the first time candidate metastable states in ${}^{136}$Cs, 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 ${}^{136}$Xe neutrinoless double beta decay. We find that one of these Hamiltonians, which also systematically underestimates the NME compared to the others, dramatically fails to describe the observed low-energy ${}^{136}$Cs spectrum, while the other two show reasonably good agreement.