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

• DOI: 10.48550/arxiv.2301.11371
• Publication Date: 2023-01-01
• Authors: Rebeiro, B. M.; Triambak, S.; Garrett, P. E.; Ball, G. C.; Brown, B. A.; Menéndez, J.; Romeo, B.; Adsley, P.; Lenardo, B. G.; Lindsay, R.; Bildstein, V.; Burbadge, C.; Coleman, R.; Varela, A. Diaz; Dubey, R.; Faestermann, T.; Hertenberger, R.; Kamil, M.; Leach, K. G.; Natzke, C.; Ondze, J. C. Nzob...; Radich, A.; Rand, E.; Wirth, H. -F.
• Keywords: FOS: Physical sciences; Nuclear Experiment (nucl-ex)

We used the $^{138}$Ba$(d,α)$ 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.