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

TL;DR

This study investigates specific nuclear states in $^{136}$Cs relevant for dark matter and solar neutrino detection in xenon detectors, identifying potential metastable states that could improve real-time detection capabilities.

Contribution

First observation of candidate metastable states in $^{136}$Cs, with implications for enhancing detection of solar neutrinos and dark matter in xenon-based experiments.

Findings

Identification of unobserved low-energy states in $^{136}$Cs.

Discovery of candidate metastable states for real-time detection.

Comparison of shell-model calculations with experimental data.

Abstract

We used the $^{138}$Ba$(d,\alpha)$ 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.