${}^{138}\mathrm{Ba}(d,\alpha )$ Study of States in ${}^{136}\mathrm{Cs}$: Implications for New Physics Searches with Xenon Detectors

We used the ${}^{138}\mathrm{Ba}(d,\alpha )$ reaction to carry out an in-depth study of states in ${}^{136}\mathrm{Cs}$, 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 ${}^{136}\mathrm{Cs}$, 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 ${}^{136}\mathrm{Xe}$ 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 ${}^{136}\mathrm{Cs}$ spectrum, while the other two show reasonably good agreement.

Figure 1

Sample ${}^{138}\mathrm{Ba}(d,\alpha )$ spectrum obtained at ${\theta }_{\mathrm{lab}}=10°$. A few prominent peaks are labeled.

Figure 2

Comparison between theory and experiment for the low-lying energy spectrum of ${}^{136}\mathrm{Cs}$. The shell-model results were obtained with the GCN5082, SN100PN, and QX effective interactions.

Figure 3

Measured ${}^{138}\mathrm{Ba}(d,\alpha )$ angular distributions compared with best-fit dwuck5 DWBA predictions (solid red curves). The blue dashed curves are from using fixed relative $L$ contributions from Eq. (1). The dominant orbitals involved in the pair transfer are specified in each plot.