Speaker
Description
Neutrinoless double beta decay (0$\nu\beta\beta$) is a rare nuclear process predicted by beyond-Standard Model theories, offering crucial insights into the nature of neutrinos and lepton number violation. A confirmed observation of 0$\nu\beta\beta$ would establish the Majorana nature of neutrinos and provide constraints on their absolute mass scale. Among candidate isotopes, the decay of $^{136}$Xe to $^{136}$Ba is extensively studied in large-scale experiments such as EXO, KamLAND-Zen, nEXO, and PandaX. However, to date, experiments have only set lower limits on the decay lifetimes [1].
A significant challenge remains in the precise determination of nuclear matrix elements (NMEs), which introduce uncertainties in extracting neutrino properties from measured decay rates. Theoretical predictions of NMEs vary considerably [2], highlighting the need for improved nuclear structure data.
This study investigates the nuclear structure of $^{136}$Ba, the daughter nucleus of $^{136}$Xe, through high-resolution gamma-ray spectroscopy using the FIPPS array at ILL. The focus is on low-spin states in $^{136}$Ba populated via the $^{135}$Ba(n,$\gamma$)$^{136}$Ba reaction, with particular emphasis on the characterization of low-spin 0$^{+}$ states. These states play a fundamental role in 0$\nu\beta\beta$ decay transitions but remain incompletely understood.
The level scheme of $^{136}$Ba has been studied through $^{136}$Cs $\beta$ decay and $^{135}$Ba(n, $\gamma$) reaction experiments. Although several (n, $\gamma$) studies have been conducted, the only published data dates back to 1969 [3]. More recently, a study of the $^{138}$Ba(p, t) $^{136}$Ba reaction [4] identified several previously unknown 0$^{+}$ states in $^{136}$Ba. The high statistics of this experiment will allow for a significant expansion of the existing data set.
The experimental setup consisted of 16 HPGe clover detectors with anti-Compton shields, achieving an efficiency of 3.5\% at 1.4~MeV and an energy resolution of $\sim$2~keV at 1.3~MeV. The experiment employed a thermal neutron beam from the ILL reactor with an intensity of $\sim$10$^{7}$~n/s/cm$^{2}$ [5]. The results will highlight newly identified transitions and spin assignments for states up to 5~MeV in excitation energy. The coincidence method was used to assign new decay lines by analyzing $\gamma\gamma$ matrices, while spin assignments were determined through angular correlation analysis of coincident $\gamma$ rays, referencing existing literature on tentative values and mixing ratios.
Additionally, the findings will be compared with theoretical calculations to provide further insights into the nuclear structure of $^{136}$Ba. Lifetime measurements will be conducted to reduce uncertainties and provide new data. The vibrational and mixed-symmetry properties of $^{136}$Ba ($N=80$) will also be explored to enhance the understanding of its collective dynamics. These results aim to reduce NME uncertainties, advance knowledge of 0$\nu\beta\beta$, and contribute to broader nuclear structure studies.
References:
[1] A. Gando et al. (KamLAND-Zen Collaboration), Phys. Rev. Lett. 117, 082503 (2016).
[2] J. Engel and J. Menéndez, Rep. Prog. Phys. 80, 046301 (2017).
[3] W. Gelletly et al., Phys. Rev. 181, 1682 (1969).
[4] B. M. Rebeiro et al., Phys. Lett. B 809, 135702 (2020).
[5] C. Michelagnoli et al., EPJ Web Conf. 193 (2018).