Two-neutrino $ββ$ decay of $^{136}$Xe to the first excited $0^+$ state in $^{136}$Ba

Abstract

We calculate the nuclear matrix element for the two-neutrino ββ decay of 136 Xe into the first excited 0 + state of 136 Ba. We use different many-body methods: the quasiparticle random-phase approximation (QRPA) framework, the nuclear shell model, the interacting boson model (IBM-2), and an effective field theory (EFT) for β and ββ decays. While the QRPA suggests a decay rate at the edge of current experimental limits, the shell model points to a half-life about two orders of magnitude longer. The predictions of the IBM-2 and the EFT lie in between, and the latter provides systematic uncertainties at leading order. An analysis of the running sum of the nuclear matrix element indicates that subtle cancellations between the contributions of intermediate states can explain the different theoretical predictions. For the EFT, we also present results for two-neutrino ββ decays to the first excited 0 + state in other nuclei.

Search for two-neutrino double-beta decay of 136Xe to the excited state of 136Ba with the complete EXO-200 dataset*

“…The current best half-life limits on 0νββ set by various experiments [1−5] for different isotopes are in the range of 10 [25][26] yr. The mode with two accompanying neutrinos (2νββ) is a standard model process and has been observed in over ten isotopes [6] with half-life between 10 19 and 10 24 yr. The 2νββ can also decay to the excited states of the daughter nucleus.…”

“…The 136 Xe decay to the ground state of 136 Ba has a Q value of 2457.83 keV [19]. The decay to the state of 136 Ba has a Q value of 878.8 keV, followed by the emission of two characteristic de-excitation γ's with energies of 760.5 keV and 818.5 keV from consecutive de-ex-…”

“…citations from the states of 136 Ba [19]. Two electrons accompanied by two γ's provide distinctive event signatures.…”