First limit on neutrinoless quadruple beta decay of ¹⁵⁰Nd to the 0⁺₁ state of ¹⁵⁰Gd

If the neutrino is a Dirac particle, then observation of lepton number violation via detection of neutrinoless double beta decay is not viable. One potential lepton-number violating process that occurs even if the neutrino is a Dirac particle is neutrinoless quadruple beta (0ν4β) decay. However, the number of nuclei that can undergo 0ν4β-decay is limited; one of these nuclei is ¹⁵⁰Nd. In 2017, NEMO-3 reported the first search for 0ν4β-decay of ¹⁵⁰Nd to the ground state [1]. Here, we report the first half-life limit of the 0ν4β-decay to the excited 0⁺₁ state of ¹⁵⁰Gd. We searched for 0ν4β-decay events to excited final states of ¹⁵⁰Gd by detecting the de-excitation gamma rays of the daughter nucleus in coincidence. The enriched Nd₂O₃ sample consisted of 40.33 ± 0.02 g ¹⁵⁰Nd and was observed for 642.8 days at the Kimballton Underground Research Facility. A half-life limit for the decay to the 0⁺₁ state of ¹⁵⁰Gd was found to be T_1/2 > 1.76 × 10²⁰ years (90% CL). Though the predicted half life of this decay is many orders of magnitude larger, constraining this value experimentally is vital to check for potential enhancements to the decay rate.

[1] https://arxiv.org/abs/1705.08847