First limit on neutrinoless quadruple β decay of Nd 150 to the 0 1 + state of Gd 150
Observation of lepton number violation via detection of neutrinoless double β decay requires that the neutrino be a Majorana particle. If the neutrino is a Dirac particle, a potential lepton-number violating process is neutrinoless quadruple β decay. Only a few nuclei can undergo neutrinoless quadruple β decay; one of these nuclei is 150 Nd. This study yields the first half-life limit of the neutrinoless quadruple β decay to the excited $0$ $^{+}_{1}$ state of 150 Gd. Here, we searched for neutrinoless quadruple β decay events to excited final states of 150 Gd by detecting the deexcitation γ rays of the daughter nucleus in coincidence. These γ rays have energies of 569.031 and 638.050 keV, and are emitted in coincidence through a $0$ $^{+}_{1}$ → $2$ $^{+}_{1}$ → $0$ $^{+}_{gs}$ transition. The enriched Nd 2 O 3 sample consisted of 40.33 ± 0.02 g 150 Nd 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 150 Gd was found to be T 1/2 > 1.76 × 10 20 years (90% CL). We report the first search for this decay to excited final states. Though the predicted half-life of this decay is many orders of magnitudes larger, constraining this value experimentally is vital to check for potential enhancements to the decay rate.