First limit on neutrinoless quadruple β decay of Nd 150 to the 01+ state of Gd 150
Background: 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 Nd150. Purpose: This study yields the first half-life limit of the neutrinoless quadruple β decay to the excited 01+ state of Gd150. Methods: We searched for neutrinoless quadruple β decay events to excited final states of Gd150 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 01+→21+→0gs+ transition. Results: The enriched Nd2O3 sample consisted of 40.33±0.02 g Nd150 and was observed for 642.8 days at the Kimballton Underground Research Facility. A half-life limit for the decay to the 01+ state of Gd150 was found to be T1/2>1.76×1020 years (90% CL). Conclusions: 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.