First limit on neutrinoless quadruple β decay of Nd150 to the 01+ state of Gd150

Abstract

Background: Observation of lepton number violation via detection of neutrinoless double $\ensuremath{\beta}$ 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 $\ensuremath{\beta}$ decay. Only a few nuclei can undergo neutrinoless quadruple $\ensuremath{\beta}$ decay; one of these nuclei is $^{150}\mathrm{Nd}$.Purpose: This study yields the first half-life limit of the neutrinoless quadruple $\ensuremath{\beta}$ decay to the excited ${0}_{1}^{+}$ state of $^{150}\mathrm{Gd}$.Methods: We searched for neutrinoless quadruple $\ensuremath{\beta}$ decay events to excited final states of $^{150}\mathrm{Gd}$ by detecting the deexcitation $\ensuremath{\gamma}$ rays of the daughter nucleus in coincidence. These $\ensuremath{\gamma}$ rays have energies of 569.031 and 638.050 keV, and are emitted in coincidence through a ${0}_{1}^{+}\ensuremath{\rightarrow}{2}_{1}^{+}\ensuremath{\rightarrow}{0}_{gs}^{+}$ transition.Results: The enriched ${\mathrm{Nd}}_{2}{\mathrm{O}}_{3}$ sample consisted of $40.33\ifmmode\pm\else\textpm\fi{}0.02$ g $^{150}\mathrm{Nd}$ 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 $^{150}\mathrm{Gd}$ was found to be ${T}_{1/2}>1.76\ifmmode\times\else\texttimes\fi{}{10}^{20}$ 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.