Search for Neutrinoless Quadruple-$\beta$ Decay of ${}^{150}\mathrm{Nd}$ with the NEMO-3 Detector

We report the results of a first experimental search for lepton number violation by four units in the neutrinoless quadruple-$\beta$ decay of ${}^{150}\mathrm{Nd}$ using a total exposure of 0.19 kg yr recorded with the NEMO-3 detector at the Modane Underground Laboratory. We find no evidence of this decay and set lower limits on the half-life in the range $T_{1/2}>(1.1–3.2)\times 10^{21}\mathrm{yr}$ at the 90% C.L., depending on the model used for the kinematic distributions of the emitted electrons.

Figure 1

Normalized distribution of the individual electron kinetic energies $T$ in each $0\nu 4\beta$ decay for the four kinematic models.

Figure 2

Energy-sum distributions for three-electron events originating in the source foils of (a) ${}^{100}\mathrm{Mo}$ and (b) ${}^{82}\mathrm{Se}$, which cannot undergo $0\nu 4\beta$ decay. The hashed areas represent the systematic uncertainty on the reconstruction efficiency.

Figure 3

Display of a decay with four reconstructed electrons in NEMO-3 data, originating in the ${}^{100}\mathrm{Mo}$ source foil, in the horizontal plane.

Figure 4

Energy sum distributions for (a) $4e$, (b) $3e$, and (c) $3e1t$ events in the ${}^{150}\mathrm{Nd}$ foil for data, the expected background, and signal. The hashed areas represent the uncertainty on the background model. The $0\nu 4\beta$ signal distributions are normalized to the 90% C.L. limits, with an additional scaling factor of 0.1 applied to the expected $4e$ signal distributions for better visibility.