Measurement of the $2\nu \beta \beta$ Decay Half-Life of ${}^{82}\mathrm{Se}$ with the Global CUPID-0 Background Model

We report on the results obtained with the global CUPID-0 background model, which combines the data collected in the two measurement campaigns for a total exposure of $8.82\mathrm{kg}\times \mathrm{yr}$ of ${}^{82}\mathrm{Se}$. We identify with improved precision the background sources within the 3 MeV energy region, where neutrinoless double $\beta$ decay of ${}^{82}\mathrm{Se}$ and ${}^{100}\mathrm{Mo}$ is expected, making more solid the foundations for the background budget of the next-generation CUPID experiment. Relying on the excellent data reconstruction, we measure the two-neutrino double $\beta$-decay half-life of ${}^{82}\mathrm{Se}$ with unprecedented accuracy: $T_{1/2}^{2\nu }=[8.69\pm 0.05(\mathrm{stat}){}_{-0.06}{}^{+0.09}(\mathrm{syst})]\times {10}^{19}\mathrm{yr}$.

Figure 1

${\mathcal{M}}_1^{\beta /\gamma }$ (left) and ${\mathcal{M}}_1^{\alpha }$ (right) spectra registered by CUPID-0 in phase-I (full histograms) and phase-II (line histograms). In ${\mathcal{M}}_1^{\beta /\gamma }$, we identify the dominant $2\nu \beta \beta$ contribution (green), and the $\gamma$-ray peaks due to cosmogenic activation of $\mathrm{Zn}{}^{82}\mathrm{Se}$ (${}^{65}\mathrm{Zn}$), and setup contaminants (${}^{40}\mathrm{K}$, ${}^{208}\mathrm{Tl}$). In ${\mathcal{M}}_1^{\alpha }$, degraded $\alpha$ particles produce a flat background at 2–4 MeV, while the $\alpha$ peaks above are the distinctive signatures of the different detector contaminants. The main differences between phase-I and phase-II traceable in the data are the ${}^{65}\mathrm{Zn}$ reduction due to its short half-life and the ${}^{40}\mathrm{K}$ and ${}^{208}\mathrm{Tl}$ reduction due to the 10 mK (1 cm of copper) shield, which also introduces an additional ${}^{210}\mathrm{Po}$ component in the 2–4 MeV region of ${\mathcal{M}}_1^{\alpha }$. The yellow band in ${\mathcal{M}}_1^{\beta /\gamma }$ marks the region of interest of ${}^{82}\mathrm{Se}$ $0\nu \beta \beta$.

Figure 2

Background index in the region of interest for the $0\nu \beta \beta$ of ${}^{82}\mathrm{Se}$, i.e., [2.8,3.2] MeV of the ${\mathcal{M}}_1^{\beta /\gamma }$ spectrum. Experimental background indexes (black, top pad) are compared to the reconstructed ones for phase-I (blue) and phase-II (orange). The total background index is the sum of all components with their correlations, and error bars represent the systematic uncertainty of the background reconstruction.

Figure 3

Measurement of the ${}^{82}\mathrm{Se}\text{−}2\nu \beta \beta$ half-life obtained in the systematic tests (bottom) and as a final result of this analysis, compared to the previous result of CUPID-0 (top). The error bars shown in the bottom plot correspond to the statistical error of each systematic test, while the red vertical lines correspond to the Reference mean value (solid line) and its statistical uncertainty (dashed line). The ${}^{90}\mathrm{Sr}/{}^{90}\mathrm{Y}$ strongly correlates with the $2\nu \beta \beta$, thus broadening the uncertainty. The statistical uncertainty is then combined with the systematic one and compared with the previous result (top).