Sensitivity of the CUPID experiment to $0νββ$ decay of $^{100}$Mo

TL;DR

CUPID targets neutrinoless double-beta decay of $^{100}$Mo using Li$_2$MoO$_4$ bolometers to achieve very low backgrounds and high energy resolution. The paper develops both Frequentist and Bayesian statistical frameworks, employing extended unbinned likelihoods and pseudo-experiments to quantify discovery and exclusion sensitivities, with explicit treatment of NMEs and a range of background and resolution scenarios. For the baseline design, a 10-year run yields a 3σ discovery sensitivity of $T_{1/2}^{0 u}\approx1.0\times10^{27}$ yr (corresponding to $m_{\beta\beta}\approx12$–36 meV) and a 90% C.L. exclusion around $T_{1/2}^{0 u}\approx1.8\times10^{27}$ yr ($m_{\beta\beta}\approx9$–26 meV), depending on NMEs; Bayesian results show consistent exclusion reach with a median $T_{1/2}^{0 u}=1.6^{+0.6}_{-0.5}\times10^{27}$ yr and $m_{\beta\beta}=9.6$–28 meV. The staged deployment (Phase I) could provide early discovery potential with roughly $3$ years at ~150 kg, before the full detector is deployed. Overall, the work demonstrates CUPID’s competitiveness in probing the inverted neutrino mass ordering and reaching sensitivities comparable to next-generation experiments, while highlighting NMEs as a major source of $m_{\beta\beta}$ uncertainty.

Abstract

CUPID is a next-generation bolometric experiment to search for neutrinoless double-beta decay ($0νββ$) of $^{100}$Mo using Li$_2$MoO$_4$ scintillating crystals. It will operate 1596 crystals at $\sim$10 mK in the CUORE cryostat at the Laboratori Nazionali del Gran Sasso in Italy. Each crystal will be facing two Ge-based bolometric light detectors for $α$ rejection. We compute the discovery and the exclusion sensitivity of CUPID to $0νββ$ in a Frequentist and a Bayesian framework. This computation is done numerically based on pseudo-experiments. For the CUPID baseline scenario, with a background and an energy resolution of $1.0 \times 10^{-4}$ counts/keV/kg/yr and 5 keV FWHM at the Q-value, respectively, this results in a Bayesian exclusion sensitivity (90% c.i.) of $\hat{T}_{1/2} > 1.6 \times 10^{27} \ \mathrm{yr}$, corresponding to the effective Majorana neutrino mass of $\hat{m}_{ββ} < \ 9.6$ -- $28 \ \mathrm{meV}$. The Frequentist discovery sensitivity (3$σ$) is $\hat{T}_{1/2}= 1.0 \times 10^{27} \ \mathrm{yr}$, corresponding to $\hat{m}_{ββ}= \ 12$ -- $36 \ \mathrm{meV}$.

Figures (12)

• Figure 1: Expected total background in the region of interest of CUPID, in red, including the $2\nu\beta\beta$ highlighted in blue. The histograms are obtained based on GEANT4 simulations.
• Figure 2: Example of two pseudo-experiments for the baseline background index of 1 $\times$ 10$^{-4}$ counts/keV/kg/yr. Top: with zero signal. Bottom: with signal rate $\Gamma$ = $1\times 10^{-27}$ yr$^{-1}$. The best fit to Eq. \ref{['model']} is also shown.
• Figure 3: Decay rates obtained from fitting pseudo-experiments with known decay rates, showing no significant bias in the reconstructed $\Gamma$ value. The background index is assumed to be $1\times 10^{-4}$ counts/keV/kg/yr.
• Figure 4: Probability distribution of the test statistic, $t_P(0)$ for zero decay rate in blue and $\Gamma=1 \times 10^{-27}$ yr$^{-1}$ in red, for $B$ = $10^{-4}$ counts/keV/kg/yr.
• Figure 5: Background-only p-value, $p_b$, against the injected signal rate. We show the median with a black curve and 1, 2$\sigma$ bands in green and orange. The red dashed line highlights the rate value for which a 3$\sigma$ discovery is expected, corresponding to a p-value $p_b$ = 0.14%.