First Results from CUORE: A Search for Lepton Number Violation via $0νββ$ Decay of $^{130}$Te

Abstract: The CUORE experiment, a ton-scale cryogenic bolometer array, recently began operation at the Laboratori Nazionali del Gran Sasso in Italy. The array represents a significant advancement in this technology, and in this work we apply it for the first time to a high-sensitivity search for a lepton-number--violating process: $^{130}$Te neutrinoless double-beta decay. Examining a total TeO$2$ exposure of 86.3 kg$\cdot$yr, characterized by an effective energy resolution of (7.7 $\pm$ 0.5) keV FWHM and a background in the region of interest of (0.014 $\pm$ 0.002) counts/(keV$\cdot$kg$\cdot$yr), we find no evidence for neutrinoless double-beta decay. The median statistical sensitivity of this search is $7.0\times10^{24}$ yr. Including systematic uncertainties, we place a lower limit on the decay half-life of $T^{0\nu}{1/2}$($^{130}$Te) > $1.3\times 10^{25}$ yr (90% C.L.). Combining this result with those of two earlier experiments, Cuoricino and CUORE-0, we find $T^{0\nu}_{1/2}$($^{130}$Te) > $1.5\times 10^{25}$ yr (90% C.L.), which is the most stringent limit to date on this decay. Interpreting this result as a limit on the effective Majorana neutrino mass, we find $m_{\beta\beta}<(110 - 520)$ meV, where the range reflects the nuclear matrix element estimates employed.

• The paper demonstrates the CUORE experiment’s innovative approach to search for neutrinoless double-beta decay in 130Te.
• Utilizing 988 TeO₂ crystals cooled to 7 mK, the study refines energy resolution and background mitigation to achieve high sensitivity.
• Results set a 90% confidence level lower limit of 1.5×10²⁵ years for the decay half-life, constraining the effective Majorana neutrino mass.

First Results from CUORE: A Search for Lepton Number Violation via Neutrinoless Double-Beta Decay of $^{130}$Te

The paper under consideration reports on pivotal experimentation from the CUORE Collaboration aimed at exploring the neutrinoless double-beta ($0\nu\beta\beta$) decay in $^{130}$Te, a hypothetical process that, should it be observed, would confirm the Majorana nature of neutrinos and offer insight into lepton number violation. The Cryogenic Underground Observatory for Rare Events (CUORE) has embarked on this exploration at a scale and sensitivity unprecedented in this field.

Experimental Setup and Sensitivity

CUORE is an extensive bolometer array, consisting of 988 tellurium dioxide (TeO$_2$) crystals, each with a mass of approximately 750 grams, cooled to 7 mK. This intricate setup allows for the detection of minute temperature increases due to energy deposition, with the primary goal of identifying the energy signature characteristic of $0\nu\beta\beta$ decay at the Q-value of $2527.515 \pm 0.013$ keV.

The large-scale experiment capitalizes on the significant natural abundance of $^{130}$Te, which is 34.167%, alongside promising $Q_{\beta\beta}$ values which optimize the sensitivity to the $0\nu\beta\beta$ decay signature. Over the course of two datasets, several technical advancements were applied, such as active noise cancellation and operational temperature optimization, to enhance the resolution, resulting in distinct refinements in the detector's effectiveness and stability.

Results and Analysis

From the datasets spanning 86.3 kg$\cdot$yr of $^{130}$Te exposure, the statistical analysis presented in the study did not reveal any conclusive evidence for $0\nu\beta\beta$ decay. Nevertheless, CUORE established a 90% confidence level lower limit on the decay half-life of $^{130}$Te of $$T_{1/2}^{0\nu}(^{130}\mathrm{Te}) > 1.5 \times 10^{25}$$ yr, incorporating data from previous experiments Cuoricino and CUORE-0. The effective Majorana neutrino mass constraint derived from this new half-life limit spans a range of 110 to 520 meV, contingent upon the nuclear matrix elements employed.

Discussion and Future Prospects

This research signifies a substantial step in the quest to observe $0\nu\beta\beta$ decay, a fundamental pursuit in neutrino physics with profound implications for our understanding of particle physics and cosmology. While no detection was achieved, the experiment solidified CUORE's position as the most sensitive and largest bolometric setup dedicated to $0\nu\beta\beta$ decay searches. The background rate achieved—at $1.3 \times 10^{-2}$ counts/(keV$\cdot$kg$\cdot$yr) at the region of interest—combined with an energy resolution of 7.7 keV FWHM, underlines substantial progress toward minimizing experimental uncertainties and external interferences.

The continuation of this endeavor involves further refinements to detector resolution and background mitigation, projecting toward potentially improved sensitivity that could significantly lower the effective mass bound of Majorana neutrinos. Results from CUORE could have far-reaching implications, not just within neutrinoless double-beta decay searches, but across broader efforts to unravel the mysteries tied to neutrino masses and their fundamental characteristics in relation to the Standard Model and cosmology.

This ongoing journey to ascertain the existence of $0\nu\beta\beta$ decay aligns with a global scientific effort marked by concurrent developments in various detector technologies and collaborative research designed to enhance both the reach and precision of experimental neutrino physics.