Exploring the keV-scale physics potential of CUORE

We present the analysis techniques developed to explore the keV-scale energy region of the CUORE experiment, based on more than 2 tonne yr of data collected over 5 years. By prioritizing a stricter selection over a larger exposure, we are able to optimize data selection for thresholds at 10 keV and 3 keV with 691 kg yr and 11 kg yr of data, respectively. We study how the performance varies among the 988-detector array with different detector characteristics and data taking conditions. We achieve an average baseline resolution of 2.54 $\pm$ 0.14 keV FWHM and 1.18 $\pm$ 0.02 keV FWHM for the data selection at 10 keV and 3 keV, respectively. The analysis methods employed reduce the overall background by about an order of magnitude, reaching 2.06 $\pm$ 0.05 counts/(keV kg days) and 16 $\pm$ 2 counts/(keV kg days) at the thresholds of 10 keV and 3 keV. We evaluate for the first time the near-threshold reconstruction efficiencies of the CUORE experiment, and find these to be 26 $\pm$ 4 % and 50 $\pm$ 2 % at 3 keV and 10 keV, respectively. This analysis provides crucial insights into rare decay studies, new physics searches, and keV-scale background modeling with CUORE. We demonstrate that tonne-scale cryogenic calorimeters can operate across a wide energy range, from keV to MeV, establishing their scalability as versatile detectors for rare event and dark matter physics. These findings also inform the optimization of future large mass cryogenic calorimeters to enhance the sensitivity to low-energy phenomena.

💡 Research Summary

The CUORE Collaboration presents a comprehensive study aimed at extending the physics reach of the CUORE experiment into the keV energy regime. Using more than 2 tonne·year of data collected over five years, the authors develop and apply a suite of analysis techniques that lower the effective energy threshold from the standard 40 keV (used for neutrinoless double‑beta decay searches) down to 10 keV and, in a more aggressive configuration, to 3 keV.

Two data‑selection strategies are defined. The “10 keV” selection retains a large exposure of 691 kg·yr by applying a moderate set of quality cuts that suppress noise while preserving most events. The “3 keV” selection imposes stricter pulse‑shape criteria, resulting in a much smaller but ultra‑clean exposure of 11 kg·yr. Both selections are applied to the full 988‑detector array, allowing the authors to study performance variations across detectors and over the three operational configurations (initial 12 mK, post‑Minus‑K upgrade, and later 15 mK operation).

Key technical advances include:

1. Offline Trigger Optimization – The trigger algorithm is re‑tuned to improve sensitivity to low‑amplitude pulses, ensuring that genuine keV‑scale events are not missed.

2. Noise Decorrelation – Auxiliary sensors (microphones, accelerometers, seismometers) are used to decorrelate vibrational noise from the thermistor waveforms, a step that is crucial for achieving stable baselines at the lowest energies.

3. Optimum Filtering at keV Scale – The standard optimum filter is adapted using detector‑specific average pulse shapes and noise power spectra, yielding an average baseline resolution of 2.54 ± 0.14 keV (FWHM) for the 10 keV selection and 1.18 ± 0.02 keV (FWHM) for the 3 keV selection.

4. Low‑Energy Calibration – Te X‑ray lines at 27 keV and 31 keV, observed in calibration runs, serve as reliable low‑energy anchors. Because CUORE’s resolution cannot separate the eight individual Te X‑ray lines, the authors model them as two Gaussian components with a shared width and a floating amplitude ratio that accounts for depth‑dependent attenuation.

5. Multi‑Site Event Tagging – By distinguishing single‑site (M1) from double‑site (M2) events, the analysis efficiently rejects background events that originate from external γ‑rays scattering between crystals, further cleaning the near‑threshold spectrum.

6. Spurious Pulse Rejection – A combination of pulse‑shape cuts, pile‑up identification, and vetoes based on auxiliary sensor activity removes electronic glitches, vibrationally induced phonon bursts, and heater‑pulse artifacts that would otherwise dominate the sub‑10 keV region.

The background rate after all cuts is reduced by roughly an order of magnitude, reaching 2.06 ± 0.05 counts/(keV·kg·day) at 10 keV and 16 ± 2 counts/(keV·kg·day) at 3 keV. Near‑threshold reconstruction efficiencies are measured for the first time in CUORE: 50 ± 2 % at 10 keV and 26 ± 4 % at 3 keV. Although the efficiency drops at the lowest energies due to the stringent quality criteria, the large exposure and low background compensate, preserving competitive sensitivity to rare low‑energy phenomena.

The authors discuss the implications of these results for a broad physics program. The demonstrated ability to operate a tonne‑scale cryogenic calorimeter over three orders of magnitude in energy establishes CUORE as a versatile platform for searches beyond neutrinoless double‑beta decay, including low‑mass WIMP dark matter, axion‑like particles, and other exotic interactions that produce keV‑scale deposits. The analysis framework also provides a template for future large‑mass cryogenic experiments such as CUPID, where optimization of low‑energy response will be essential for maximizing scientific reach.

In summary, this work shows that with careful offline processing, noise mitigation, and calibrated low‑energy event selection, CUORE can reliably probe the keV regime while maintaining excellent energy resolution and dramatically reduced background. This opens a new window for rare‑event searches with tonne‑scale cryogenic detectors and informs the design of next‑generation experiments targeting low‑energy new physics.