Latest results from CUORE and prospects for CUPID

This paper presents the latest results from the CUORE experiment, which has achieved leading limits on neutrinoless double beta decay in $^{130}$Te, and outlines the prospects for the upcoming CUPID upgrade that will utilize $^{100}$Mo-enriched crystals and particle identification to significantly enhance sensitivity to the Inverted Hierarchy of neutrino masses.

The Gist

Imagine the universe is a giant, quiet library. For decades, scientists have been trying to find a single, specific book that was never supposed to exist. This "book" is a rare event called neutrinoless double beta decay.

Finding this event would be like discovering a ghost in the library. It would prove that:

1. Neutrinos are their own antiparticles (like a mirror image that is actually the same person).
2. The universe broke a fundamental rule (conservation of lepton number), which might explain why we have matter instead of just empty space.

This paper, written by Kang Zhao on behalf of the CUORE and CUPID collaborations, tells the story of two high-tech "libraries" built deep underground to catch this ghost.

1. The First Library: CUORE (The Giant Ice Cube)

The Setup:
Think of CUORE as a massive, ultra-cold refrigerator sitting 1,400 meters underground inside a mountain in Italy. Inside this fridge, there are 988 giant crystals made of a material called Tellurium Oxide (TeO₂).

How it works:

• The Temperature: The fridge is kept at a temperature colder than outer space (about 15 millikelvin). At this temperature, the crystals are so sensitive that they act like super-thermometers.
• The Detection: If a particle hits a crystal, it creates a tiny amount of heat. Because the crystal is so cold, even a tiny bit of heat causes a noticeable temperature spike. It's like trying to feel a single drop of hot water fall into a bucket of ice water.
• The Goal: They are watching for a specific type of atomic decay in the element Tellurium. If they see a "heat spike" at exactly the right energy level, it's a sign of the "ghost" (neutrinoless double beta decay).

The Results:
After running for over 7 years and collecting data equivalent to 2.9 tonnes of crystals, CUORE found no ghosts.

• The Verdict: They didn't find the decay, but they set a very strict "speed limit" on how fast it could be happening. They proved that if it happens, it's incredibly rare (happening less than once every 35 septillion years).
• The Bonus: Even though they didn't find the ghost, they measured a different, more common type of decay (two-neutrino double beta decay) with incredible precision, like a master clockmaker tuning a watch.

2. The Next Generation: CUPID (The Smart Upgrades)

The Problem:
CUORE was great, but it had a flaw. Sometimes, the crystals would get "tricked" by background noise, like alpha particles (tiny radioactive specks) bouncing off the surface of the crystals. It was like trying to hear a whisper in a room where someone is constantly tapping on the walls.

The Solution:
Enter CUPID (CUORE Upgrade with Particle IDentification). This is the "remodeled" version of the library, using the same building (the cryostat) but upgrading the equipment.

The New Tricks:

1. New Crystals: Instead of Tellurium, CUPID will use Lithium Molybdate crystals enriched with a specific isotope of Molybdenum. This isotope has a higher "energy signature," meaning the ghost's signal will appear in a part of the spectrum where there is less background noise (a quieter corner of the library).
2. Two Eyes, Not One: CUORE only measured "heat." CUPID will measure both heat AND light.
   • The Analogy: Imagine a burglar breaking a window. A normal alarm (CUORE) just hears the crash (heat). CUPID has a second sensor that sees the flash of light.
   • Why it matters: Alpha particles (the noise) make very little light when they hit the crystal. Beta particles (the signal) make a lot of light. By looking at the ratio of heat to light, CUPID can instantly tell the difference between a "ghost" and a "burglar," rejecting the noise with high precision.
3. The Amplifier: They use a special trick called the Neganov-Trofimov-Luke effect. Think of this as a microphone that amplifies the faint "light" signal so it's loud enough to be heard clearly over the background hum.

The Future:
CUPID plans to start in 2030. If everything goes according to plan, it will be sensitive enough to detect the "ghost" if it exists within the range of the "Inverted Hierarchy" of neutrino masses. Essentially, it will be the most powerful neutrino detector ever built, capable of seeing a single event in a trillion years.

Summary

• CUORE was the pioneer. It proved we can build massive, ultra-cold detector arrays and operate them for years. It didn't find the neutrinoless decay, but it narrowed the search significantly.
• CUPID is the upgrade. It takes the same building but adds "light sensors" and "noise-canceling" technology to look harder and smarter.

If CUPID finds the decay, it will rewrite the textbooks of physics, explaining why the universe exists. If it doesn't, it will tell us that the "ghost" is even more elusive than we thought, forcing scientists to invent even wilder theories. Either way, we are getting closer to the truth.

Technical Summary

1. Problem Statement

The primary physics goal is the search for neutrinoless double beta decay ($0\nu\beta\beta$). This hypothetical process is crucial because:

• It violates lepton number conservation by two units, indicating physics beyond the Standard Model.
• It would confirm that neutrinos are Majorana particles (their own antiparticles).
• It provides a pathway to determine the absolute mass scale and mass hierarchy of neutrinos.
• It offers potential explanations for the matter-antimatter asymmetry in the universe via leptogenesis.

Despite observing the standard two-neutrino double beta decay ($2\nu\beta\beta$) in 11 nuclides, no confirmed observation of $0\nu\beta\beta$ exists. Current limits on the half-life ($T_{1/2}$) are around $10^{26}$ years. To probe the Inverted Hierarchy (IH) of neutrino masses, experiments must achieve sensitivities in the range of $10^{27}$ years. This requires:

• Ultra-low background rates in the Region of Interest (ROI).
• Excellent energy resolution to distinguish the mono-energetic $0\nu\beta\beta$ peak from the continuous $2\nu\beta\beta$ spectrum.
• Scalability to large target masses (tonne-scale).

2. Methodology

The CUORE Experiment (Current Status)

• Technique: Cryogenic bolometry. CUORE utilizes 988 cubic Tellurium Dioxide ($TeO_2$) crystals operating at $\sim15$ mK.
• Detection Mechanism: Particle interactions generate phonons (heat) in the crystal, causing a minute temperature rise measured by Neutron Transmutation Doped (NTD) Germanium thermistors.
• Target Isotope: $^{130}Te$ (Q-value = 2527.5 keV). No enrichment was used due to high natural abundance (34%).
• Background Mitigation:
  • Located at Gran Sasso National Laboratories (LNGS) with 3600 m water equivalent rock overburden to reduce cosmic rays.
  • Passive shielding against natural radioactivity.
  • Detailed offline analysis using Bayesian fitting to model background components (e.g., $^{60}Co$ sum peaks, $^{214}Bi$, $^{208}Tl$).
• Data Analysis: The experiment accumulates exposure (mass $\times$ time) and applies selection cuts (Base cuts, Anti-Coincidence, Pulse Shape Discrimination) to isolate the ROI around the Q-value.

The CUPID Experiment (Future Upgrade)

• Concept: "CUORE Upgrade with Particle IDentification." It reuses the CUORE cryostat and infrastructure but replaces the detector array.
• Target Isotope: $^{100}Mo$ (Q-value = 3034 keV). The higher Q-value places the signal in a background-free region of the spectrum.
• Detection Mechanism: Scintillating Bolometers.
  • Absorber: Lithium Molybdate ($Li_2MoO_4$) crystals enriched to $\ge95\%$ in $^{100}Mo$.
  • Dual Readout: Simultaneous measurement of Heat (phonons) and Light (scintillation).
  • Light Detection: High-purity Germanium (Ge) wafers coated with SiO, utilizing the Neganov-Trofimov-Luke (NTL) effect. An applied electric field amplifies the light signal by generating additional phonons from drifting electron-hole pairs, improving signal-to-noise ratio and time response.
• Background Rejection Strategy:
  • Alpha/Beta Discrimination: Alpha particles (from surface contamination) produce significantly less light than beta/gamma events due to quenching. Dual readout allows for the rejection of alpha backgrounds.
  • Surface Event Identification: The absence of reflective layers around crystals allows surface events to be identified via anti-coincidence cuts.
• Design: 1596 crystals arranged in 57 towers (14 floors each), totaling 240 kg of $^{100}Mo$.

3. Key Contributions

1. Operational Milestone: CUORE successfully operated a tonne-scale cryogenic detector array for over 7 years, demonstrating the long-term stability and scalability of bolometric technology.
2. Precise $2\nu\beta\beta$ Measurement: CUORE provided the most precise measurement of the $^{130}Te$ two-neutrino double beta decay half-life to date.
3. Background Modeling: The paper details a robust background reconstruction across a broad energy range, establishing a baseline for future experiments.
4. CUPID Design Validation: The paper outlines the finalized design for CUPID, including the transition to NTL-amplified light detectors and the specific mechanical modifications (screwless assembly, laser-cut copper frames) to minimize intrinsic background.
5. Sensitivity Projections: Detailed projections for CUPID's ability to cover the Inverted Hierarchy region of neutrino masses.

4. Results

CUORE Results

• Exposure: Accumulated 2.9 tonne-years of raw $TeO_2$ exposure (2039.0 kg·yr analyzed).
• $0\nu\beta\beta$ Search: No statistically significant evidence of $0\nu\beta\beta$ decay was found.
  • Half-life Limit: $T_{1/2}^{0\nu} > 3.5 \times 10^{25}$ years (90% C.I.).
  • Effective Mass Limit: $m_{\beta\beta} < 70–250$ meV (depending on the nuclear matrix element model).
• $2\nu\beta\beta$ Measurement:
  • $T_{1/2}^{2\nu} = (9.32^{+0.05}_{-0.04} \text{ (stat)} ^{+0.07}_{-0.07} \text{ (syst)}) \times 10^{20}$ years.
• Performance: Achieved an average energy resolution of 7.54 keV FWHM at the 2615 keV calibration peak. The background index (BI) in the ROI is $\sim1.42$ counts/(keV·kg·yr).

CUPID Projections

• Target Sensitivity:
  • Discovery Sensitivity: $1 \times 10^{27}$ years ($3\sigma$) corresponding to $m_{\beta\beta} \approx 12–21$ meV.
  • Exclusion Sensitivity: $1.8 \times 10^{27}$ years (90% C.L.) corresponding to $m_{\beta\beta} \approx 9–15$ meV.
• Background Goal: Target BI of $1.0 \times 10^{-4}$ counts/(keV·kg·yr) in the ROI (a reduction of $\sim10^4$ compared to CUORE).
• Timeline:
  • Stage 1 (Partial): Start ~2030 (1/3 of crystals).
  • Stage 2 (Full): Start ~2034 (Full 240 kg $^{100}Mo$).

5. Significance

• Scientific Impact: CUORE has set one of the world's leading limits on $0\nu\beta\beta$ decay, ruling out certain regions of the parameter space for Majorana neutrino masses. Its precise measurement of the $2\nu\beta\beta$ half-life provides a critical benchmark for nuclear theory.
• Technological Legacy: CUORE proved that tonne-scale cryogenic bolometer arrays are feasible and stable, paving the way for the next generation of experiments.
• Future Physics Reach: CUPID represents a paradigm shift by combining the scalability of CUORE with particle identification (light readout). By targeting $^{100}Mo$ with a higher Q-value and utilizing NTL amplification, CUPID aims to fully cover the Inverted Hierarchy of neutrino masses. If successful, it could either discover $0\nu\beta\beta$ or push the limits into the Normal Hierarchy region, fundamentally altering our understanding of particle physics and cosmology.