糖心视频

In a , the HOLMES collaboration has achieved the most stringent upper bound on the effective electron neutrino mass ever obtained using a calorimetric approach, setting a limit of less than 27 eV/c虏 at 90% credibility.

This result validates a decades-old experimental vision and demonstrates the scalability needed for next-generation neutrino mass experiments.

While oscillation experiments have measured the differences between neutrino mass states, the actual individual mass values鈥攖he absolute neutrino mass scale鈥攔emain unknown. Pinning down these values would help complete our understanding of the Standard Model of particle physics.

Direct kinematic measurements, which rely solely on energy and momentum conservation in nuclear beta decays, provide the most model-independent approach to this fundamental question. The HOLMES experiment employs low-temperature microcalorimetry to measure the electron capture decay of holmium-163 (鹿鈦堵矵o), a technique first proposed over 40 years ago.

糖心视频 spoke to Angelo Nucciotti, Professor in the Department of 糖心视频ics at the Universit脿 di Milano-Bicocca and spokesperson for the international HOLMES experiment.

"My passion for this work began during my Ph.D. in the 1990s, when I was introduced to the world of thermal detectors by Professor Ettore Fiorini," said Nucciotti.

"Fiorini was a true pioneer who first proposed using these detectors for rare events, including the measurement of neutrino mass. Now, after over 30 years of effort, the publication of this result on holmium-163 proves the long-term viability of that original vision."

How the calorimetric approach works

Microcalorimeters are microscopic devices that measure energy by detecting the tiny temperature changes that occur when particles are absorbed.

The HOLMES experiment uses an array of 64 transition-edge sensor (TES) microcalorimeters operated at approximately 95 mK in a 鲁He/鈦碒e dilution refrigerator. The 鹿鈦堵矵o nuclei are ion-implanted directly into the gold absorbers of these ultrasensitive superconducting detectors.

When holmium-163 undergoes electron capture decay, the energy released鈥攅xcept for the portion carried away by the neutrino鈥攊s absorbed by the gold layer. Because the absorber's heat capacity is minuscule at millikelvin temperatures, even the small energy from a single decay produces a measurable temperature spike.

"The fundamental principle is simple: the energy released in the holmium decay hits the gold absorber, causing its temperature to increase," explained Nucciotti. "The TES thermometer, kept precisely within its superconducting transition, measures this temperature rise as a sharp change in electrical resistance and current, which is proportional to the energy released."

The signature of neutrino mass appears as a corresponding reduction in the maximum energy detected, which is the endpoint of the decay spectrum. By precisely measuring this upper end of the spectrum, researchers can determine the neutrino's mass.

Holmium-163 is ideal for this measurement thanks to its low Q-value of roughly 2,863 eV. Since the total energy available in the decay (Q-value) is less, the neutrino mass signature becomes more prominent in the spectrum. Additionally, its half-life of approximately 4,750 years yields higher specific activity than other candidates, making it more suitable for use in microcalorimeters.

Enabling precision

One of the innovations enabling the success of this experiment is the scalable microwave multiplexed readout system. The 64 detectors encode their signals at different frequencies in the 4鈥�8 GHz range, allowing them to share read-out infrastructure. This is like multiple radio stations broadcasting simultaneously on separate channels.

This frequency multiplexing is essential for future experiments requiring thousands of detectors.

The detectors achieved an average energy resolution of 6 eV鈥攑recise enough to resolve the detailed spectral features near the decay endpoint where neutrino mass signatures would appear. This level of precision, limited primarily by intrinsic detector noise, demonstrates the feasibility of the calorimetric approach for measuring neutrinos' mass.

Producing and preparing the 鹿鈦堵矵o source presented its own challenges. The isotope doesn't exist in nature and must be produced in a nuclear reactor, generating a complex mix of radioactive byproducts, particularly the problematic 鹿鈦垛伓岬怘o contaminant.

"First, a specialized chemical purification is performed using resin-based techniques to separate holmium from highly radioactive isotopes of other rare earth elements," explained Nucciotti.

"Then, a custom-built implantation system selects the correct isotope鈥斅光伓鲁Ho鈥攂ased on its mass and embeds it precisely into the detector absorbers. This machine had to operate with minimal losses, as 鹿鈦堵矵o is extremely scarce and valuable."

Setting an upper limit

"Recording 70 million decay events from 鹿鈦堵矵o in just two months was the culmination of over a decade of technical development," said Nucciotti. "While the final measurement may appear straightforward, every component of the experiment had to be pushed to its limit."

To analyze the data, the researchers employed Bayesian parameter estimation using a Poisson likelihood, with the spectrum analyzed in the region of interest between 2,250 and 3,500 eV. This method finds the most likely values for unknown parameters by comparing predicted and observed data patterns.

The team found the electron neutrino's mass must be under 27 eV/c虏 at 90% credibility. In statistical terms, this means the researchers can be 90% certain this represents the actual upper limit on the mass.

Monte Carlo simulations validated the analysis approach, demonstrating that potential systematic effects remain negligible compared to current statistical uncertainties.

"This result sets the most stringent bound ever achieved using a truly scalable calorimetric approach鈥攁 milestone that definitively validates the viability and maturity of holmium-based technology for future experiments," said Nucciotti.

"Currently, the best limit comes from KATRIN (鈮� 0.45 eV/c虏), which studies the electron antineutrino. However, KATRIN is reaching its technical limit."

The experiments differ fundamentally: KATRIN measures the electron antineutrino by analyzing tritium beta decay, while HOLMES determines the electron neutrino mass by studying electron capture decay.

Comparing these independent results provides a critical test of CPT (Charge-Parity-Time) symmetry, which requires neutrino and antineutrino masses to be identical. Any discrepancy would indicate physics beyond the Standard Model.

What the future holds

The significance extends beyond this result. The demonstrated scalability of the technology provides a clear path toward the ultimate goal: sub-eV sensitivity.

"This achievement marks the beginning of a new phase for HOLMES, focused on massive scaling to reach the ultimate target sensitivity in the sub-eV range," said Nucciotti.

"Our approach, based on holmium-163 calorimetry, offers ample perspectives of growth and scalability that, in the long term, promise to push sensitivity beyond the current state of the art."

Future progress will leverage improved detector fabrication and signal readout techniques. As sensitivity improves, systematic uncertainties related to the environment of the holmium atoms inside detectors will require dedicated investigation.

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