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The heart of the HOLMES experiment, housed in its gold-plated copper box: an array of 64 TES microcalorimeters, each ion-implanted with \u00b9\u2076\u00b3Ho, alongside the microwave multiplexed readout system. The imaged area spans approximately 2 by 3 cm\u00b2. Credit: Marco Faverzani.<\/p>\n
In a Physical Review Letters study<\/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\u00b2 at 90% credibility.<\/p>\n This result validates a decades-old experimental vision and demonstrates the scalability needed for next-generation neutrino mass experiments.<\/p>\n While oscillation experiments have measured the differences between neutrino mass states, the actual individual mass values\u2014the absolute neutrino mass scale\u2014remain unknown. Pinning down these values would help complete our understanding of the Standard Model of particle physics.<\/p>\n 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 (\u00b9\u2076\u00b3Ho), a technique first proposed over 40 years ago.<\/p>\n Phys.org spoke to Angelo Nucciotti, Professor in the Department of Physics at the Universit\u00e0 di Milano-Bicocca and spokesperson for the international HOLMES experiment.<\/p>\n “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.<\/p>\n “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.”<\/p>\n \t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\tHow the calorimetric approach works<\/p>\n Microcalorimeters are microscopic devices that measure energy by detecting the tiny temperature changes that occur when particles are absorbed.<\/p>\n The HOLMES experiment uses an array of 64 transition-edge sensor (TES) microcalorimeters operated at approximately 95 mK in a \u00b3He\/\u2074He dilution refrigerator. The \u00b9\u2076\u00b3Ho nuclei are ion-implanted directly into the gold absorbers of these ultrasensitive superconducting detectors.<\/p>\n When holmium-163 undergoes electron capture decay, the energy released\u2014except for the portion carried away by the neutrino\u2014is 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.<\/p>\n “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.”<\/p>\n 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.<\/p>\n 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.<\/p>\n Left: copper box containing the 64 TES array in the middle. The two chips on either sides of the array are the bias network and the microwave multiplexer (\ud835\udf07MUX), respectively. The array dimensions are approximately (20\u00d710)\u2009\u2009mm2. The multiplexer has the feedline aligned with the coaxial connectors used for feeding the readout tones. For readout, two connectors on one side are connected via a short semirigid coaxial cable. Right: schematic, not to scale, representation of the HOLMES TES microcalorimeter used in the experiment. Credit: Physical Review Letters (2025). DOI: 10.1103\/s9vl-7n24<\/p>\n \n Discover the latest in science, tech, and space with over 100,000 subscribers<\/strong> who rely on Phys.org for daily insights. \t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\tEnabling precision<\/p>\n 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\u20138 GHz range, allowing them to share read-out infrastructure. This is like multiple radio stations broadcasting simultaneously on separate channels.<\/p>\n This frequency multiplexing is essential for future experiments requiring thousands of detectors.<\/p>\n The detectors achieved an average energy resolution of 6 eV\u2014precise 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.<\/p>\n Producing and preparing the \u00b9\u2076\u00b3Ho 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 \u00b9\u2076\u2076\u1d50Ho contaminant.<\/p>\n “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.<\/p>\n “Then, a custom-built implantation system selects the correct isotope\u2014\u00b9\u2076\u00b3Ho\u2014based on its mass and embeds it precisely into the detector absorbers. This machine had to operate with minimal losses, as \u00b9\u2076\u00b3Ho is extremely scarce and valuable.”<\/p>\n \t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\tSetting an upper limit<\/p>\n “Recording 70 million decay events from \u00b9\u2076\u00b3Ho 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.”<\/p>\n 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.<\/p>\n The team found the electron neutrino’s mass must be under 27 eV\/c\u00b2 at 90% credibility. In statistical terms, this means the researchers can be 90% certain this represents the actual upper limit on the mass.<\/p>\n Monte Carlo simulations validated the analysis approach, demonstrating that potential systematic effects remain negligible compared to current statistical uncertainties.<\/p>\n “This result sets the most stringent bound ever achieved using a truly scalable calorimetric approach\u2014a milestone that definitively validates the viability and maturity of holmium-based technology for future experiments,” said Nucciotti.<\/p>\n “Currently, the best limit comes from KATRIN (\u2248 0.45 eV\/c\u00b2), which studies the electron antineutrino. However, KATRIN is reaching its technical limit.”<\/p>\n 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.<\/p>\n 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.<\/p>\n \t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\tWhat the future holds<\/p>\n The significance extends beyond this result. The demonstrated scalability of the technology provides a clear path toward the ultimate goal: sub-eV sensitivity.<\/p>\n “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.<\/p>\n “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.”<\/p>\n 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.<\/p>\n \n Written for you by our author Tejasri Gururaj<\/a>, edited by Sadie Harley<\/a>, and fact-checked and reviewed by Robert Egan<\/a>\u2014this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive.![\"Calorimetric](\"https:\/\/www.europesays.com\/us\/wp-content\/uploads\/2025\/10\/calorimetric-experimen.jpg\" "\\"Left:")
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