With this achievement, the HOLMES experiment layed the foundation for an exciting new era in precision neutrino physics. It has set the strongest current upper limit on the effective electron neutrino mass. The experiment employs a state-of-the-art, multiplexed array of 64 transition-edge sensor (TES) microcalorimeters. It functions at a rather extreme temperature of ∼ 95mK within a helium dilution refrigerator. This groundbreaking work will have direct and important applications to continuing research in both the field of particle physics and neutrinos.
The relatively rare nature of this decay makes this experiment the most sensitive measure to date of the electron capture decay of holmium-163 (¹⁶³Ho). This radioactive isotope would be the perfect candidate for analysis, with a low Q-value of ~2,863 eV. With a half-life just over 4,750 years, holmium-163 offers a steady source for exacting measurements. With their careful, ingenious experiments, the HOLMES team truly opened a new door to the field by providing an upper bound on the effective electron neutrino mass. They set it to be lower than 27 eV/c² at 90% credible limit.
Technical Innovations in Detector Design
Our HOLMES experiment uses a close-packed array of 64 TES microcalorimeters, which are individually ion-implanted with holmium-163. The resulting array dimensions of ~ (20×10) mm² enable a high resolution measurement capability. These microcalorimeters are central to measuring small energy fluctuations underlining the electron capture decay process.
Passing through such extremely low temperatures is essential to the detectors’ spectacular performance. The apparatus operates housed within a helium dilution refrigerator where we cool our apparatus to base temperature of 95 mK. Unlike a conventional room, this environment greatly suppresses thermal noise. It further improves the sensitivity of the detectors, a key element to achieve precise measurements of the neutrino mass.
Aside from their innovative design, the advanced detectors reached a spectacular average energy resolution of 6 eV. This outstanding level of resolution is absolutely critical for discerning the small energy changes associated with neutrino interactions. It further contributes to minimizing background noise, increasing the reliability of results we achieve.
Microwave Multiplexed Readout System
One of the key elements of the HOLMES experiment is its highly scalable microwave multiplexed readout system. This unique laser timing system enables the correlated simultaneous readout of 512 detectors in parallel. It debugs their signals, then encodes those signals at different frequencies within the 4–8 GHz range. This multiplexing capability will be key to future experiments, which will likely need thousands of these detectors to improve precision of measurements.
In addition to protecting sensitive data, this readout system greatly simplifies the data collection process. It demonstrates the scalability requirement for next-generation neutrino mass experiments. The HOLMES experiment confirms a decades-old experimental dream. This impressive development establishes an excellent precedent for future research to better understand the nature of neutrinos.
Data Analysis and Results
At least as impressive as the experimental scope was the breadth of analytical methods employed in the HOLMES experiment. To get a proper read on the data, the team used a Bayesian parameter estimation approach based on Poisson likelihood. This new statistical approach gives researchers the ability to make more refined estimates about the mass of neutrinos and better confidence levels in their conclusions.
All in all, the results from this experimental produced essential experimentation which confirmation neutrinos’ illusive nature and presence of neutrino mass. Researchers have realized the tightest upper limit on the effective electron neutrino mass with calorimetric method. This innovative effort lays the foundation for much more in-depth modeling of essential particle physics.