NIST-Developed Sensors Aid Breakthrough in Unraveling Neutrino Mass Mystery

Researchers utilizing highly sensitive detectors developed at the National Institute of Standards and Technology (NIST) have made significant strides in determining the elusive mass of the electron neutrino. This fundamental particle, known for its minimal interaction with matter, plays a crucial role in the universe’s formation and structure. Understanding its mass is a long-standing goal in physics, with implications for how elementary particles acquire their mass.

The experiment, dubbed HOLMES and conducted in Italy, employed arrays of miniature sensors known as transition edge sensors (TES), fabricated at NIST. These devices function as exceptionally sensitive thermometers, operating at the precise temperature where thin superconducting films transition between having zero electrical resistance and exhibiting ordinary resistance. Within this critical range, TES can detect minute temperature fluctuations, as small as a few millionths of a degree Kelvin. For the HOLMES experiment, tiny quantities of the radioactive isotope holmium-163 were integrated into a gold film, which was then thermally connected to each TES. Approximately once every second, a holmium-163 nucleus would capture an electron from its orbit, initiating a radioactive decay process that transforms the holmium into dysprosium-163 and releases an electron neutrino. While the neutrino escapes, the energy it carries away is directly related to its mass. The remaining energy, absorbed by the newly formed dysprosium atoms, causes a slight temperature increase. This temperature rise is meticulously measured by the TES arrays.

By analyzing the temperature changes recorded by the TES arrays over a two-month period, scientists were able to quantify the energy retained by the dysprosium atoms across millions of radioactive decay events. This analysis allowed them to establish an upper limit for the mass of the electron neutrino. The findings indicate that the maximum possible mass for an electron neutrino is no greater than 27 electron volts (eV). For context, a standard electron has a mass of 511,000 eV. While another experiment, the KATRIN neutrino experiment in Germany, has set an even lower upper bound, HOLMES serves as a pioneering effort. Future, larger-scale versions of HOLMES operating for extended periods could potentially yield even more precise measurements than KATRIN. The HOLMES collaboration involves scientists from multiple institutions across Switzerland, France, and Italy, including the National Institute of Physics in Milan, the University of Milan-Bicocca, the National Laboratory of Gran Sasso, the University of Genova, and the University of Colorado Boulder. The results of this groundbreaking research were published in the journal Physical Review Letters.

The development and application of these advanced NIST sensors highlight the critical role of precision measurement technologies in fundamental scientific research. The ability of the TES to detect minuscule energy depositions is key to pushing the boundaries of our understanding of particle physics.

Article by Mel Anara, based upon information from the National Institute of Standards and Technology

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