Neutrinos are the most abundant massive particles in the Universe. Remnants of the Big Bang or the burning core of a star, they are also produced in certain radioactive decays, such as that of tritium, an unstable isotope of hydrogen.
On a cosmological scale, these featherweights of the Universe play a crucial role in the distribution of galaxies. In the realm of the infinitely small, the origin of the mass of neutrinos is not yet theoretically understood and could therefore be a key element for unveiling a new physics beyond the standard model. Determining the infinitesimally small mass of neutrinos has thus been a major concern of particle physics, astrophysics and cosmology for decades.
The international KATRIN experiment, located at the Karlsruhe Institute of Technology (KIT), has taken up this challenge. It uses the beta decay of tritium to measure the mass of the neutrino, via the energy distribution of the electrons released during the decay process. This precision measurement requires a considerable technological effort: the 70-meter-long experiment house the most intense source of tritium in any fundamental research facility, as well as a giant spectrometer to measure the energy of the decaying electrons with unprecedented accuracy. Following the first results published in 2019, the quality of the data has been continuously improved.
The results are unequivocal: to date, no trace of a neutrino mass has been detectable in the collected data. Consequently, a new upper limit on neutrino mass has been established at 0.8 eV. This is the first time ever that a direct experiment has gone below the electronvolt threshold, an essential range for both particle physics and for understanding the structure of our cosmos.
The CEA has played a major role in the analysis and interpretation of the data. Indeed, an Irfu researcher designed and implemented the global structure of the analysis, which includes many steps to check the robustness of the results. In addition, the Irfu conducted one of the three independent analyses that led to the new 0.8 eV/c2 limit, a world first in neutrino physics.
The neutrino mass measurements will continue until the end of 2024.
Additional improvements to further reduce the background noise will also be implemented. After the neutrino mass measurement, a new detection system (TRISTAN) will be installed in KATRIN to begin tracking "sterile" neutrinos with a mass in the keV range – a candidate for dark matter in the Universe.
Click here to read the press release (in French).