Researchers from the Karlsruhe Tritium Neutrino experiment (KATRIN) have announced the most precise upper limit yet on the neutrino’s mass. Thanks to new data and upgraded techniques, the new limit – 0.45 electron volts (eV) at 90% confidence – is half that of the previous tightest constraint, and marks a step toward answering one of particle physics’ longest-standing questions.
Neutrinos are ghostlike particles that barely interact with matter, slipping through the universe almost unnoticed. They come in three types, or flavours: electron, muon, and tau. For decades, physicists assumed all three were massless, but that changed in the late 1990s when experiments revealed that neutrinos can oscillate between flavours as they travel. This flavour-shifting behaviour is only possible if neutrinos have mass.
Although neutrino oscillation experiments confirmed that neutrinos have mass, and showed that the masses of the three flavours are different, they did not divulge the actual scale of these masses. Doing so requires an entirely different approach.
Looking for clues in electrons
In KATRIN’s case, that means focusing on a process called tritium beta decay, where a tritium nucleus (a proton and two neutrons) decays into a helium-3 nucleus (two protons and one neutron) by releasing an electron and an electron antineutrino. Due to energy conservation, the total energy from the decay is shared between the electron and the antineutrino. The neutrino’s mass determines the balance of the split.
“If the neutrino has even a tiny mass, it slightly lowers the energy that the electron can carry away,” explains Christoph Wiesinger, a physicist at the Technical University of Munich, Germany and a member of the KATRIN collaboration. “By measuring that [electron] spectrum with extreme precision, we can infer how heavy the neutrino is.”
Because the subtle effects of neutrino mass are most visible in decays where the neutrino carries away very little energy (most of it bound up in mass), KATRIN concentrates on measuring electrons that have taken the lion’s share. From these measurements, physicists can calculate neutrino mass without having to detect these notoriously weakly-interacting particles directly.
Improvements over previous results
The new neutrino mass limit is based on data taken between 2019 and 2021, with 259 days of operations yielding over 36 million electron measurements. “That’s six times more than the previous result,” Wiesinger says.
Other improvements include better temperature control in the tritium source and a new calibration method using a monoenergetic krypton source. “We were able to reduce background noise rates by a factor of two, which really helped the precision,” he adds.
At 0.45 eV, the new limit means the neutrino is at least a million times lighter than the electron. “This is a fundamental number,” Wiesinger says. “It tells us that neutrinos are the lightest known massive particles in the universe, and maybe that their mass has origins beyond the Standard Model.”
Despite the new tighter limit, however, definitive answers about the neutrino’s mass are still some ways off. “Neutrino oscillation experiments tell us that the lower bound on the neutrino mass is about 0.05 eV,” says Patrick Huber, a theoretical physicist at Virginia Tech, US, who was not involved in the experiment. “That’s still about 10 times smaller than the new KATRIN limit… For now, this result fits comfortably within what we expect from a Standard Model that includes neutrino mass.”
Model independence
Though Huber emphasizes that there are “no surprises” in the latest measurement, KATRIN has a key advantage over its rivals. Unlike cosmological methods, which infer neutrino mass based on how it affects the structure and evolution of the universe, KATRIN’s direct measurement is model-independent, relying only on energy and momentum conservation. “That makes it very powerful,” Wiesinger argues. “If another experiment sees a measurement in the future, it will be interesting to check if the observation matches something as clean as ours.”
KATRIN’s own measurements are ongoing, with the collaboration aiming for 1000 days of operations by the end of 2025 and a final sensitivity approaching 0.3 eV. Beyond that, the plan is to repurpose the instrument to search for sterile neutrinos – hypothetical heavier particles that don’t interact via the weak force and could be candidates for dark matter.
“We’re testing things like atomic tritium sources and ultra-precise energy detectors,” Wiesinger says. “There are exciting ideas, but it’s not yet clear what the next-generation experiment after KATRIN will look like.”
The research appears in Science.
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