In the invisible world of subatomic particles, few are as mysterious—or as difficult to pin down—as the neutrino. These ghostly particles pass through your body by the trillions every second, without leaving a trace. They barely interact with matter, travel at nearly the speed of light, and until recently, their mass remained one of physics’ biggest open questions.
Now, researchers from the Karlsruhe Tritium Neutrino (KATRIN) experiment in Germany have made the most precise measurement of the neutrino’s mass, placing a new upper limit: no more than 0.45 electron volts (eV). For comparison, an electron—the next lightest known particle—weighs 511,000 eV. That means the neutrino is at least a million times lighter than an electron.
The findings, published recently in Science, push the boundaries of what we know about these elusive particles.
“To this day, neutrinos are the only elementary particles whose mass is unknown,” said Susanne Mertens, a physicist at the Max Planck Institute for Nuclear Physics and spokesperson for the KATRIN collaboration in a recent statement. “Measuring it could provide a crucial clue to how the particle acquires its mass in the first place.”
Understanding how neutrinos gain mass could even hint at physics beyond the Standard Model—the framework scientists use to describe fundamental particles and forces. While most particles get their mass via the Higgs boson, it’s still unclear if neutrinos follow the same rule, or if an entirely new mechanism is at play.
The Power of Tritium
To measure a particle that’s practically weightless, scientists at KATRIN (short for Karlsruhe Tritium Neutrino) used a clever approach. They studied the decay of tritium, a rare isotope of hydrogen. When tritium decays, it emits an electron and an antineutrino. Although the antineutrino escapes undetected, the energy of the emitted electron gives physicists a window into its mass.
These electrons are funneled into a 23-meter-long, airship-shaped detector in a vacuum, where their energy is measured with incredible precision. It’s a straightforward idea, but building a machine capable of such sensitivity was anything but simple.
“The concept is simple on paper, but in practice the team had to overcome many technical difficulties to ‘tame’ their apparatus,” explained Loredana Gastaldo, a physicist at Heidelberg University.
After analyzing 259 days of data, the KATRIN team has now halved their previous upper estimate of 0.8 eV, made in 2022. But their work isn’t done.
The Neutrino Finish Line—and Beyond
KATRIN is expected to wrap up data collection later this year. With a full 1,000 days’ worth of data, the team hopes to lower the upper limit on neutrino mass to 0.3 eV or even 0.2 eV.
“If the particle’s mass had been closer to 1 eV, an experiment such as KATRIN might have pinned down the actual value,” said Alexey Lokhov, a physicist at the Karlsruhe Institute of Technology. “Instead, more-sensitive experiments will be required.”
One such possibility is KATRIN++, a future upgrade that could further push the limits of precision and sensitivity.
Why Neutrinos Matter
Neutrinos might be tiny, but their impact on the universe is profound. They were created in massive numbers during the Big Bang and could hold secrets about how galaxies formed and evolved.
“Knowing their mass could also shed light on how neutrinos originating from the Big Bang have influenced the formation of galaxies,” Mertens added.
Kenna Hughes-Castleberry is a freelance science journalist and staff writer at The Debrief. Follow and connect with her on BlueSky or contact her via email at kenna@thedebrief.org