Physicists have placed a new limit on how big the elusive neutrino can be—one of the universe’s smallest known particles—a limit that makes other subatomic particles look as big as black holes by comparison.
In a new result published this week in Science, researchers have put a new upper limit on the mass of this itsy-bitsy particle: no more than 0.45 electron volts (eV). For context, that’s less than one-millionth the mass of an electron, which clocks in at a comparatively gargantuan 511,000 eV. So, yeah—neutrinos are ridiculously lightweight.
Trillions of neutrinos pass through your body every second, but they are so small and so weakly interacting that you don’t feel a thing.
Neutrinos are the only known elementary particles whose mass remains unknown, though questions remain about how well those elementary particles cooperate with the Standard Model. Determining the neutrino’s mass with precision could offer profound insights into the universe’s laws. Are neutrinos getting their mass from the Higgs boson, as other particles do? Or is there some entirely new mechanism at play?
Enter the Karlsruhe Tritium Neutrino Experiment, or KATRIN, a 75-foot-long (23-meter-long) blimp-shaped vacuum chamber. Scientists monitor the radioactive decay of tritium inside the vacuum chamber; as the tritium decays, it spits out electrons and antineutrinos. Researchers can’t measure the antineutrinos directly (they ghost through matter like it’s nothing), but they can (and do) study the leftover electron’s energy to make inferences about the mass of the missing particles.
After analyzing 259 days of data, the KATRIN team was able to cut their previous best estimate for the neutrino’s mass (0.8 eV) nearly in half. But they’re not finished; by the time the full 1,000-day dataset is crunched, the team hopes to push that mass limit down to 0.3 eV, maybe even 0.2 eV.
Neutrinos still have plenty of tricks up their subatomic sleeves. As physicist Susanne Mertens from the Max Planck Institute puts it, the KATRIN Collaboration’s measurement could be a backdoor into new physics—and possibly a better understanding of how the early universe evolved.
In February, a different team detected the most energetic neutrino (also called “ghost particles” for their enigmatic nature) deep in the Mediterranean Sea, indicating that the particles may be emitted by interactions between matter and the cosmic microwave background—the oldest visible light in the universe.
If the neutrino mass were more—around one electronvolt—KATRIN could have found its actual value. But with the particle being so freaking small, a new and improved detector—KATRIN++—may be required to measure its mass with precision.
Few things in life are certain, but among the things we can reliably expect are death, taxes, and the neutrino being smaller than ever.
