The result, published recently in the journal Science, does not define the exact mass of a neutrino, just its upper limit. But the finding helps bring physicists closer to figuring out just what is wrong with the so-called Standard Model, their best — albeit incomplete — theory of the laws that rule the subatomic realm. One way physicists know it is not accurate is that it suggests that the neutrino should not have any mass at all.

Learning more about neutrinos will also help cosmologists fill in their ever-hazy picture of the universe, including how galaxies clustered together and what influences the expansion of the cosmos.

“We’re looking at trying to understand why we are here,” said John Wilkerson, a physicist at the University of North Carolina, Chapel Hill, US and an author of the new study. “That’s something neutrinos may have a key role in.”

Physicists know a few things about neutrinos. They are prolific across the cosmos, created virtually anytime atomic nuclei snap together or rip apart. But they carry no electric charge and are notoriously difficult to detect.

Neutrinos also come in three types, which physicists describe as flavours. And, oddly, they morph from one flavour to another as they move through space and time, a discovery recognised by the Nobel Prize in physics in 2015. The underlying mechanism that makes these transformations possible, physicists realised, meant that neutrinos must have some mass.

But only just so. Neutrinos are mind-boggingly light, and physicists don’t know why. Uncovering the exact mass of neutrinos could lead to “some kind of portal” to new physics, said Alexey Lokhov, a scientist at the Karlsruhe Institute of Technology in Germany. “This is, for now, the world’s best limit,” he said of his team’s measurement.

Lokhov and his colleagues used the Karlsruhe Tritium Neutrino, or Katrin, experiment to narrow down the mass of a neutrino. At one end of the 70-metre-long apparatus was a source of tritium, a heavier version of hydrogen with two neutrons in its nucleus. Because tritium is unstable, it decays into helium: one neutron converts into a proton, which spits out an electron in the process. It also spits out an antineutrino, the antimatter twin of a neutrino. The two should have identical mass.

The mass of the original tritium is split among the products of the decay: the helium, electron and antineutrino. Neither neutrinos nor antineutrinos can be directly detected, but a sensor at the other end of the experiment recorded 36 million electrons shed by the decaying tritium over 259 days. By measuring the energy of the electron’s motion, they could indirectly deduce the maximum mass possible for the antineutrino.

They found that value to be no more than 0.45 electron volts, a million times lighter than an electron. This was measured for only one flavour of neutrino. But Wilkerson said that makes it possible to calculate the others. The latest measurement pushes the possible mass of the neutrino lower than the previous limit set in 2022 by the Katrin collaboration, of no more than 0.8 electron volts. It is also nearly twice as precise.

Elise Novitski, a physicist at the University of Washington, US, who was not involved in the work, commended the effort. The Katrin team is working on an even tighter boundary on the neutrino mass from 1,000 days of data, which it expects to collect by the end of the year. That will give the physicists more electrons to measure, leading to a more precise measurement.

Other experiments will also contribute to a better understanding of the neutrino’s mass, including Project 8 in Seattle, US, and the Deep Underground Neutrino Experiment in the US Midwest.

Astronomers studying the structure of the cosmos, thought to be influenced by the vast collection of neutrinos flooding the universe, have their own measurement of the particles’ maximum mass. But according to Wilkerson, the boundaries set by astronomers staring out into the void don’t match up with what particle physicists calculate in the lab.

“There’s something really interesting going on,” he said. “And the likely solution to that is going to be physics beyond the Standard Model.”

NYTNS