Neutrino Mass Mystery Shrinks with Latest KATRIN Results

By Gayoung Lee edited by Lee Billings

The neutrino is a notorious troublemaker in the world of particle physics. This tiny, elusive particle with no electric charge likely permeates every corner of the universe, but you’d be hard-pressed to know that without extremely specialized instruments. Trillions pass through you every second, in fact, all without interacting with a single atom of your body. That is but one of the reasons why, for something so supposedly abundant and fundamental, we know painfully little about the neutrino—not even something so basic as its mass.

But neutrino physics might be on the verge of an experimental breakthrough: physicists with the Karlsruhe Tritium Neutrino (KATRIN) experiment in Germany have succeeded in measuring the upper limit of the neutrino’s mass to a mere 0.45 electron volts (eV), which is less than one millionth of the mass of an electron. These results, published last week in Science, represent just a fraction of KATRIN’s investigations; about three quarters of the detector’s planned data haul from its ongoing 1,000-day campaign remains to be analyzed and revealed.

Another reason for excitement is that KATRIN has achieved a twofold increase in sensitivity from just last year, when some researchers raised questions as to whether the experiment would even be able to make progress on physicists’ decades-long quest to gauge the neutrino’s mass. And the KATRIN team intends to push the detector even further, says Alexey Lokhov, a co-author of the new study and an experimental physicist at Karlsruhe Institute of Technology in Germany. By the conclusion of KATRIN’s campaign, he says, the detector’s sensitivity is targeted for a lower-end neutrino mass of 0.3 eV, another significant boost.

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With their eyes on that prize, for this particular round of data analysis, Lokhov and his colleagues, including co-author Christoph Wiesinger, performed several technical overhauls to significantly improve the instrument’s capabilities. “By the end of this year, we’ll have this new, really big chunk of data to look at,” says Wiesinger, a physicist at Technical University of Munich, Germany. “Now KATRIN is in a more stable, near-final configuration, [so] I’m very confident we’ll manage this [sensitivity boost] in upcoming years.”

The KATRIN experiment began operations in 2019. It seeks to constrain the neutrino’s mass by looking at the energy spectrum of electrons and electron antineutrinos emitted by decaying tritium, a radioactive isotope of hydrogen. As detailed in the new paper, during the experiment’s first 259 days, KATRIN performed energy measurements of about 36 million electrons. From the energy spectrum of these electrons, physicists were able to infer the mass of the neutrino by identifying what would appear to be a “distortion” in the energy spectrum of electrons, Lokhov explains. “The trick is that to produce a neutrino in this decay process, one needs to at least produce a mass that a small, nonzero mass [that] would influence how much energy [would be] left for the electrons.” And that “leftover” energy, he says, would hint at the presence of something else—the neutrino—present in the decay process.

To be clear, KATRIN still hasn’t locked in on an absolute value for neutrino mass—nor is it supposed to. But that may be more the product of the neutrino’s innate weirdness rather than any representation of KATRIN’s shortcomings. The neutrino’s mass is particularly elusive quarry because it stubbornly refuses to abide by the tenets of the Standard Model of particle physics. Famously, almost all of this theory’s predictions have been experimentally confirmed, yet some of its forecasts for the neutrino have notoriously fallen flat. The model predicts that neutrinos should be completely massless, but this was ultimately refuted by a Nobel-winning experiment that showed neutrinos not only have mass but also, for whatever reason, change mass by oscillating between three different neutrino varieties, or “flavors.”

“You know, when everything is settled and we are all happy, [neutrinos] are like that one person in the room saying, no, not quite,” muses Carlos Argüelles Delgado, a physicist at Harvard University, who is unaffiliated with KATRIN.

But the neutrino’s troublesome nature is precisely why physicists are so enamored with it; the tiny particle, theorists say, must be a rebel with a cause, with some deeper and more fundamental explanation for its quirks that could open vast new realms of physics beyond the Standard Model’s increasingly bland confines. And results like KATRIN’s are part of a steady flow of theoretical and experimental advances bringing us incrementally closer to those long-awaited breakthroughs.

“There’s a tricky business here because the neutrinos are superpositions of mass states—they have three—and what [KATRIN] shows is that this mass combination can be no larger than 0.45 eV,” Argüelles Delgado says. Now, with the experiment’s lengthy campaign set to conclude by the end of this year, the clock is ticking towards a final countdown. Time is running out to further boost its sensitivity and tighten its snare around this slippery subatomic subversive. “If the true mass of the neutrinos is within the sensitivity range of KATRIN, then KATRIN should be able to measure it,” explains Georgia Karagiorgi, a particle physicist at Columbia University, who is not part of the research team.

That said, KATRIN is probably going to be the last of its kind, Argüelles Delgado says, noting the diminishing returns associated with scaling up such experiments. Major investments to make bigger, longer-running experiments risk only delivering marginal advances—which is all the more reason why KATRIN’s sunk costs and ongoing success now call for urgency. “Given KATRIN’s projections, it’s clear that additional data will help get it to [the researchers’] target sensitivity, so they absolutely need to run it now,” Karagiorgi says.

If or when KATRIN achieves its intended higher sensitivity, this particular experiment will end—but the hope is that future instruments will be able to continue its mission by taking heed of what physicists have learned from KATRIN. Despite its end, the data will be a treasure trove that physicists will mine for discoveries for many years to come. After all, the greater quest to measure the neutrino’s mass is unquestionably a marathon.

“Neutrinos are so elusive that, well, you need either these big detectors or very, very sophisticated technologies,” Wiesinger says. “But even though that the [neutrino’s] mass is so small, we expect today that just by there being so many [of them], they have a large influence on the cosmos—how structures are forming and how they evolve.”

It’s fascinating to realize that such an infinitesimal and rebellious particle can have such profound effects on both subatomic and cosmic scales—and that it can be robustly studied in earthbound laboratories at all.

“Neutrinos are one-of-a-kind portals to new discoveries in physics—they were always like this from the beginning, when they were first postulated,” Lokhov says. “And even now, they’re still bringing some new, exciting discoveries that [further] our understanding of nature.”