A cosmic neutrino of unknown origins smashes energy records

A neutrino from space recently plunged into the Mediterranean Sea with an energy that blows all other known neutrinos out of the water.

Packing a punch of some 220 million billion electron volts, this particle was around 20 times as energetic as the highest energy cosmic neutrinos seen before, researchers report in the Feb. 13 Nature. The particle was glimpsed by the partially built Cubic Kilometre Neutrino Telescope, or KM3NeT.

“They hit the jackpot,” says Francis Halzen, a physicist at the University of Wisconsin–Madison and principal investigator of the IceCube Neutrino Observatory in Antarctica. “We have been taking data with a much bigger detector for 10 years. We’ve never seen such an event.”

Physicists are keen to catalog cosmic neutrinos because these lightweight, electrically neutral particles can cross vast stretches of space nearly undisturbed. The most energetic ones could offer unmatched insights into the powerful phenomena that spit them out, such as supermassive black holes. But netting particles that barely interact with matter requires giant telescopes made of sensors encased in ice, like IceCube, or submerged in water, like KM3NeT.

KM3NeT’s two neutrino detectors — one off the coast of Sicily, the other near southern France — are still under construction but already collecting data. Both contain cables hundreds of meters tall, which are strung with bundles of light sensors anchored to the seafloor.

When cosmic neutrinos interact with matter in or near a KM3NeT detector, they spawn charged particles such as muons. As those muons careen through water, they give off feeble flashes of bluish light that KM3NeT’s sensors can pick up. Clocking when different sensors spot this light can reveal a particle’s path; the brightness of the blue hue reflects the particle’s energy.

On February 13, 2023, the detector near Sicily was run through by an extremely energetic muon traveling nearly parallel to the horizon. At the time, only 21 of the planned 230 sensor cables were in place. Based on the muon’s energy and trajectory, KM3NeT scientists determined it must have been spawned by a neutrino from space rather than a particle from the atmosphere.

Simulations suggest the neutrino’s energy was around 220 petaelectron volts. The previous record holder boasted around 10 petaelectron volts.

“It’s a kind of shocking situation,” says KM3NeT team member Luigi Antonio Fusco, a physicist at the University of Salerno in Fisciano, Italy. It’s like neutrino physicists have only ever seen fires fueled by a few sticks of kindling, “and then someone comes with a flamethrower.” The KM3NeT researchers estimate that they expect to see a neutrino of this caliber once every 70 years or so.

“I definitely went in kind of skeptically,” says Erik Blaufuss, a physicist at the University of Maryland in College Park who wrote a commentary on the study in the same issue of Nature. “But they make a pretty convincing case in the paper that it’s real.”

To trace the neutrino’s origins, the KM3NeT team scoured data from gamma ray, X-ray and radio telescopes. Twelve objects stood out in the region of the sky from whence the neutrino came. “Most of them are active galactic nuclei,” Fusco says — bright cores of galaxies where supermassive black holes are guzzling gas and dust. “The problem is that there are so many,” he says. “You cannot really pinpoint a single one.”

Another possibility is that this is the first observed cosmogenic neutrino, created when ultrahigh energy cosmic rays mingle with photons from the cosmic microwave background, the afterglow of the Big Bang.

“At this point, it’s very difficult to make conclusions about the origins,” says Kohta Murase, a theoretical physicist at Penn State not involved in the research. “It’s dangerous to rely on one event.”

The expansion of KM3NeT should improve its ability to nab neutrinos and pinpoint their origins. Other neutrino telescopes are also in the works, such as a planned expansion of IceCube, a proposed observatory off Canada’s Vancouver Island and one under construction in the South China Sea. Those tools, Murase says, may help researchers home in on the birthplaces of neutrinos with staggeringly high energies.