The Most Energetic Neutrino Ever Seen Makes a Mediterranean Splash todayheadline

The Most Energetic Neutrino Ever Seen Makes a Mediterranean Splash

By Meghan Bartels edited by Lee Billings

For the fishes swimming deep in a particular patch of the Mediterranean Sea off the coast of Sicily in the wee hours of February 13, 2023, it was a night like any other—at least until a sudden azure shimmer, invisible to human eyes, shot through the dark water. The event signaled something extraordinary: the detection of the most energetic particle of its kind that has been measured to date.

The flash was the calling card of a cosmic neutrino, a tiny and typically mild-mannered “ghost particle,” so named because of its astronomical unlikelihood of interacting with the ordinary matter that makes up our world. A neutrino might pass through a light-year of lead unscathed. And each second some 100 trillion of these particles (most of which have been emitted by our sun) pass through your body. That makes them difficult to catch—but also potent messengers of otherwise-occluded astrophysical processes at work in the opaque hearts of stars and the dust-darkened cores of galaxies.

This one’s discovery and characterization comes from a predominantly European collaboration dubbed KM3NeT, a sprawling neutrino telescope that is still under construction and that, once fully built, will use about a cubic kilometer of instrument-laced Mediterranean seawater as the basis of its two distinct detectors. Yet even in its incomplete state, the project has delivered a stunning result—a neutrino that likely hails from beyond the galaxy and that contains unprecedented power.

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“It’s in a completely unexplored region of energy, 30 times higher than any previous observation of neutrinos,” said Paschal Coyle, a neutrino physicist at the French National Center for Scientific Research and a member of the KM3NeT team, during a press conference about the research that was held on Tuesday.

Of KM3NeT’s two detectors, one is dedicated to more mundane atmospheric neutrinos. The other, dubbed ARCA, is located under nearly 3.5 kilometers of water off the coast of Sicily and is designed to detect astrophysical neutrinos by observing the debris of their rare interactions with water molecules.

“These things have so much energy—they hit so hard—that you get this really enormous spray of particles,” says Kate Scholberg, a physicist at Duke University, who studies neutrinos but was not involved in the new research. “It doesn’t interact much, but when it does, it makes a gigantic, spectacular splash of all kinds of particles spraying everywhere. And it’s the light from those particles that you see.”

When the newly discovered neutrino hit, ARCA was observing with just 21 of its planned 230 detection lines. The neutrino bashed into a water molecule outside the detector, creating a burst of particles, including a high-energy muon—a type of subatomic particle that’s similar to an electron but 200 times heavier. That muon then created its own fragmentary debris, sparking a ripple of telltale pale blue photons dubbed Cherenkov radiation that passed over ARCA’s instruments. By analyzing that light, physicists were able to reconstruct the muon’s submarine path, estimate the energy of the original neutrino and pinpoint its origins to a particular region of space.

The researchers estimate that the neutrino’s energy was on the order of 220 peta electron volts, more than 30 times higher than the most energetic neutrino detected prior to the new observations. To help people conceptualize it during the press conference, Aart Heijboer, a physicist at the Nikhef National Institute for Subatomic Physics in the Netherlands and a co-author of the new research, offered the image of a Ping-Pong ball dropping about one meter in Earth’s gravity. The newly detected neutrino held about that much energy packed into a single subatomic particle, he said. Or one can compare it with the fiercest particle accelerators scientists have built: “This is about 1,000 times more energetic than anything we could produce on Earth,” says Bryan Ramson, a neutrino physicist at the Fermi National Accelerator Laboratory in Illinois, who was not involved in the new research.

The detection is tantalizing, but it also poses more questions than it answers. KM3NeT is joining a long-running neutrino telescope called IceCube that has been gathering data from near the South Pole since 2011. IceCube was designed to catch this sort of high-energy neutrino just as effectively as KM3NeT, but its current record-holding observation carried just one thirtieth of the energy of KM3NeT’s new find, which has raised eyebrows among some experts.

“My first impression is that this is very unexpected. And how can this be possible without IceCube having seen something [similar] before?” says Ignacio Taboada, a physicist at the Georgia Institute of Technology and current spokesperson of the IceCube collaboration.

In addition, the KM3NeT scientists were unable to pin their high-energy neutrino to a particular source. The researchers scanned the small patch of sky from which the neutrino likely came but saw no smoking gun such as a type of active galactic nucleus called a blazar—an immediate suspect for celestial shenanigans that scientists expect could create such a powerful particle. That could mean the neutrino may have instead come from a superspeedy cosmic ray that careened off a photon of extragalactic background light or from the cosmic microwave background, the researchers argue.

“This event is weird; I think that is a good takeaway.”

Such esoteric possibilities make studying astrophysical neutrinos vexingly complicated but are also a major part of why scientists are drawn to study them in the first place. Most astronomical observations remain tethered to photons—yet photons can easily be blocked. In contrast, neutrinos’ ghostly nature means they travel unimpeded in a straight line over vast distances, offering a different lens on the universe that even looks back to its earliest days. With light, “there’s a limit on how far you can look,” Ramson says—namely, the photonic fog of the cosmic microwave background, emitted some 380,000 years after the big bang. “Neutrinos are one way you can sort of pierce that veil and look further back than ever before.”

Whether scientists are on the brink of piercing that veil depends on whether KM3NeT continues to make stunning observations like the 2023 detection and whether IceCube can match it after going so long with no sightings of such high-energy particles. Right now, the apparent discord in the detectors’ sightings is confusing, to say the least. “They might have just gotten lucky; it’s hard to tell,” Scholberg says. “It’s very intriguing, and it clearly means that we need more data.”

Taboada agrees that the detection in hand is tantalizing but that neutrino scientists need more observations to know how to interpret KM3NeT’s catch. “If it were to prove an astrophysical neutrino, that would be monumental,” Taboada says. But he wants to see more. “This event is weird; I think that is a good takeaway,” he says. “It is unexpected, more or less whichever way you look at it.”