Underwater detector spots the most energetic neutrino yet

On Feb. 13, 2023, something extraordinary happened deep beneath the Mediterranean Sea. KM3NeT’s Astroparticle Research with Cosmics in the Abyss (ARCA) telescope, a sprawling underwater array of ultra-sensitive photodetectors, caught sight of the telltale sign of an incredibly rare cosmic messenger: a highly energetic fundamental particle known as a neutrino. With an estimated energy of 220 peta-electron volts (PeV), it is the most energetic neutrino ever detected. The discovery was published Feb. 12 in Nature.

Scientists didn’t immediately realize its significance. It was just another data point in a sea of millions. But as they sifted through months of observations afterward, something about this one stood out.

“I first realized how spectacular it was when I looked at our event display,” says Paschal Coyle, a researcher at the French National Centre for Scientific Research (CNRS) and a spokesperson for KM3NeT at the time of the detection. “It had so many more photons (light particles) than anything we had ever seen.” In fact, Coyle’s program crashed when he attempted to analyze the unusually high number of photons. “But that’s more because my programming isn’t very good,” he says, chuckling.

Catching ghost particles

Neutrinos — sometimes called ghost particles — are famously elusive. They have no electric charge, barely any mass, and can pass through entire planets without interacting. KM3NeT (short for the Cubic Kilometer Neutrino Telescope) is a detector spanning a cubic kilometer (0.24 cubic mile) anchored to the seafloor 2.2 miles (3.5 km) below the surface, off the southeastern coast of Sicily.

The setup is ingenious. Strings of glass spheres, each packed with highly sensitive photodetectors, await the faintest flash of blue light, called Cherenkov radiation. This blue glow is emitted when a high-energy particle, created by a neutrino interacting with a particle in the ocean, zips through the water faster than the speed of light in water (which is slower than the familiar 186,000-miles-per-second [300,000 km/s] speed of light in a vacuum). 

The 220 PeV neutrino likely struck somewhere 6 to 19 miles (10 to 30 km) away, creating a slightly lower-energy particle called a muon, which then streaked through the detector, lighting up over a third of the sensors. The flash lasted about two microseconds, but KM3NeT captured every photon with nanosecond precision.

“This was a super-duper mega event,” Coyle says. The neutrino’s energy was off the charts, and its path was almost horizontal. “Both of these pieces of information combined is what makes this so special.” 

The direction this neutrino came from makes it particularly intriguing. As previously mentioned, neutrinos typically pass straight through everything — including Earth. Neutrino telescopes primarily use the planet as a filter to separate cosmic neutrinos from noisy background events.

But higher-energy neutrinos are more likely to interact with matter inside Earth. They aren’t as likely to travel unscathed through the planet. That means the only way to detect one is to catch it arriving from the side: skimming through the atmosphere or the shallow layers of Earth’s crust.

That’s what happened here. The neutrino came in almost horizontally, grazing the Maltese continental shelf before reaching the detector. “It was exactly the right type of configuration,” Coyle says. 

The hunt for cosmogenic neutrinos

For decades, astrophysicists have been searching for so-called cosmogenic neutrinos. These are super-energetic neutrinos created when a cosmic ray created by a high-energy event (such as matter falling into a supermassive black hole or a star exploding as a supernova) interacts with a low-energy photon of the cosmic microwave background left over from the Big Bang. Analyzing such a neutrino could help scientists better understand many aspects of our universe, but they haven’t been able to detect one yet. 

“This neutrino is definitely in the energy range where we expect cosmogenic neutrinos to be,” Coyle says. If it is cosmogenic, it won’t point back to a particular object in the sky. That’s because the cosmic ray that created it, likely consisting of a single proton, would have had its direction bent by magnetic fields before creating the neutrino. 

The neutrino could also have come from a specific astrophysical source, like an active galactic nucleus (AGN), a galaxy with a supermassive black hole at the center shooting out jets of particles. Three AGNs near the position astronomers think the neutrino came from on the sky were flaring at the time the detector spotted it, says Coyle. So, if the neutrino points back to one of these AGNs, it would suggest an astrophysical origin, but if it points to nothing, it could be the first cosmogenic neutrino ever detected. 

At the moment, the origins of the neutrino are still uncertain. The KM3NeT team is refining their measurements, narrowing down the source region in the sky. 

Just getting started

The KM3NeT telescope is still under construction. It was only one-tenth complete at the time of the detection. Currently, the ARCA detector consists of just 21 strings of optical sensors. Eventually, it will have more than 200. “That’s the great thing about neutrino telescopes,” Coyle says. “You don’t need the full detector to start collecting interesting data.”

The team is eager to detect more ultra-high-energy neutrinos to determine whether this was a one-off or the beginning of a new era in neutrino astronomy. More events will help clarify whether these particles originate from known astrophysical sources or if they are truly cosmogenic.

Beyond ARCA’s hunt for high-energy neutrinos, KM3NeT also operates the Oscillation Research with Cosmics in the Abyss (ORCA), a second undersea detector under construction near Toulon, France. ORCA is optimized for studying low-energy neutrinos to investigate the fundamental properties of these particles, which we still barely understand. 

KM3NeT is also an unexpected ally for marine scientists. “Since our detectors sit at the bottom of the sea, we continuously monitor ocean conditions like temperature and oxygen levels and even listen to marine life such as whales and dolphins,” Coyle adds. “We offer a free electric plug and ethernet connection three and a half kilometers into the sea so other sciences can study the ocean better.”

Neutrinos offer a unique way to explore the universe. Unlike light or charged particles (including the cosmic rays that produce them), they travel in straight lines, unbothered by magnetic fields or cosmic dust. If this event points directly to an AGN, it will help scientists better understand the mysterious jets produced by supermassive black holes. Otherwise, it may be the first cosmogenic neutrino ever detected. 

Either way, this neutrino is a landmark discovery. “It’s a completely unexplored energy range,” Coyle says. “We don’t know what to expect, and that’s the most exciting part.”

KM3NeT is poised to reveal even more about the high-energy cosmos. For now, scientists will keep watching the deep sea, waiting for the next ghost particle to light up the darkness.