Universe Today
In the exotic world of particle physics, neutrinos may be the most mysterious members. They rarely interact with other matter, have almost no mass, and have no electrical charge. These characteristics make them extremely difficult to study. Even detecting them requires specialized facilities in deep caves, in thick Antarctic ice, or on the ocean floor.
One of the foremost neutrino detectors is called KM3NeT, which stands for the Cubic Kilometre Neutrino Telescope. It's on the sea-floor in the Mediterranean, and in February 2023, it detected the most energetic neutron ever observed. It's called KM3-230213A, and it's estimated energy was 220 PeV (220 x 1015 electron volts or 220 million billion electron volts). That's an unbelievable amount of energy, and ever since it was detected, physicists have been trying to determine its source.
Neutrinos come from the high-energy Universe. This is the realm of cataclysmic supernovae, gamma-ray bursts, kilonovae, and other extraordinarily energetic events. Only they have the power to impart such high energies to particles. But tracing KM3-230213A back to one of these has been a scientific challenge.
If detecting neutrinos is challenging, determining their source might be even more challenging. Neutrino detectors don't actually detect the neutrinos themselves. Instead, they detect secondary particles or Cherenkov radiation that comes from the rare times that a neutrino interacts with other matter. In the case of KM3-230213A, it was a muon that was detected.
After meticulous research into the high-energy event, researchers associated with KM3NeT have published their results in Nature. The research is titled "Observation of an ultra-high-energy cosmic neutrino with KM3NeT." The KM3NeT Collaboration is listed as the author.
*This illustration shows KM3NeT, the Cubic Kilometre Neutrino Telescope. It consists of strings of detectors anchored the Mediterranean sea floor. A mass of detectors are needed since neutrinos seldom interact with other matter. Image Credit: KM3NeT Collaboration*
"The detection of cosmic neutrinos with energies above a teraelectronvolt (TeV) offers a unique exploration into astrophysical phenomena," the authors write. "Electrically neutral and interacting only by means of the weak interaction, neutrinos are not deflected by magnetic fields and are rarely absorbed by interstellar matter: their direction indicates that their cosmic origin might be from the farthest reaches of the Universe."
High-energy neutrinos have specific sources. They're created when ultra-relativistic cosmic-ray protons or nuclei interact with matter or photons. When scientists observe these neutrinos, it's like looking at the signature of the process itself, according to the researchers.
“Neutrinos are one of the most mysterious of elementary particles. They have no electric charge, almost no mass and interact only weakly with matter. They are special cosmic messengers, bringing us unique information on the mechanisms involved in the most energetic phenomena and allowing us to explore the farthest reaches of the Universe”, explained Rosa Coniglione in a press release. Coniglione was the KM3NeT Deputy-Spokesperson at the time of the detection.
The researchers were able to trace the high-energy neutrino back to where it came from, but not precisely. Their work revealed four types of potential sources: galactic, local Universe, transient and extragalactic origin.
This figure shows some of the potential sources for the high-energy neutrino. The red star indicates KM3-230213A, and the error regions within R(68%), R(90%) and R(99%) are shown with dotted, dashed and solid contours, respectively. The directions of the selected source candidates are shown as coloured markers. The colours and marker type indicate the criterion according to which the source was selected. The sources are numbered according to their proximity to KM3-230213A. Image Credit: The KM3NeT Collaboration 2026. Nature.
In their paper, the authors remind us that the energy in KM3-230213A was far greater than any other detection so far. There are only a couple of reasons that it could be so energetic. Either it originated from a different cosmic object than other less energetic neutrinos, or it's an example of a cosmogenic neutrino. Cosmogenic neutrinos are largely hypothetical at this point, with no clear detections. They're created when ultra-high-energy cosmic rays, which are protons or heavier nuclei traveling at near light-speed, slam into photons from the Cosmic Microwave Background, the relic radiation from the Big Bang. The impact creates a decay chain and a cascading flood of other particles, including ultra-high energy neutrinos like KM3-230213A.
Cosmogenic neutrinos are fascinating for several reasons. They can point back directly to their sources, which could be active galactic nuclei, gamma-ray bursts, even galaxy mergers. Since they're produced throughout the Universe's history, they can serve as probes of the early Universe. And since they're far more energetic than anything we can produce and study in a particle accelerator, studying them could reveal aspects of physics that are beyond the Standard Model. In short, they're a scientific bonanza.
So, was KM3-230213A a cosmogenic neutrino? It sits in the energy range that physicists think cosmogenic neutrinos inhabit. Is that enough?
In their paper, the researchers explain that the event could be a cosmogenic neutrino, and that explanation is "A viable alternative hypothesis..."
It all comes down to the neutrino's extraordinarily high energy. "This suggests that the neutrino may have originated in a different cosmic accelerator than the lower-energy neutrinos, or this may be the first detection of a cosmogenic neutrino, resulting from the interactions of ultra-high-energy cosmic rays with background photons in the Universe," they write.
So for now, there's no clear conclusion.
Understanding these high-energy neutrinos will depend on future neutrino observatories and upgrades to current ones. KM3NeT is being expanded to include more detectors. That will make it more effective at not only detecting more neutrinos, but more accurately determing their sources in the cosmos.
