Primordial Could Be the Most Energetic Neutrino Ever Found

Neutrinos might be the most enigmatic particles in the fascinating field of particle physics. They have virtually no mass, no electrical charge, and hardly ever interact with other stuff. They are quite challenging to research because of these traits. In deep caverns, thick Antarctic ice, or on the ocean floor, even their detection requires specialist facilities.

The Cubic Kilometre Neutrino Telescope, or KM3NeT, is one of the most advanced neutrino detectors. It is located on the Mediterranean sea floor and found the highest energetic neutron ever recorded in February 2023. The calculated energy of KM3-230213A was 220 PeV (220 x 1015 electron volts, or 220 million billion electron volts). Physicists have been attempting to identify the source of that astounding amount of energy ever since it was discovered.

The high-energy Universe is the source of neutrinos. This is the domain of extremely intense phenomena such as kilonovae, gamma-ray bursts, and cataclysmic supernovae. They are the only ones capable of giving particles such tremendous energy. However, it has proven difficult for scientists to link KM3-230213A to one of them.

Determining the source of neutrinos may be even more difficult than detecting them. The neutrinos themselves are not detected by neutrino detectors. Rather, they find Cherenkov radiation or secondary particles that result from the few interactions of neutrinos with other matter. A muon was found in the instance of KM3-230213A.

Researchers connected to KM3NeT have published their findings in Nature after carefully examining the high-energy event. The KM3NeT Collaboration is identified as the author of the study titled “Observation of an ultra-high-energy cosmic neutrino with KM3NeT.”

*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.

Was KM3-230213A a cosmogenic neutrino, then? It falls within the energy range that cosmogenic neutrinos are thought to occupy by physicists. Is that sufficient?

The researchers describe the possibility that the event was caused by a cosmogenic neutrino as “A viable alternative hypothesis…” in their study.

It all boils down to the neutrino’s incredibly 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.

Future neutrino observatories and improvements to existing ones will be necessary to comprehend these high-energy neutrinos. More detectors are being added to KM3NeT. This will increase its efficiency in both detecting more neutrinos and precisely identifying their sources in the universe.