‘Impossible’ particle that hit Earth may have been dark matter

An unprecedented high-energy particle detected traversing through Earth has left scientists puzzled ever since its observation. Initially believed to be an atypical neutrino, some researchers are now proposing a more radical theory – suggesting that it could be a particle of dark matter journeying across the universe.

The detection of this “impossible” neutrino took place in 2023 at the KM3NeT detector located off the coast of Italy, even while it was still in its construction phase. This particle stood out due to its immense energy, being 35 times more energetic than any previously recorded. The origin of this particle remains elusive, with potential sources including a galaxy housing an extremely active central black hole known as a blazar, or a background origin of high-energy neutrinos dispersed throughout the cosmos.

Bhupal Dev from Washington University in St. Louis, Missouri, along with his team, propose an alternative hypothesis. They suggest that the event might not have been a neutrino as initially assumed, but rather a dark matter particle colliding with Earth, originating from a blazar. Dev explains, “It opens up a new way you can really test dark matter. We can convert these neutrino telescopes into dark matter detectors.”

Neutrino detectors face challenges due to the minuscule size and almost negligible mass of these particles, making interactions with matter rare as they travel across space. Upon reaching Earth, they may occasionally collide with atoms, generating a particle called a muon that can be detected by neutrino detectors like KM3NeT and IceCube located at the South Pole.

While IceCube has observed evidence of hundreds of cosmic neutrinos since 2011, it had never encountered something as energetic as the discovery made by KM3NeT. This anomaly sparked confusion as IceCube should have detected the source that KM3NeT observed. Dev suggests that if the incoming particle was dark matter instead of a neutrino, it could provide an explanation for this discrepancy. The trajectory of the incoming particle, passing through more of Earth to reach KM3NeT than IceCube, increased the likelihood of it being deflected into a muon. Dev elaborates, “The dark matter goes through lots of Earth’s matter, and we can explain why IceCube didn’t see it.”

The theory postulates that the particle was generated in a blazar and directed towards Earth in a beam. Dev favors this concept as high-energy protons in a blazar are more efficient in transferring their energy into dark matter compared to neutrinos. While the majority of events detected by KM3NeT and IceCube are likely neutrinos, this exceptional event could be attributed to dark matter.

Although not everyone is fully convinced yet, with some suggesting that it may simply be an exceptionally energetic neutrino, the prospect of utilizing these detectors to study dark matter particles is met with enthusiasm. If proven correct, this method could offer a means to detect and analyze dark matter particles, a feat that has eluded scientists thus far. “Everybody would be pretty thrilled if these machines can study not only neutrinos but also dark matter,” says Dan Hooper from the University of Wisconsin–Madison.

Shirley Li at the University of California, Irvine, points out that the hypothesis could be tested in the future. The impact of an incoming dark matter particle on Earth should produce two muons in the detector, rather than just one. However, the current detectors lack the precision to differentiate between two particles from a single signal at these energy levels. “That’s potentially testable, but at these energies muon reconstruction is very difficult,” she explains.