Physicists at the University of Massachusetts Amherst argue that an ultra-high-energy neutrino detected by the KM3NeT experiment could be the signature of an explosion of a ‘quasi-extremal primordial black hole,’ pointing toward new physics beyond the Standard Model.
The KM3NeT experiment has recently observed a neutrino with an energy around 100 PeV, and IceCube has detected five neutrinos with energies above 1 PeV; while there are no known astrophysical sources, exploding primordial black holes could have produced these high-energy neutrinos. Image credit: Gemini AI.
Black holes exist, and we have a good understanding of their life cycle: an old, large star runs out of fuel, implodes in a massively powerful supernova and leaves behind an area of spacetime with such intense gravity that nothing, not even light, can escape. These black holes are incredibly heavy and are essentially stable.
But, as physicist Stephen Hawking pointed out in 1970, another kind of black hole, a primordial black hole, could be created not by the collapse of a star, but from the Universe’s primordial conditions shortly after the Big Bang.
Primordial black holes exist only in theory so far, and, like standard black holes, are so massively dense that almost nothing can escape them. However, despite their density, these objects could be much lighter than the black holes we have so far observed.
Furthermore, Hawking showed that primordial black holes could slowly emit particles via what is now known as Hawking radiation if they got hot enough.
“The lighter a black hole is, the hotter it should be and the more particles it will emit,” said Dr. Andrea Thamm, a physicist at the University of Massachusetts Amherst.
“As primordial black holes evaporate, they become ever lighter, and so hotter, emitting even more radiation in a runaway process until explosion.”
“It’s that Hawking radiation that our telescopes can detect.”
“If such an explosion were to be observed, it would give us a definitive catalog of all the subatomic particles in existence, including the ones we have observed, such as electrons, quarks and Higgs bosons, the ones that we have only hypothesized, like dark matter particles, as well as everything else that is, so far, entirely unknown to science.”
In 2023, the KM3NeT experiment captured that impossible neutrino — exactly the kind of evidence Dr. Thamm and colleagues hypothesized we might soon see.
But there was a hitch: a similar experiment, called IceCube, also set up to capture high-energy cosmic neutrinos, not only didn’t register the event, it had never clocked anything with even one hundredth of its power.
If the Universe is relatively thick with primordial black holes, and they are exploding frequently, shouldn’t we be showered in high-energy neutrinos? What can explain the discrepancy?
“We think that primordial black holes with a ‘dark charge’ — what we call quasi-extremal primordial black holes — are the missing link,” said Dr. Joaquim Iguaz Juan, a physicist at the University of Massachusetts Amherst.
“The dark charge is essentially a copy of the usual electric force as we know it, but which includes a very heavy, hypothesized version of the electron — a dark electron.”
“There are other, simpler models of primordial black holes out there,” added Dr. Michael Baker, also from the University of Massachusetts Amherst.
“Our dark-charge model is more complex, which means it may provide a more accurate model of reality.”
“What’s so cool is to see that our model can explain this otherwise unexplainable phenomenon.”
“A primordial black hole with a dark charge has unique properties and behaves in ways that are different from other, simpler primordial black hole models,” Dr. Thamm said.
“We have shown that this can provide an explanation of all of the seemingly inconsistent experimental data.”
The team is confident that, not only can their dark-charge model primordial black holes explain the neutrino, it can also answer the mystery of dark matter.
“Observations of galaxies and the Cosmic Microwave Background suggest that some kind of dark matter exists,” Dr. Baker said.
“If our hypothesized dark charge is true, then we believe there could be a significant population of primordial black holes, which would be consistent with other astrophysical observations, and account for all the missing dark matter in the Universe,” Dr. Iguaz Juan said.
“Observing the high-energy neutrino was an incredible event,” Dr. Baker said.
“It gave us a new window on the Universe. But we could now be on the cusp of experimentally verifying Hawking radiation, obtaining evidence for both primordial black holes and new particles beyond the Standard Model, and explaining the mystery of dark matter.”
The findings appear in the journal Physical Review Letters.
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Michael J. Baker et al. Explaining the PeV neutrino fluxes at KM3NeT and IceCube with quasi-extremal primordial black holes. Phys. Rev. Lett, published online December 18, 2025; doi: 10.1103/r793-p7ct
