A Ghost Particle Flew Through Earth. It Might Have Come From Dark Matter.

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Here’s what you’ll learn when you read this story:

In February of 2023, the Cubic Kilometer Neutrino Telescope (KM3NeT) detected a neutrino some 35 times higher in energy than any previous detection.
A new study posits that this muon might be the result of a dark matter particle that decayed as it passed through the Earth, which would also explain why the IceCube Observatory in the South Pole hasn’t detected any such particle.
For now, the most likely explanation is that the particle is just a high-energy neutrino, but we’re likely to learn more about these elusive particles when KM3NeT is fully online.

In the middle of the night on February 13, 2023, the Astroparticle Research with Cosmics in the Abyss (ARCA) array—part of the larger Cubic Kilometer Neutrino Telescope (KM3NeT), located in the Mediterranean off the coast of Italy—detected a high-energy particle unlike any other. After crunching the numbers for two years, scientists earlier this year announced that this particle likely originated from a blazar, and was the highest-energy neutrino ever detected by a factor of 35. Even more impressive, the array documented this particle while it was only partially finished.

However, there was one strange attribute of this neutrino event, which was dubbed KM3-230213A—the IceCube Neutrino Observatory in the South Pole has never recorded such an event, and did not detect this high-energy event, either. Now, a new study (posted to the preprint server arXiv) suggests that one way to resolve this conundrum is to consider if the ultra-energetic muon neutrino was actually produced by dark matter.

“It opens up a new way you can really test dark matter,” Bhupal Dev, lead author of the study from Washington University, said to New Scientist. “We can convert these neutrino telescopes into dark matter detectors.”

Because KM3-230213A traveled along a shallow path as it passed through out planet, it likely flew through more of the Earth’s crust than it takes for neutrinos to reach IceCube—and that might be the key. The researchers tried to recreate a dark matter scenario that KM3NeT could detect, but IceCube could not.

“We propose a novel solution to this conundrum in terms of dark matter (DM) scattering in the Earth’s crust,” the paper reads. “We show that intermediate dark-sector particles that decay into muons are copiously produced when high-energy (∼100 PeV) DM propagates through a sufficient amount of Earth overburden.”

So, the high-energy particle wasn’t directly a dark matter particle, but rather a muon that decayed from dark matter likely fired from a blazar (a type of active galactic nucleus) right at Earth. As the paper notes, this is just one of many theories that have propagated since KM3NeT announced the discovery of the particle. These ideas suggest, according to the paper, that the particle could be “decaying heavy DM, primordial black hole evaporation, Lorentz invariance violation, neutrino non-standard interactions.”

The KM3NeT collaboration has detailed how the ARCA array—along with the Oscillation Research with Cosmics in the Abyss (ORCA) array—could provide indirect observations of dark matter similar to this event. However, other physicists speaking with New Scientist said that the most likely explanation (at least, as of right now) is that KM3-230213A is simply a high-energy neutrino, and not evidence of decaying dark matter.

While hypotheses proliferate, scientists are still working with a sample size of one. But with K3MNeT finishing construction by the end of the decade, that likely won’t be the case for long.

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