IceCube researchers discover a rare high-energy neutrino emitted by a powerful celestial body

About one trillion tiny particles called neutrinos pass through you every second. These relic neutrinos were created during the Big Bang and are found throughout the universe, but they can’t harm you. In fact, only one of them is likely to tap an atom in your body during your lifetime.

Most neutrinos produced by objects like black holes have more energy than the remnant neutrinos floating around in space. While it’s much rarer, it’s more likely that these high-energy neutrinos will hit something and produce a signal that physicists like me can detect. But to detect them, neutrino physicists must build very large experiments.

IceCube is one such experiment, recording a particularly rare type of high-energy astrophysical neutrino in a study published in April 2024. But my colleagues and I succeeded in discovering them for the first time, extracting some from nearly 10 years of data.

Their existence brings researchers like me one step closer to solving the mystery of how high-energy particles like astrophysical neutrinos are created.

IceCube Observatory

The IceCube Neutrino Observatory is the 800-pound gorilla of a massive neutrino experiment. It has about 5,000 sensors and has been keeping tabs on the billions of tons of ice beneath Antarctica for more than a decade. When neutrinos collide with atoms in the ice, they create balls of light that are recorded by sensors.

IceCube has detected neutrinos produced in a variety of places, such as black holes in Earth’s atmosphere, the center of the Milky Way, and other galaxies many light-years away.

But tau neutrinos are particularly high-energy neutrinos that IceCube has so far been unable to detect.

Neutrino flavor

There are three different types of neutrinos, which physicists call flavors. Each flavor leaves a unique imprint on a detector like IceCube.

When a neutrino strikes another particle, it usually creates a charged particle that corresponds to its flavor. Muon neutrinos produce muons, electron neutrinos produce electrons, and tau neutrinos produce tau.

Neutrinos with a muon flavor have the most unique characteristics, so it was natural that my colleagues in the IceCube collaboration and I would look for these neutrinos first. Mesons emitted by muon-neutrino collisions will travel through hundreds of meters of ice, forming a long trail of detectable light before decaying. This trajectory allows researchers to trace the neutrino’s origin.

The team next looked at electron neutrinos, whose interactions create a roughly spherical ball of light. The electron produced by an electron-neutrino collision never decays; it hits every particle in the ice near it. This interaction leaves behind an expanding ball of light before the electron finally comes to rest.

Because the direction of electron neutrinos is difficult to discern with the naked eye, IceCube physicists applied machine learning techniques to pinpoint where electron neutrinos were likely produced. These techniques use complex computing resources and adjust millions of parameters to separate the neutrino signal from all known background.

The third type of neutrino, the tau neutrino, is the chameleon of the three. One tau neutrino can behave like a light track, while the next can behave like a ball. The tau particle produced in the collision travels for a small amount of time before decaying, and when it does, it typically creates a ball of light.

These tau neutrinos create two spheres of light, one where they originally hit something and created tau, and another where the tau neutrino itself decays. Most of the time, tau particles decay after traveling only a short distance, leaving the two spheres of light to overlap so much that they are indistinguishable from a single sphere.

But at higher energies, the emitted tau particles can travel tens of meters, creating two separate spheres of light. Physicists armed with these machine learning techniques can see through this and find the needle in the haystack.

High energy tau neutrino

Using these computational tools, the team successfully extracted seven candidate tau neutrinos from approximately 10 years of data. The energy of these tauons is higher than even the most powerful particle accelerators on Earth, which means they must come from an astrophysical source, such as a black hole.

These data confirm IceCube’s earlier discovery of astrophysical neutrinos and confirm the suggestion that IceCube had previously discovered astrophysical tau neutrinos.

These results also show that even at the highest energies and great distances, neutrinos behave in much the same way as they do at lower energies.

In particular, the detection of tau neutrinos in astrophysics confirmed that high-energy neutrinos from distant sources can change the flavor or oscillation. Neutrinos with much lower energy, traveling much shorter distances, would oscillate in the same way.

As IceCube and other neutrino experiments collect more data and scientists are better able to distinguish between the three neutrino flavors, researchers will eventually be able to guess how neutrinos from black holes are produced. We also want to know whether the space between Earth and these distant astrophysical neutrino accelerators treats particles differently depending on their mass.

There are always fewer high-energy tau neutrinos and their close cousins ​​muons and electrons than the more common neutrinos from the Big Bang. But there is enough to help scientists like me search for the most powerful neutrino emitters in the universe and study the infinite space in between.