by About a trillion tiny particles called neutrinos pass through you every second. Created during the Big Bang, these “remnant” neutrinos are all over the universe, but they can’t harm you. In fact, it is unlikely that only one of them will benefit from a light atom in your body during your entire lifetime.
Most neutrinos produced by objects such as black holes have much more energy than the remnant neutrinos floating through space. Although much rarer, these energetic neutrinos are much more likely to crash into something and create a signal that physicists like me can detect. But to detect them, neutrino physicists had to carry out very large experiments.
IceCube, one such experiment, documented a very rare type of highly energetic astrophysical neutrino in a study published in April 2024. These energetic neutrinos are often masquerading as other, more common types of neutrino. But for the first time, my colleagues and I managed to detect them, pulling out a few from almost 10 years of data.
Their presence brings researchers like me one step closer to solving the mystery of how highly energetic particles like astrophysical neutrinos are produced in the first place.
IceCube Observatory
The IceCube Neutrino Observatory is the 800-pound gorilla of large neutrino experiments. There are about 5,000 sensors that have been closely watching a gigaton of ice under the South Pole for more than a decade. When a neutrino collides with an atom in the ice, it produces a ball of light that is recorded by the sensors.
IceCube has discovered neutrinos created in many places, such as the Earth’s atmosphere, the center of the Milky Way galaxy and black holes in other galaxies many light years away.
But the tau neutrino, one type of extremely energetic neutrino, has eluded IceCube – until now.
Neutrino flavors
Neutrinos come in three different types, which physicists call flavors. Each flavor leaves a distinct trace on a detector like IceCube.
When a neutrino collides with another particle, it usually produces a charged particle corresponding to its flavor. A muon produces a muon neutrino, an electron neutrino produces an electron, and a tau neutrino produces a tau.
Muon-flavored neutrinos have the most distinctive signature, so my colleagues and I in the IceCube collaboration looked for those first. The muon emitted from a muon-neutrino collision will pass through hundreds of meters of ice, making a long trail of visible light, before it decays. This trace allows researchers to trace the origin of the neutrino.
The team next looked at electron neutrinos, whose interactions produce a roughly spherical ball of light. The electron that produces an electron-neutrino collision doesn’t cheat, and hits every particle in the ice it comes close to. This interaction leaves behind an expanding ball of light before the electron finally comes to rest.
Since the direction of the electron neutrino is very difficult to discern by eye, the IceCube physicists applied machine learning techniques to direct it back to where the electron neutrino might have been created. These techniques use sophisticated computing resources and tune millions of parameters to separate neutrino signals from all known backgrounds.
The third flavor of neutrino, the tau neutrino, is the chameleon of the trio. One tau neutrino can appear as a trail of light, and the next as a ball. The tau particle created in the collision travels for a fraction of a second before decaying, and when it decays, it usually produces a ball of light.
Those neutrinos create two balls of light, one when they first hit something and form a tau, and one where the tau itself disappears. Most of the time, the tau particle decays after traveling only a very short distance, causing the two balls of light to overlap so much that they are indistinguishable from one.
But at higher energies, the emitted tau particle can travel tens of meters, making two balls of light separate from each other. Physicists armed with those machine learning techniques can see through this to find the needle in the haystack.
Energetic tau neutrinos
With these computational tools, the team succeeded in extracting seven strong tau neutrinos from approximately 10 years of data. These taus had higher energies than even the world’s most powerful particle accelerators, which means they must be from astrophysical sources, such as black holes.
These data confirm IceCube’s earlier discovery of astrophysical neutrinos, and confirm a hint that IceCube previously captured astrophysical tau neutrinos.
These results also suggest that even at the highest energies and over vast distances, neutrinos behave in the same way as they do at lower energies.
In particular, the detection of astrophysical tau neutrinos confirms that energetic neutrinos from distant sources vary in flavor, or oscillate. A neutrino oscillates in the same way with much lower energy traveling much shorter distances.
As IceCube and other neutrino experiments collect more data, and scientists get better at distinguishing between the three neutrino flavors, researchers will finally be able to guess how neutrinos coming from black holes are produced. We also want to know if the space between Earth and these distant neutrino accelerators treats particles differently depending on their mass.
There will always be less energetic tau neutrino and its muon and electron cousins compared to the more common neutrinos from the Big Bang. But there are plenty out there to help scientists like me search for the most powerful neutrino emitters in the universe and study the infinite space between them.
This article is republished from The Conversation, a non-profit, independent news organization that brings you reliable facts and analysis to help you make sense of our complex world. Written by: Doug Cowen, Penn State
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Doug Cowen receives funding from the National Science Foundation.
