Finding the origin of cosmic particles that pass through our bodies billions of times per second

In fact, during your lifetime, there are probably a few atoms in your body that will touch the primordial neutrino. That's because the atom, although small, is 'hollow' with a large distance between the nuclei , so neutrino particles, which are electrically neutral, can easily pass through the body without leaving a collision.

Neutrinos produced by objects such as black holes often have more energy than primordial neutrinos floating in space. Although much rarer, these high-energy neutrinos are more likely to collide with something and produce a signal that physicists can detect. But to detect them, neutrino physicists had to design very large experiments.

IceCube , one such experiment, tracked a particularly rare type of neutrino from space in a study published in April this year. These high-energy neutrinos often 'disguise' as other, more common types of neutrinos. But for the first time, Penn State Professor of Physics and Professor of Astronomy and Astrophysics Doug Cowen and colleagues discovered them from nearly 10 years of data collected.

Their presence brings researchers one step closer to unraveling the mystery of how high-energy particles like neutrinos are created.

IceCube Observatory

The IceCube neutrino observatory is where large-scale neutrino experiments are performed.

The IceCube neutrino observatory is where large-scale neutrino experiments are performed. The station receives signals from about 5,000 sensors that have closely monitored tons of ice under Antarctica for more than a decade. The reason for choosing to build in Antarctica is to avoid man-made air pollution, helping to receive cosmic rays most effectively.

When a neutrino collides with an atom in the ice, it creates a sphere of light that sensors record. Thanks to this, IceCube has detected neutrinos produced in a number of places, such as the Earth's atmosphere, the center of the Milky Way, and black holes in other galaxies many light years away from us. But tau neutrinos, a type of neutrino with special energy, have eluded IceCube – until now.

Neutrinos come in three different types that physicists call 'flavours' or 'flavors' (electron neutrinos, muon neutrinos or tau neutrinos). Each flavor leaves its own distinct mark on a detector like IceCube.

When a neutrino collides with another particle, it usually creates a charged particle corresponding to its flavor. Muon neutrinos create muon particles, electron neutrinos create electron particles, and tau neutrinos create tau particles.

Neutrinos with muon flavor have the most distinctive signatures , so scientists in the IceCube collaboration naturally looked for those particles first. Muons emitted from a muon neutrino collision will travel through hundreds of meters of ice, creating a long trail of light that can be detected before it can decay. This fingerprint allows researchers to trace the neutrino's origin.

Next, the team observed electron neutrinos, whose interactions create a nearly spherical point of light. The electron created by an electron neutrino collision never decays, and it hits every ice particle it comes close to. This interaction leaves behind a spot of light that expands spherically.

Because the direction of electron neutrinos is difficult to discern visually , IceCube physicists applied techniques to show where electron neutrinos might have been created. These techniques use complex computational resources and tune millions of parameters to separate neutrino signals from all known background signals.

The third flavor of neutrino, neutrino tau, is the most elusive 'chameleon'. One tau neutrino may appear as a streak of light, while the next neutrino may appear as a bright spot. The tau particle created in the collision travels for an infinitesimal fraction of a second before decaying, and when it does it typically creates a spherical point of light.

Those tau neutrinos create two balls of light, one initially where they hit something and create tau particles, and one where the tau particles themselves decay. Most of the time, the tau particle decays after traveling only a very short distance, causing the two bright spheres to overlap to the point where they are indistinguishable from each other.

But at higher energies, the emitted tau particle can travel dozens of meters, causing the two bright spheres to separate. Physicists equipped with cutting-edge techniques can see this through – a task that has been described as finding the needle in the haystack.

As neutrinos move through IceCube, a tiny fraction of them interact with atoms in the ice and create light that sensors record. In the video, the spheres represent individual sensors, with the size of each sphere proportional to the amount of light it detects. The colors represent the relative arrival times of light, according to the colors of the rainbow, with red arriving earliest and purple arriving latest.

With these computational tools, the team extracted seven outstanding tau neutrinos from about 10 years of data. These tau neutrinos have energies higher than even the most powerful particle accelerators on Earth, meaning they must come from cosmic sources, such as black holes.

As IceCube and other neutrino experiments collect more data and scientists increasingly distinguish between the three types of neutrinos, researchers will eventually be able to predict how neutrinos come from black holes. how.

Energetic tau neutrinos (including muon neutrinos) are usually less energetic than the more common neutrinos from the Big Bang. But beyond the infinite space, there is still plenty to help scientists search for the most powerful neutrino emitters in the universe.