In physics, tempting hints of the existence of sterile (inert) neutrinos, a theoretical fourth type of neutrino, differing from the other three predicted by the Standard Model, periodically emerge. Researchers are trying to find it using the IceCube neutrino observatory (Ice Cube). It is a powerful neutrino detector in Antarctica, capable of recognizing neutrinos coming from space . Can it happen that we discover this particle and rush into a new exciting era of physics?

Not. Searches on IceCube did not lead to anything, judging by the results published in early August 2016. The absence of detections does not mean that sterile neutrinos do not exist, but impose the strictest restrictions on them, strongly narrowing the range of possible energies and determining the direction of future searches.
')
If sterile neutrinos were found, they would explain the anomalies in the old studies, open up new physics to us beyond the Standard Model, and provide clues to such mysteries as the nature of dark matter and the imbalance between matter and antimatter in the Universe. “If you add the fourth neutrino, everything changes,” says Francis Halzen, project manager for IceCube and one of the authors of the paper.

Ghostly presence

To feel the nature of the neutrino, you only need to imagine that during the time you read this sentence, at least 100 trillion neutrinos passed through your body. Without resistance, they pass through solid matter, since they do not interact by means of electromagnetic forces. Electromagnetic forces, among other things, connect the atoms of your body and prevent you from falling through the floor.

One of the reasons for detecting neutrinos is that they are subject to weak nuclear interactions. Such interactions occur very rarely: a neutrino collides with an atom as it passes through matter, resulting in a flash. This is what modern neutrino detectors, including the IceCube, are observing.

Sterile neutrinos would be even more elusive than conventional ones, since they do not even experience weak interactions, so it is impossible to detect them in this way. They have mass, so that they interact through gravity, but a detector cannot be built on this principle. Sterile neutrinos can fly through your body right now, but you won’t find out. At least directly.

Who to call for help

This does not mean that we are doomed to eternal ignorance. Neutrinos constantly oscillate and change the “flavors” - they switch between the states of electron neutrino, muon neutrino and tau neutrino. Sterile neutrinos can turn into ordinary ones, which allows detecting their presence.

Neutrinos passing through very dense matter, such as the core of the earth, often interact with matter. Because of this, their oscillation pattern changes and they come to a resonance, which increases the chances of their becoming sterile neutrinos, so the researchers are looking for neutrinos coming at an angle indicating their passage through the core of the earth and further hit the detector. Such neutrinos appear in the atmosphere above the northern hemisphere due to cosmic radiation, and then pass through the planet and enter the IceCube.

“Oscillations mix all four flavors that have sterile neutrinos and all the other flavors, so if you play with one of them, it will affect all others,” says Haltzen.

Neutrinos that have become sterile will eventually disappear - they will not appear on the detector. If several neutrinos passing through the nucleus become sterile, then the energies of the arriving muon neutrinos will be a failure, just in the place corresponding to the mass of the sterile neutrino.

Fortunately, this energy - about 1 TeV - easily falls into the sensitivity of IceCube, covering energies from 10 GeV to 10 PeV. If sterile neutrinos existed in this energy gap, IceCube would detect them.

Searches cover data obtained for two years, during which neutrinos moving in the right direction were captured every six minutes. The data were analyzed by two independent groups that came to the same conclusion: no energy failure was detected in the muon neutrinos, which would indicate the presence of sterile ones.

Riddles and possibilities

At first it was believed that neutrinos have no mass and move at the speed of light. This is predicted by the standard model. But after 30 years of uncertainty, neutrino oscillations were detected. For massless particles, they would be impossible, so they have mass.

“So far, this is the only sign that the Standard Model is not complete,” says Hulzen. - There must be a physicist outside the Standard Model. Specifically, the new physics should hide in neutrinos and it will help explain the greatest mysteries of modernity. ”

These puzzles, in particular, the existence of dark matter and the imbalance between matter and antimatter in the Universe, are part of the motivation for physicists searching outside the Standard Model. And although the Standard Model is surprisingly successful, it could not cover large parts of the universe. “Sterile neutrinos could be the way to understand these problems,” says Hulzen.

The Large Hadron Collider is also used to search for ways outside the Standard Model and it also recently could not find traces of a new particle that looked so promising last year. “Neutrino specialists and the LHC are simultaneously attacking the Standard Model in search of new branches of physics,” says Hulzen.

The way forward

Failure to detect sterile neutrinos does not cancel their existence, but every time the search does not detect them, faith in their existence decreases. “People see hints of the existence of sterile neutrinos everywhere, like hints that Elvis is alive,” says Hulzen. “There is a collection of hints and theorists are confident of their existence.”

Thanks to such valuable rewards, researchers are not ready to give up yet. “In the absence of discoveries, we still continue to search and, of course, IceCube studies neutrinos on a large dynamic range, and we will continue to study all of these neutrinos at all energies in the hope that somewhere the Standard Model will give up slack, and we will start to find a new physics” - says Hulzen in the above video.

In addition, the researchers showed that IceCube can be used for a range of work that goes beyond the search for cosmic neutrinos alone.

“This result emphasizes the versatility of the neutrino observatory,” said Olga Botner, a representative of the IceCube collaboration, and another one of the authors of the work. “This is not only a tool for studying the intense Universe, but also the possibility of studying the properties of the neutrinos themselves.”