The building that houses the IceCube servers.
Tantalizing hints have regularly surfaced pointing to the existence of a sterile neutrino — a theoretical fourth type of neutrino separate from the three predicted by the Standard Model. Researchers have now looked for it using the IceCube Neutrino Observatory, a powerful neutrino detector in Antarctica that can detect neutrinos of cosmic origin. Could this particle finally be found, ushering in an exciting new era of physics?
No. IceCube’s search turned up nothing, as evidenced by the results published today. The lack of detection doesn’t necessarily mean that sterile neutrinos don’t exist, but it does place the most severe constraints on them, narrowing the range of energies they could have and informing future studies where to look.
Had sterile neutrinos been found, they would have explained anomalies in old research, revealed new physics beyond the Standard Model, and may have provided clues to mysteries such as the nature of dark matter and the imbalance between matter and antimatter in the universe. “If you throw in a fourth neutrino, that changes everything,” said Francis Halzen, IceCube’s principal investigator and one of the paper’s authors.
Ghostly presence
To get an idea of the nature of neutrinos, just consider that about 100 trillion neutrinos have passed through our bodies in less than the time it took to read this sentence. They pass through solid matter as if it were not there, because they do not interact through the electromagnetic force. Among other things, electromagnetic force holds the atoms in your body together and prevents you from sinking through the floor.
The only reason neutrinos can be detected at all is because they interact via the weak nuclear force. This interaction happens very rarely: a neutrino will “bump” into an atom as it passes through solid matter, releasing a flash of light. That’s what current neutrino detectors, including IceCube, look for.
Sterile neutrinos would be even spookier than regular neutrinos because they don’t even interact with the weak force, meaning it’s impossible to detect them in the same way. They do have mass, so they interact through gravity, but we can’t use that to build a detector. Sterile neutrinos may be passing through your body right now, but you don’t know that. At least not directly.
IceCube are you going to call?
Some researchers describe the search for sterile neutrinos.
But that doesn’t mean we’re doomed to be in the dark forever. Neutrinos constantly oscillate, changing from one flavor — electron, muon, or tau — to another. Sterile neutrinos can turn into regular neutrinos, allowing their presence to be detected.
Neutrinos passing through a very dense piece of matter, such as the Earth’s core, are affected by their relatively frequent interactions with matter. This changes the oscillation pattern of the neutrinos, putting them in a resonance that is more likely to oscillate in sterile neutrinos. detector. These neutrinos were generated by cosmic rays in the atmosphere above the northern hemisphere and then traveled through the planet to reach IceCube.
“Fluctuations [mix] all four flavors now containing the sterile plus the old flavors, so if you mess with one it will affect all the others,” Halzen told Ars.
Neutrinos transformed into sterile ones would essentially disappear – they wouldn’t show up in the detector. If some of the neutrinos passing through the nucleus become sterile, then there would be a dip in the energy of arriving muon neutrinos, with the energy corresponding to the mass of sterile neutrinos.
Fortunately, this energy – about one Tera-electronVolt – easily fits into IceCube’s sensitivity, which ranges from about 10 Giga-electronVolts to 10 PeV. If sterile neutrinos existed within that energy range, IceCube would have detected them.
The search represents two years of IceCube data, in which one such upward-traveling neutrino was detected every six minutes. Two independent analyzes were performed on the data, each yielding the same conclusion: no energy dip among muon neutrinos that would have meant the presence of sterile neutrinos.
Possibilities and mysteries
Initially, neutrinos were thought to be massless particles traveling at the speed of light. In fact, this is what the Standard Model predicts. But after 30 years of ambiguity, the oscillations of the neutrinos were discovered. The oscillations would be impossible if neutrinos are massless, so they do have mass.
“This is, in fact, the only indication so far that the Standard Model is not the whole story,” Halzen told Ars. “There has to be physics that goes beyond the Standard Model. In particular, neutrinos should hide new physics that will hopefully explain the great mysteries of today’s physics.”
Those mysteries, particularly the existence of dark matter and the imbalance between matter and antimatter in the Universe, are part of physicists’ motivation to look beyond the Standard Model. While the Standard Model has been remarkably successful, it has failed to explain large parts of the Universe. “Sterile neutrinos could have been a gateway to understanding these problems,” Halzen said.
The Large Hadron Collider is also being used to find ways beyond the Standard Model, and it also recently failed to detect signs of a new particle that looked promising last year. “Neutrino physicists and the LHC have parallel lines of attack in discovering physics beyond the Standard Model,” Halzen told Ars.
The path forward
Failure to detect sterile neutrinos does not mean they have been completely ruled out, but any time a search fails to detect them, belief in their existence will diminish. “Like Elvis, people are seeing hints of the sterile neutrino everywhere,” Halzen said. “There was a collection of hints and theorists were convinced that it exists.”
With rewards this rich, researchers aren’t ready to give up. “If there’s no discovery, we’ll keep looking, and of course IceCube studies neutrinos over a really wide dynamic range, and we’ll continue to study all these neutrinos at all these energies in hopes that somewhere the Standard Model gives and we start discovering new physics. ,” Halzen said in the video above.
The research also shows that IceCube can be used for a range of things beyond the cosmic neutrinos it was built for.
“This new result highlights the versatility of the IceCube Neutrino Observatory,” said Olga Botner, the spokesperson for the IceCube Collaboration and another author of the paper. “It is not only a tool for exploration of the violent universe, but also enables detailed studies of the properties of the neutrinos themselves.”
ArXiv2015. Abstract: arXiv:1605.01990v1 (About DOIs).
Correction: PeV, not TeV.