Tantalizing hints have regularly turned up to indicate the existence of a sterile neutrino—a theoretical fourth type of neutrino separate from the three predicted by the Standard Model. Researchers have now searched for it using the IceCube Neutrino Observatory, a powerful neutrino detector in Antarctica that is able to spot neutrinos of cosmic origin. Could this particle finally be found, ushering in a thrilling new era of physics?

No. IceCube’s search has turned up nothing, as revealed in results published today. The lack of detection doesn’t necessarily mean sterile neutrinos don’t exist, but it does put the strictest constraints on them yet, narrowing down the range of energies they could have and informing future studies on where to look.

Had sterile neutrinos been found, they would have explained anomalies in old research, revealed new physics beyond the Standard Model, and potentially provided clues for mysteries such as the nature of dark matter and the imbalance between matter and anti-matter in the Universe. “If you throw in a fourth neutrino, it changes everything,” said Francis Halzen, principal investigator for IceCube and one of the paper’s authors.

Ghostly presence

To get a sense of the nature of neutrinos, one need only consider that roughly 100 trillion neutrinos have passed through our bodies in less than the time it took one to read this sentence. They pass through solid matter as if it wasn’t there because they don’t interact via the electromagnetic force. Electromagnetic force, among other things, holds the atoms in your body together and prevents you from falling through the floor.

The only reason neutrinos can be detected at all is because they interact via the weak nuclear force. This interaction takes place very infrequently: a neutrino will “bump into” an atom as it’s passing through solid matter, releasing a flash of light. That’s what current neutrino detectors, including IceCube, look for.

Sterile neutrinos would be even more ghostly than ordinary neutrinos, as 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 via gravity, but we can’t use that to build a detector. Sterile neutrinos could be passing through your body at this moment, but there’s no way to know. At least not directly.

IceCube you gonna call?



But that doesn’t mean we’re doomed to remain forever in the dark. Neutrinos are constantly oscillating, changing from one flavor—electron, muon, or tau—into another. Sterile neutrinos can transform into ordinary ones, allowing their presence to be detected.

Neutrinos that pass through a very dense bit of matter, such as the Earth’s core, are affected by their relatively frequent interactions with matter. This changes the neutrinos' pattern of oscillation, bringing them into a resonance that has a higher chance of oscillating into sterile neutrinos, so the researchers looked for neutrinos traveling at an angle that indicated they’d passed through the core and were traveling upwards through the detector. These neutrinos were generated in the atmosphere above the Northern Hemisphere by cosmic rays, and then they would travel through the planet to reach IceCube.

“Oscillations [mix] all four flavors which now includes the sterile plus the old flavors, so if you mess with one, it will affect all others,” Halzen told Ars.

Neutrinos that had transformed into sterile ones would essentially disappear—they wouldn’t show up in the detector. If some of the neutrinos passing through the core turned sterile, then there’d be a dip in the energy of arriving muon neutrinos, at the energy corresponding to the mass of sterile neutrinos.

Luckily, this energy—about one Tera-electronVolt—easily fits into IceCube’s sensitivity, which covers 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, during which one such upward-traveling neutrino was detected every six minutes. Two independent analyses were performed on the data, each turning up the same conclusion: no energy dip among muon neutrinos that would have signified the presence of sterile neutrinos.

Possibilities and mysteries

Neutrinos were initially thought to be massless particles that travel at the speed of light. In fact, this is what the Standard Model predicts. But after 30 years of ambiguity, the neutrinos’ oscillations were discovered. The oscillations would be impossible if neutrinos are massless, so they do have mass.

“This is in fact the only indication to date that the Standard Model is not the whole story,” Halzen told Ars. “There must be physics beyond the Standard Model. Specifically, neutrinos must hide new physics that hopefully will explain the great mysteries of physics today.”

Those mysteries, specifically the existence of dark matter and the imbalance between matter and anti-matter in the Universe, are part of the motivation of physicists to look beyond the Standard Model. While the Standard Model has been remarkably successful, it has failed to account for large swaths 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 past the Standard Model, and it has also recently failed to turn up signs of a new particle that had looked promising last year. “Neutrino physicists and the LHC have parallel lines of attack on discovering physics beyond the Standard Model,” Halzen told Ars.

The path forward

The failure to turn up sterile neutrinos doesn't mean they're ruled out entirely, but each time a search fails to turn them up it will lower credence in their existence. “Like Elvis, people see hints of the sterile neutrino everywhere,” said Halzen. “There was this collection of hints, and theorists were convinced it exists.”

With rewards so rich, researchers aren’t ready to give up just yet. “In the absence of a discovery, we’ll still keep looking, and of course IceCube is studying neutrinos over really large dynamic range, and we’ll keep studying all these neutrinos at all these energies with the hope that somewhere the Standard Model gives and we begin to discover new physics,” Halzen said in the video above.

The research also demonstrates that IceCube can be used for a range of things beyond the cosmic neutrinos it was built to find.

“This new result highlights the versatility of the IceCube Neutrino Observatory,” said Olga Botner, the spokesperson for the IceCube Collaboration and another of the paper’s authors. “It is not only an instrument for exploration of the violent universe but allows detailed studies of the properties of the neutrinos themselves.”

ArXiv, 2015. Abstract: arXiv:1605.01990v1 (About DOIs).

Correction: PeV, not TeV.