On the frigid central plain of Antarctica, where the sun only rises once a year, a set of 5,160 light sensors encased in a cubic kilometer of crystal clear ice sits poised to register the flash of passing quantum particles.
In a study published Monday in the journal Physical Review Letters, a team led by Francis Halzen, a professor of physics at the University of Wisconsin-Madison, reported with 99 percent certainty that a hypothetical quantum particle called a sterile neutrino doesn’t actually exist — ending a 20-year debate among physicists.
The study analyzed two years of data taken from the IceCube Neutrino Observatory, a South Pole instrument designed to detect neutrinos, the highest energy particles in the natural world. If the sterile neutrino had been identified, it would have been the first time a particle would have been found to exist outside the mathematical system used by physicists.
“I’ve said that the sterile neutrino is kind of like Elvis,” Halzen said. “We see hints of it but we can never get a picture.”
A neutrino is an incredibly low mass particle that was named for its neutral charge. Most are produced by radiation in the sun or are left over from the birth of the universe, though a few are made by nuclear reactors on Earth.
Neutrinos have very few interactions with physical forces and matter, so they are able to pass through the cosmos unhindered by giant stars or black holes. This makes them the perfect messengers to relay astronomical information from the edges of the universe.
According to the Standard Model, the mathematical system that physicists use to describe particles and the forces with which they interact, there are three flavors of neutrinos: tau, muon and electron.
But neutrinos are also one of the weirder particles in quantum physics and undergo a peculiar phenomenon called neutrino oscillation.
“Basically, if you start with one neutrino, as it travels, or flies, it can become a different flavor of neutrino,” said Ben Jones, a particle physicist and collaborator on the study at the University of Texas at Arlington.
In the 1990s, a new oscillation was detected that couldn’t be explained by the three existing flavors of neutrinos.
Sterile neutrinos were thus hypothesized as a way to explain the new oscillation. The problem is that sterile neutrinos are just that — “sterile.” They aren’t predicted to interact with anything other than the force of gravity so finding them is a difficult task.
“We look for neutrinos from cosmic rays on the other side of Earth,” said Carlos Arguelles, a particle physicist at the Massachusetts Institute of Technology. “These cosmic rays interact with the upper atmosphere and only the neutrinos are able to pass through the Earth because they don’t interact with anything.”
According to Jones, a very small fraction of these neutrinos will interact with the ice and produce a muon. That muon produces light that can be detected by sensors.
Back at the South Pole, IceCube takes advantage of the largest continuous ice sheet in the world to measure the glow of muons derived from the neutrinos in cosmic rays.
“We know that this muon comes from neutrinos because they can’t make it through the Earth,” Halzen said.
If sterile neutrinos existed, then a large dip would be easily detected in the muon spectrum.
“It was easy to look at the data and see that the muons didn’t disappear,” Halzen said. “We will keep looking, of course, but I think the evidence is pretty conclusive.”
“One of our major goals is to understand cosmic rays,” Jones said. “Even though the sterile neutrino doesn’t exist, it still helps us figure out what’s going on in these high energy beams.”
