Scientists at the IceCube Neutrino Observatory in Antarctica who have been searching for mysterious particles known as sterile neutrinos have come up empty.
The results, described in Physical Review Letters, leave some troubling gaps in our understanding of particle physics and may force some theorists to reimagine a model that explains longstanding mysteries from dark matter to the universe's missing antimatter.
"Everybody, including me and my collaborators, would have loved to see this thing," said Francis Halzen, IceCube's principal investigator and a theoretical physicist at the University of Wisconsin-Madison. But the data, he said, were pretty emphatic. "Unfortunately, I think we have closed the door on that."
Neutrinos are as tiny as they are plentiful. Even though they have almost no mass, they're the second most common particle in the universe after photons, which means that in cosmic bulk, they could really throw their weight around. But neutrinos hardly ever interact with matter, passing through everything from the Earth to the stars like ghosts. About 100 billion neutrinos from the sun pass through your fingertip every second.
There are three flavors of neutrino — electron, muon and tau — and each has a slightly different mass. Neutrinos can switch from one flavor to another at any time — a quantum mechanical phenomenon that implies that neutrinos have mass. That already makes them an uncomfortable fit in what's known as the standard model of particle physics, which assumes that neutrinos don't have mass. (It was designed before scientists realized that neutrinos were not massless, a discovery that earned last year's Nobel Prize in physics.)
But scientists have long wondered whether there is a fourth flavor of neutrino, a "sterile" neutrino that wouldn't interact with matter. Experiments at Los Alamos National Laboratory in the 1990s hinted at the existence of such a particle; follow-ups by other teams further whetted researchers' appetites.
If the sterile neutrino does exist, then it creates an even bigger snag in the tapestry of the standard model — one that could undo it entirely. The rules surrounding the tiny building blocks of the universe, as we understand them, may have to be rewritten. While that may sound like a problem for scientists, it's also an opportunity to build a new model that might describe many phenomena unexplained by the standard model, such as the existence of dark matter or the reason for the matter-antimatter asymmetry in the universe.
But neutrinos, even when they can be caught, are exceedingly difficult to measure. Any signal from an individual neutrino particle is so faint that researchers typically have to build their giant detectors underground to block out the "noise" caused by the deluge of particles bombarding Earth.
IceCube waits for neutrinos to hit a nucleus in the ice and send up a telltale flash of light, which is picked up by sensors embedded in the frozen water. But because sterile neutrinos theoretically wouldn't interact, IceCube scientists actually look for what's missing. If one of the other flavors switches to a sterile neutrino mid-flight, it will leave a distinct dip in the total number of measured neutrinos.
The scientists paid particular attention to the muon neutrinos; if a bunch of them seemed to go missing at a particular energy level, it would be a pretty clear sign that they'd flipped into the undetectable sterile neutrinos.
But after two separate analyses of about 100,000 neutrino events, the researchers found no such dip. All known neutrinos seemed to be present and accounted for. Sterile neutrinos, at least in the regime tested by IceCube, do not exist.
"Sterile neutrinos could have been our gateway to detecting that new physics," Halzen said. But now, "that door is closed."
The researchers haven't ruled out every theory that includes sterile neutrinos, however.
"They may live somewhere where we haven't looked, and we'll keep looking," Halzen added.
It's been a downbeat few days for particle physics: Monday's IceCube findings came on the heels of an announcement last week by scientists with CERN's Large Hadron Collider that a signal in the data long rumored to represent a new particle was a fluke.
MORE IN SCIENCE