Still No Sign of 'Sterile Neutrino' Particle, Candidate for Dark Matter

An experiment buried deep under the ice of Antarctica that was designed to study distant cosmic objects, has come up empty in a search for a strange particle. 

The submerged instrument, called the IceCube Neutrino Observatory, is pioneering the field of particle astrophysics — that is, detecting particles (other than light) that come from cosmic events such as star explosions. Because the instrument was built to detect particles called neutrinos, it has also provided some insight into the nature of these mysterious particles.

Today, the IceCube team announced the publication of a new paper showing that the detector found no sign of a theoretical particle called the sterile neutrino, which is a potential candidate for dark matter, the material that makes up more than 80 percent of the mass in the universe. [Neutrinos from Beyond the Solar System Found (Images)]

Neutrinos are particles that don't make up normal matter, but they are ubiquitous in the universe. The sun produces a heavy stream of neutrinos that showers down on Earth, but these particles interact with regular matter very rarely. So rather than colliding with regular matter (that is, the atoms that make up the planet or the people living on it), they slip through the planet like ghosts. 

There are three known types of neutrinos, and some theoretical models have predicted the existence of an even more elusive fourth neutrino. It's named the "sterile neutrino" because it would never physically collide with particles that make up regular matter. A sterile neutrino would only interact with regular matter via gravity, which is why it's a possible candidate for dark matter — a substance that does not radiate or reflect light, and also appears to interact only with regular matter via gravity. 

So, to detect a sterile neutrino with IceCube requires a slightly different approach. Neutrinos are shape-shifters; one type of neutrino can suddenly become another type of neutrino as it travels through space. The neutrinos passing through the Earth and interacting with IceCube would effectively disappear if they were to transform into sterile neutrinos, researchers with IceCube said.

"It turns out that it is more likely to morph into a sterile neutrino if it goes through a very dense region of matter," Janet Conrad, a professor of physics at MIT and a member of the IceCube collaboration, said in a video released today by Ice Cube explaining the finding. "And so the [Earth's] core is ideal for producing much more morphing than you would get for the neutrinos that do not pass through the core. And so what we're looking for is neutrinos that are on the trajectory that come through the core to disappear." 

IceCube can observe the sterile neutrino only if the particle has a mass within a particular range (and no other experiment can look for sterile neutrinos in that entire range, the researchers said in the video). If the neutrino did fall within that range, then the observed effect in the detector would be "dramatic," and "you either see or you don't see [it]; it's as simple as that," Francis Halzen, a professor of physics at the University of Wisconsin-Madison and the principal investigator for IceCube, said in the video. 

The results don't completely rule out the existence of the sterile neutrino but show that many currently operating neutrino experiments most likely won't be able to find it, the researchers said. Furthermore, the results appear to negate some "hints" of a sterile neutrino that have appeared in past years — that is, weak detections that looked like they could have been caused by the sterile neutrino but were not strong enough to confirm its existence. In particular, Halzen pointed to a claim by the Liquid Scintillator Neutrino Detector at Los Alamos National Laboratory from about 20 years ago. 

"I think that we should keep pursuing any hints of new physics in the neutrino data, but our result shows that they are unlikely to be associated with the existence of a sterile neutrino," Halzen told Space.com in an email. 

"What [the new result] is going to mean is that our belief in the sterile neutrino decreases, but it also is telling us where a sterile neutrino may not be, and where it may still survive," Carlos Argüelles Delgado, a postdoctoral researcher at MIT and a member of the IceCube team, said in the video. 

Conrad said in the video that the new results still help scientists refine models that describe the universe. Any models that include a sterile neutrino in this particular mass range may need to be rewritten, and scientists hope it will help them get closer to an accurate description of the physical world. 

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