IceCube’s vertical strings of optical sensors pick up on some strong neutrino-impact energy, later dubbed Ernie (Image: IceCube Collaboration)

A pair of neutrinos detected in Antarctica may be the first of these ghostly particles seen coming from outside the solar system since 1987. If the finding is confirmed, it could lead to a new way of looking at the universe that may solve a number of cosmic puzzles.

Neutrinos have no charge and negligible mass, which means they can travel through space largely unimpeded by matter and electromagnetic fields. Detecting astrophysical neutrinos would offer an unprecedented way of studying comic objects across vast distances, similar to the way infrared light allows us to peer into opaque cosmic dust clouds to see stars forming.

For instance, neutrino detection would be a way to pinpoint sources of the highest energy cosmic rays. It is thought these rays come from some of the universe’s most powerful particle accelerators, such as intense supernovae. But the charged cosmic rays are deflected by magnetic fields as they travel, making them hard to track. The same processes that generate the rays should also create neutrinos, which can be traced directly back to their origins.


Lower energy neutrinos have been seen coming from the sun and as products of cosmic rays colliding with Earth’s atmosphere. In 1987, three underground detectors also saw neutrinos coming from a supernova in the nearby Large Magellanic Cloud. Since then, all attempts to detect neutrinos from beyond the solar system have been unsuccessful.

Open Sesame

In June last year the IceCube neutrino telescope at the South Pole reported the sighting of two candidate neutrinos, found somewhat by accident as the team was combing through the data. Bert and Ernie, as they are affectionately dubbed, each had energies of about 1 petaelectronvolt (PeV).

IceCube monitors a cubic kilometre of ice at the South Pole. It is made of 5160 digital optical modules embedded like vertical strings of pearls at depths ranging from 1.45 to 2.45 kilometres below the surface. These modules look for light emitted when neutrinos strike the ice.

According to an analysis by the IceCube collaboration, released on Monday, the odds that Bert and Ernie were the results of atmospheric shenanigans are greater than 370-to-1. In other words, there is a reasonable chance they came from more distant reaches of the galaxy – or beyond.

Of course, there is still the chance that the high-energy neutrinos were made in our atmosphere, but that could be just as exciting: until now IceCube had only seen atmospheric neutrinos with energies up to a few hundred teraelectronvolts (TeV).

Charming couple

In theory, when cosmic rays hit the upper atmosphere they can produce particles called charm mesons, which can then decay into high-energy neutrinos like Bert and Ernie. In practice, none has ever been seen.

“But you have to allow for the possibility that they exist,” says IceCube principal investigator Francis Halzen at the University of Wisconsin-Madison. While seeing neutrinos made by charm meson decay would not be overly surprising, says Halzen, it would help us understand the process better, allowing us to distinguish between all the possible atmospheric signals and the neutrinos from more distant sources.

For now, the collaboration is being cautious. “Two events are not a discovery of anything, it’s too few,” says Halzen. “What’s interesting is that because of their sheer energy, they are clearly candidates to be cosmic neutrinos.”

Dream team

The IceCube team has already started sifting through the existing data looking for more detections of such high-energy neutrinos. Assuming they find enough of them, the team can use these particles to rule out charm mesons as a source of the signal.

This can be done by studying the energy distribution of these neutrinos coming from all over the sky. If they are being generated by charm mesons in the atmosphere, the particles should be clustered around 100 TeV, and the numbers should tail off as the energy reaches 1 PeV. Bert and Ernie would be outliers.

If the neutrinos do not match this pattern, the likelihood that they are from farther out in space would go up significantly.

“It is an incredibly exciting result, the beginning of real high-energy neutrino astronomy, a dream I have had for many decades. I am very happy for them,” says John Learned at the University of Hawaii in Manoa, who is not on the IceCube team. “I can’t wait to see how this plays out.”

Journal reference: arxiv.org/abs/1304.5356