South Pole's Icy Eye Detects Extraterrestrial Neutrinos

Scientists in Antarctica have discovered neutrinos with energies much higher than neutrinos that originate in our Solar System, according to a new report in Science. The researchers hope that these tiny, massless particles could tell us something about cosmic rays, the origins of which have long been mysterious.

Cosmic rays are some of the highest energy particles in nature. Scientists have speculated that they originate in supernovas, black holes, or other energetic phenomena in the universe's distant corners. But, they haven't been able to confirm this because as these rays zip around space, their electric charge causes them to be deflected by magnetic fields and makes them hard to track.

This is where neutrinos, which lack an electric charge and thus can be tracked, come into play. "We expect that anything that is making high-energy cosmic rays will also make neutrinos," explained University of Wisconsin-Madison researcher Nathan Whitehorn. "So if we are able to identify the origin of the highest energy neutrinos, that will provide some very powerful clues about the origin of cosmic rays."

The problem is scientists have not detected any particularly high-energy neutrinos — those from sources other than the Sun or the Earth's atmosphere — since 1987.

Now, in research reported in the 22 November issue of Science, Whitehorn and a massive team of international researchers in the IceCube Collaboration make a leap forward, reporting the first very high-energy, extraterrestrial neutrinos — particles over a million times more energized than those found in1987.

These exciting results were first hinted at in 2012. Team members analyzing data from the South Pole's IceCube Neutrino Observatory, which had been scanning for high-energy neutrinos since 2010, reported two neutrino events with energies above what's expected in Earth's atmosphere.

"Such neutrinos are quite rare," said Whitehorn, "and until now, we haven't had a detector sensitive enough to see them."

Scientists weren't satisfied with these first two neutrino finds, however. "Without more data it was almost impossible to be sure they were part of a larger pattern indicating an extraterrestrial population instead of some statistical fluke," Whitehorn explained.

So team members continued to analyze results from IceCube — the largest neutrino detector in the world. Like a huge eye, it scans the clear Antarctic ice for light that results when neutrinos strike. Lots of neutrinos strike the icy surface, including those from the Sun and Earth's atmosphere, but researchers at IceCube focus only on high-energy neutrino events.

Using more than 5,000 sensors strung vertically below the ice, the detector's computers collect raw data on neutrino activity. Interesting neutrino events are sent from the South Pole back to the University of Wisconsin-Madison for further study by members of the IceCube Collaboration.

"Working at the South Pole is very different from our usual duties back in Wisconsin," said team member and researcher Claudio Kopper, who lived in the harsh Antarctic environment for a time. "We usually don't do data analysis in the South Pole, but rather perform detector maintenance, which is continually required."

After identifying the two high-energy neutrino events in 2012, he and other members of the collaboration started sifting through the rest of the data from IceCube, looking for more high-energy signals. They found 26 more, including the most energetic neutrinos ever observed. Each had characteristics similar to those scientists predicted would be found in neutrinos with extraterrestrial origins.

The scientists suspect the high-energy neutrinos originate in the same luminous sources that produce cosmic rays. Confirming this origin, their next step, would move them closer to solving the riddle of the origin of cosmic rays.

 

 
University of Wisconsin researchers discuss their work on the IceCube Collaboration. | Courtesy of University Communications/University of Wisconsin-Madison