Courtesy of the IceCube Collaboration
Ghostly high-energy particles from beyond the solar system have been snagged for the first time by a giant experiment buried under Antarctic ice. The sightings of the particles, called neutrinos, represent a major step toward identifying mysterious astrophysical phenomena that hurl subatomic particles across the universe at extraordinary speeds.
Neutrinos are an intriguing tool for exploring the cosmos. The wispy particles have no charge and rarely interact with matter, allowing astronomers to trace a straight path back to their source. However, neutrinos’ inertness also makes them hard to detect. Scientists are confident that alien neutrinos packing more energy than particles from any human-made accelerator are constantly pelting Earth, perhaps pointing to some of the universe’s most violent objects. But until now researchers had managed to detect only relatively puny neutrinos streaming from the atmosphere, the sun and a 1987 supernova.
Looking to end decades of frustration, physicists put their hopes in IceCube, a vast array of neutrino detectors buried as deep as 2.5 kilometers beneath the surface of the Antarctic ice sheet near the South Pole. Beginning in May 2010, more than 5,000 light sensors embedded in a volume equivalent to 400,000 Olympic-sized swimming pools have looked for subtle flashes indicating that neutrinos had struck atoms within the dark depths of the ice.
Now physicists’ perseverance has paid off. During its first two years of operation, IceCube captured 28 high-energy neutrinos, including the eight most energetic ever detected, researchers report in the Nov. 22 Science. The researchers filtered out about 200,000 neutrinos that came from the atmosphere, most of which had lower energies and were accompanied by a shower of other particles, to confirm that these 28 originated light-years away. “There’s nothing in the solar system that should be producing these,” says Nathan Whitehorn, a study coauthor and physicist at the University of Wisconsin–Madison.
The two speediest neutrinos, nicknamed Bert and Ernie when they were announced in April, struck the ice with more than a million billion electron volts of energy. That’s about 150 times as much energy as carried by each proton whizzing around the Large Hadron Collider, the world’s most powerful particle accelerator. (Continuing the tradition set with Bert and Ernie, the other 26 neutrinos also received Muppet names, including Mr. Snuffleupagus and Dr. Strangepork.) Even the most sluggish of the 28 neutrinos packed about a million times as much energy as those from the 1987 supernova. “It’s really nice work,” says Kate Scholberg, a particle physicist at Duke University not involved in the research. “It’s been a long time coming.”
The next step, Whitehorn says, is using the neutrinos to solve a long-standing cosmic mystery. For decades, astronomers have detected protons, electrons and other charged particles arriving from space at tremendous speeds. Yet nobody knows what object or event could accelerate particles to those energies. Charged particles don’t point to their source because they get deflected by magnetic fields, but they can decay into neutrinos, which travel in a straight line. That means the incoming neutrinos collected by IceCube could finally expose the universe’s remarkably powerful particle accelerators, which could be supermassive black holes, cataclysmic stellar explosions or unknown violent objects.
The 28 neutrinos detected so far came from all directions, Whitehorn says, noting that some particles approached from the Northern Hemisphere and traveled through Earth’s interior before striking the ice. But more data could eventually point to regions of the sky as primary sources. In addition, neutrinos come in three varieties, or flavors, that offer clues to the particles’ origins. The early data confirm that IceCube has captured all three neutrino flavors. Now researchers are busy analyzing the last year-and-a-half of data not covered in the Science study.
IceCube Collaboration. Evidence for high-energy extraterrestrial neutrinos at the IceCube detector. Science. Vol. 342, November 22, 2013, p. 947. doi: 10.1126/science.1242856.
N. Drake. Hunt for cosmic ray source falls short. Science News. Vol. 181, May 19, 2012, p. 18.
M.G. Aartsen et al. First observation of PeV-energy neutrinos with IceCube. Physical Review Letters. Vol. 111, July 12, 2013, 021103. doi: 10.1103/PhysRevLett.111.021103.
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