IceCube detector finds first solid evidence for cosmic neutrinos
Thu 21 November 2013
Scientists have found the first solid evidence for cosmic neutrinos, ghostly particles created in violent events in the far reaches of the universe.
Neutrinos are subatomic particles that hardly ever interact with the atoms that make up stars, planets and us. Detecting them is tough: in the latest study, researchers detected 28 at the IceCube detector, built under the ice of the south pole.
“This is a huge result. It could mark the beginning of neutrino astronomy,” said Darren Grant, assistant professor of physics at the University of Alberta and one of the leaders of the IceCube Collaboration, which involves more than 250 physicists and engineers from a dozen countries.
Neutrinos are electrically uncharged particles that have a tiny mass, formed in the nuclei of atoms. Travelling at near the speed of light, they hardly interact with anything and could easily fly through a light year of lead. But there are unimaginable numbers of them in the universe: trillions of them from the sun pass through each of us every day.
Scientists know that neutrinos with even higher energy than those already observed should come from cosmic explosions, such as gamma ray bursts, black holes and active galactic nuclei, far away in the universe. Detecting these high-energy neutrinos would give scientists a way to peer inside some of the most violent processes going on at the farthest reaches of the cosmos.
Until now, scientists have used other detectors to see low-energy neutrinos created in cosmic-ray collisions in the Earth’s upper atmosphere and particles from a nearby supernova known as 1987A. The 28 neutrinos detected at IceCube are much higher energy and come from as yet unidentified sources far out in the cosmos. The results were published in the journal Science on Thursday.
“I’ll bet that 20 years from now we’ll look back and say, yeah, this was the start of neutrino astronomy,” John Learned, of the University of Hawaii, Manoa, told Science magazine.
To find the particles, scientists built a detector into a cubic kilometre of ice in Antarctica. After melting holes in the ice, they lowered 86 strings of light detectors, around 5,000 in total, to depths between 1.5km and 2.5km. Neutrinos can interact with atomic nuclei, and when that happens in the ice around a detector the collisions create an avalanche of charged light-emitting particles. That light can be measured by the detectors and, the brighter the light, the more energetic the original neutrino was.
IceCube has been on the hunt for neutrinos since 2010. Since then scientists have found evidence for 28 neutrinos with energies higher than 30 teraelectronvolts (TeV). Two of the particles had energies greater than 1,000 TeV. In comparison, the biggest particle accelerator ever made, the Large Hadron Collider at Cern, will collide particles at 14TeV when its upgrade is completed in 2015.
Since they do not interact with anything, the cosmic neutrinos found at IceCube are useful to scientists because they point in straight lines to where they came from. The few they detected are not enough to pinpoint any location in particular but, according to the project scientist Gregory Sullivan, of the University of Maryland, the IceCube team will look for further detections in coming years, “like waiting for a long exposure photograph”, to fill in their emerging picture of the faraway cosmos.