After years of waiting, the world’s biggest and best neutrino detector has started its search for the source of ultrahigh-energy cosmic rays that constantly bombard the Earth’s atmosphere.
And it’s seen exactly zilch.
In 13 months of observing with the half-complete IceCube detector at the South Pole, “we didn’t see anything,” said neutrino physicist Nathan Whitehorn of the University of Wisconsin-Madison, co-author of a new Physical Review Letters paper describing the hunt. “We didn’t even have any close calls.”
The origins of cosmic rays, a constant shower of fast-moving particles from space, have long baffled physicists. Some of these particles are 100 million times more energetic than those produced at the Large Hadron Collider, the most powerful particle smasher on Earth.
Yet after nearly a century of research, scientists have no firm idea what they are or where they come from. “It’s one of these big unsolved mysteries in physics,” Whitehorn said. “What can possibly be making them?”
A top theoretical contender is gamma-ray bursts, equally mysterious cosmic explosions that can briefly outshine everything else in the observable universe. Although relatively little is known about what causes gamma-ray bursts, theory predicts that a certain fraction of their energy should show up as neutrinos.
Neutrinos are tiny, neutral particles that are extremely reluctant to interact with other types of matter. They’re extremely difficult to detect — a neutrino produced in the center of the sun would have to travel through several light-years’ worth of lead before having a 50 percent chance of interacting with a lead atom.
But every so often, a neutrino will smash into an atomic core, and send out a spray of nuclear particles. If these particles zip through water or ice, they leave faint blue trails of light that can be seen by sensitive photon detectors.
IceCube, which was completed in December 2010 after a decade of construction, is an array of 5,160 such detectors arrayed more than a mile deep in Antarctic ice. Unlike earlier neutrino detectors, like Superkamiokande in Japan and SNO in Canada, IceCube is big enough to sense neutrinos with energies higher than a trillion electron volts, which are produced by the very highest-energy cosmic rays. If gamma-ray bursts are responsible for cosmic rays, IceCube should be able to tell.
In the new study, the IceCube team compared data from April 5, 2008, through May 20, 2009, when the detector was only half complete, to 117 gamma-ray bursts detected in the Northern Hemisphere during that time. (The team had to ignore Southern Hemisphere bursts, as particles that come from the atmosphere can look a lot like neutrinos. By using Earth as a shield and only counting particles that pass through the entire planet, researchers can be sure they’re really neutrinos.)
Nothing happened. Following each of the gamma-ray bursts, it took more than half an hour for any neutrinos to arrive. Even those came at statistically insignificant levels, and none was of the anticipated high-energy variety.
The non-detection puts limits on the fraction of cosmic rays that can be traced back to gamma-ray bursts, Whitehorn said. It could mean that gamma-ray bursts produce less than 82 percent of high-energy cosmic rays.
Data from the next few years will be crucial to testing this possibility. According to Eli Waxman, a theoretical physicist at Israel’s Weizmann Institute who wrote the theory predicting how many neutrinos should be produced in gamma-ray bursts, this 117-burst dataset should have turned up at most four neutrinos.
That they didn’t show up is notable, but not shocking. “Once they expand the sample by a factor of 10, then that will be time to start asking questions,” said Waxman, who wasn’t involved in the study.
“In two years we’ll have an answer, or a lot of scratching our heads,” Whitehorn said. “We’ll either see neutrinos, or something will be strange with the universe.”
Image: NSF, IceCube/University of Madison-Wisconsin.
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