Research unlocks clues to cosmic rays

21:48, Nov 25 2013

An international team of scientists led by researchers at the University of Wisconsin-Madison has observed 28 neutrinos that travelled millions of light-years through space, crashing into the South Pole ice and emitting a flash of blue light the size of six city blocks.

The discovery, published on Thursday in the journal Science, represents an important advance in the effort to answer a question that has bedevilled scientists for the last century.

Where and how are high-energy cosmic rays generated?

The question provides a pointed reminder of the power of nature over even the most impressive man-made devices.

The multibillion-dollar Large Hadron Collider near Geneva, a product of the finest scientific minds in the world, generates high-energy beams.

Yet those beams have only one-ten-millionth of the energy of cosmic rays produced in space.


"Somewhere, nature builds accelerators that can do this, and we have no idea where they are," said Francis Halzen, a UW physics professor and the principal investigator for the project known as IceCube. "We have been unable to identify these accelerators, to find out how they work or even what they are."

"This is the big thing, the thing we have been dreaming about for some time," said John Learned, a neutrino physicist at the University of Hawaii, referring to the new study. "The bigger question will be: Where does this fit into the grand scope of particle physics and astronomy?"

The Science paper was based on two years of data from IceCube. Researchers are now analysing another year's worth, raising hopes that they may make significant progress toward the answers they've been chasing.

Neutrinos, subatomic particles with no charge and very little mass, may well lead scientists to the answers. By observing and pinpointing neutrinos it is possible to begin drawing straight lines back to their origins.

That's because these particles have no charge and travel through space in a straight line without being bent or warped. The vast majority pass through the earth, but one in a million or so collides with the nucleus of the hydrogen or oxygen atoms in ice, producing a nuclear reaction.

Observing these events has been a goal for decades, one that UW has played a key role in pursuing. The university was a leader in construction of the neutrino telescope dubbed AMANDA (Antarctic Muon and Neutrino Detector Array). That's between 40 and 100 times smaller than the IceCube Neutrino Observatory, which was used to detect the 28 neutrinos.

UW was the lead institution on the IceCube project and did most of the original construction. The actual device is a cubic-kilometre array of glass spheres with instruments inside that resemble cameras and circuit boards. But none of that is visible from the ground.

What is visible are flags at the top of drill holes. The actual instrument is located about one mile below the surface ice and stretches down a kilometre or so. Nathan Whitehorn, a 28-year-old post-doctoral student and one of the authors of the Science paper, spent about a month at the South Pole station.

"It's a really neat place to be," he said, explaining that he was drawn by the opportunity to examine "one of the great unsolved mysteries of the universe."

Whitehorn was at the station between December and January, summer at the Pole and a period when it is actually warmer than in Madison. The sun is up for 24 hours a day. Whitehorn could hear generators and the roar of airplanes landing at the station. Yet a short distance from the station, nature provides the only sound: a shuffling caused by snow crystals tumbling in the wind.

At the observatory, optical sensors capture the intensity and precise time of these one-in-a-million collisions between the long-travelling neutrinos and the polar ice. The reaction emits blue light, and instruments send this information to a computer that reconstructs the size and shape of the light pattern.

The data is relayed by satellite to Madison and then distributed to about 40 institutions in 12 countries, mostly universities and national laboratories. Worldwide, IceCube faces at least two competing projects seeking to detect neutrinos: a French-Italian-Greek collaboration involving a detector in the Mediterranean Sea; and a Russian detector deep beneath the surface of Lake Baikal in Siberia.

At present, the quest to find and understand the generators of these cosmic rays has no clear applications.

"This is purely curiosity-driven," Halzen said.

However, he added that the same was true of quantum mechanics when it became the brightest frontier in physics in the 1920s.

"Now, most of the US economy is based on quantum mechanics - things like computers and chips," Halzen said.

For now, the work has the appeal of sheer wonder. The neutrinos the IceCube project has detected, for example, have a billion times the energy of those scientists have detected in the sun.

"In terms of energy," Halzen said, "it is really mind-boggling, what we are detecting."