Scientists Track Neutrinos Through Ice to Their Source in the Cosmos

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The photo released by the National Science Foundation shows the IceCube detector, the world's largest neutrino observatory. An international team of researchers said Thursday using a particle detector buried below the ice at the South Pole they have observed the first solid evidence for high-energy neutrinos that originate outside of our own solar system.

The photo released by the National Science Foundation shows the IceCube detector, the world’s largest neutrino observatory. An international team of researchers said Thursday using a particle detector buried below the ice at the South Pole they have observed the first solid evidence for high-energy neutrinos that originate outside of our own solar system.


Photo:

NSF/Xinhua/Zuma Press

Astronomers for the first time traced a burst of powerful cosmic particles called neutrinos to a black hole firing like a ray gun aimed at Earth, by using an unusual observatory buried in a billion tons of ice under the South Pole.

The international research team said it believes the discovery, announced Thursday at the National Science Foundation in Washington, D.C., may pinpoint the first known source of high-energy cosmic rays, putting to rest a mystery that has bedeviled astrophysicists for decades.

Unaffected by normal matter, radiation or gravity, ghostly neutrinos are the most abundant, energetic and least-understood particles in the universe, hurtling through space rarely interacting with ordinary stuff. Always traveling in an inflexible straight line, though, they lead unerringly back to the point where they were created—a likely source of the high-energy cosmic rays that shower Earth, the scientists said.

Scientists are eager to learn all they can about cosmic rays because they produce cascades of subatomic particles, X-rays and other electromagnetic radiation when they hit Earth’s atmosphere.

Ice-Fishing for Neutrinos

Under the ice of the South Pole, researchers constructed a $274 million observatory to map the universe using neutrinos. These unusual subatomic particles are almost impossible to detect because they usually are unaffected by normal matter, radiation or gravity.

IceCube lab

In-ice sensor network

Engineers used hot-water drills to melt 86 holes, a mile deep or more, into the ice. There they lowered 5,160 electric-optical sensors into the holes, then allowing them to freeze in place.

Deep core

The sensors detect the flare from the rare collision of a neutrino and a normal atom as the particle speeds through the array and relay the signal to the surface, accurate to within five-billionths of a second.

Bedrock

Neutrino astronomy

By plotting the direction of a neutrino through the ice, researchers expect to reconstruct its route back across the universe to its origin in a supernova or other cosmic cataclysm.

Muons radiate blue light as they move through ice on the same path as the neutrino.

On rare occasions, a neutrino will come in contact with a proton or neutron.

When the two collide, a particle known as a muon emerges.

2

3

1

Muon

Muon

Neutrino

Collision

Blue light

Proton

IceCube lab

In-ice sensor network

Engineers used hot-water drills to melt 86 holes, a mile deep or more, into the ice. There they lowered 5,160 electric-optical sensors into the holes, then allowing them to freeze in place.

Deep core

The sensors detect the flare from the rare collision of a neutrino and a normal atom as the particle speeds through the array and relay the signal to the surface, accurate to within five-billionths of a second.

Bedrock

Neutrino astronomy

By plotting the direction of a neutrino through the ice, researchers expect to reconstruct its route back across the universe to its origin in a supernova or other cosmic cataclysm.

Muons radiate blue light as they move through ice on the same path as the neutrino.

On rare occasions, a neutrino will come in contact with a proton or neutron.

When the two collide, a particle known as a muon emerges.

2

3

1

Neutrino

Muon

Muon

Collision

Proton

Blue light

IceCube lab

In-ice sensor network

Engineers used hot-water drills to melt 86 holes, a mile deep or more, into the ice. There they lowered 5,160 electric-optical sensors into the holes, then allowing them to freeze in place.

Deep core

The sensors detect the flare from the rare collision of a neutrino and a normal atom as the particle speeds through the array and relay the signal to the surface, accurate to within five-billionths of a second.

Bedrock

Neutrino astronomy

By plotting the direction of a neutrino through the ice, researchers expect to reconstruct its route back across the universe to its origin in a supernova or other cosmic cataclysm.

When the two collide, a particle known as a muon emerges.

On rare occasions, a neutrino will come in contact with a proton or neutron.

Muons radiate blue light as they move through ice on the same path as the neutrino.

2

3

1

Muon

Neutrino

Muon

Blue light

Collision

Proton

1

2

3

Bedrock

IceCube lab

1

In-ice sensor network

2

Engineers used hot-water drills to melt 86 holes, a mile deep or more, into the ice. There they lowered 5,160 electric-optical sensors into the holes, then allowing them to freeze in place.

Deep core

3

The sensors detect the flare from the rare collision of a neutrino and a normal atom as the particle speeds through the array and relay the signal to the surface, accurate to within five-billionths of a second.

Neutrino astronomy

By plotting the direction of a neutrino through the ice, researchers expect to reconstruct its route back across the universe to its origin in a supernova or other cosmic cataclysm.

On rare occasions, a neutrino will come in contact with a proton or neutron.

1

Neutrino

Proton

When the two collide, a particle known as a muon emerges.

2

Muon

Collision

Muons radiate blue light as they move through ice on the same path as the neutrino.

3

Muon

Blue light

Source: IceCube Project, University of Wisconsin at Madison

“This is one of the oldest problems in astronomy,” said

Francis Halzen,

a physicist at the University of Wisconsin in Madison and lead scientist for the IceCube Neutrino Observatory in Antarctica. “By identifying the source of these neutrinos, we identified a source of cosmic rays.”

By their reckoning, one of the most luminous objects in the known universe hurls these high-energy neutrinos toward Earth—a galaxy called a blazar located in the constellation Orion about four billion light years away. This cloud of stars is being devoured by a black hole—a maw with the mass of a million suns compressed into a space no larger than our own solar system, said physicist

Kam-Biu Luk

at the University of California at Berkeley who wasn’t part of the project.

In its astrophysical agony, the blazar spits a jet of charged cosmic ray particles coupled with neutrinos a million times more energetic than any particle accelerator on Earth could produce, but no one is sure how or why. “There is something complicated going on inside this source,” said astrophysicist

Naoko Kurahashi Neilson

at Drexel University in Philadelphia, who analyzed the neutrino data. “This observation only deepens the mystery.”

One of the neutrino-catching probes at the IceCube Neutrino Observatory. They had to be quickly lowered into the ice before it completely froze around them.

One of the neutrino-catching probes at the IceCube Neutrino Observatory. They had to be quickly lowered into the ice before it completely froze around them.


Photo:

Jim Haugen/IceCube/National Science Foundation

The find caps almost 20 years of work by the IceCube Collaboration, comprising more than 300 astrophysicists and astronomers at 48 research centers in a dozen countries, led by scientists at the University of Wisconsin in Madison. Funded by the National Science Foundation, the $274 million Ice Cube Observatory at the U.S. Amundsen-Scott South Pole Station is the largest astronomy project ever undertaken on the isolated southernmost continent.

Their initial discovery was confirmed by astronomers at 20 observatories, including NASA’s orbiting Fermi Gamma-ray Space Telescope. They spotted intense flares of gamma rays erupting at the same spot in space as the high-energy neutrinos. The astronomers and astrophysicists documented their work in a pair of research papers published in Science on Thursday.

Until now, astronomers had been able to identify sources of only the weakest neutrino particles, which trickle from the sun and a nearby supernova.

It takes a strange observatory to spot a particle that normally shies away from normal matter. In the hunt, scientists have erected detectors in a South Dakota cavern a mile underground, at the bottom of Lake Baikal in Siberia, under a mountain in Japan, and on the floor of the Mediterranean Sea.

Researchers uncover a mile-deep bore hole for the IceCube Neutrino Observatory at the South Pole.

Researchers uncover a mile-deep bore hole for the IceCube Neutrino Observatory at the South Pole.


Photo:

Robert Lee Hotz/The Wall Street

But the largest is the IceCube, created from a cubic kilometer of the purest and most transparent ice in the world. Frozen in place are more than 5,000 basketball-size optical sensors designed to catch a trace of a passing neutrino on the rare occasion one of them interacts with the ice.

Last Sept. 22, the IceCube detectors registered the eerie blue glow of light that signaled the passage of a single high-energy neutrino through the ice. In any given second, trillions of the high-energy neutrinos are passing by the sensors, but only about 10 a year trigger an alarm.

“We sent out a neutrino alert within a minute or two,” said particle astrophysicist

Dawn Williams

at the University of Alabama in Tuscaloosa, who coordinates IceCube data analysis. “The telescopes were able to follow up on it and find compelling evidence that this blazar was associated with this high-energy neutrino.”

The discovery heralds the next step in what scientists call multi-messenger astronomy, which probes the cosmos with telescopes working across different wavelengths and now with detectors of gravitational waves as well. The neutrinos, which penetrate matter, offer a way to explore a universe of material and energy hidden from ordinary light and the conventional electromagnetic spectrum, said physicist

Olga Botner

at Sweden’s Uppsala University, a senior member of the project.

“This is the first real step in being able to utilize neutrinos as a tool to view the most extreme astrophysical processes in the universe,” said IceCube physicist

Darren Grant

at the University of Alberta in Canada.

Write to Robert Lee Hotz at sciencejournal@wsj.com