Neutrinos from deep space detected on Earth
Discovery at Antarctica's IceCube opens up new area of astronomy, may solve cosmic ray puzzle
High-energy subatomic particles called neutrinos from beyond our solar system have been detected on Earth for the first time ever.
The researchers involved say the discovery opens up a new area of astronomy and has the potential to answer a question that has puzzled astronomers for a century: Where do cosmic rays come from?
"It really is the dawn of a new field," said Darren Grant, a University of Alberta physicist, who was part of an international scientific collaboration called IceCube that reported the recent detection of 28 extremely high-energy neutrinos in Antarctica that are thought to have come from space.
The results were published online Thursday in the journal Science.
Astrophysicists had theorized that extremely high-energy extraterrestrial neutrinos – more energetic than any produced on Earth or by the sun - would be blasted out by the same catastrophic events in deep space that are thought to generate cosmic rays.
Now, Grant says, scientists have finally detected some.
"They're the highest energy neutrino events that have ever been measured… It's proof that they came from outside the Earth."
Two of the neutrinos had energies above a whopping petaelectron volt. That's 125 times the 8 teraelectronvolt energy of the record proton collisions generated by world's biggest particle accelerator, the Large Hadron Collider, and billions of times the energy of neutrinos produced by the sun.
Cosmic ray mystery
Cosmic rays, discovered more than a century ago, are extremely high-energy radiation that travels through space and strikes the Earth. Astrophysicists have theorized that they might be produced by extreme catastrophic events in deep space, such as supernovas, black holes, pulsars, or galactic nuclei — the merger of two black holes.
But it's hard to determine the origin of cosmic rays because they are made up of charged particles and therefore are deflected by magnetic fields that send them on a circuitous path through space.
Consequently, "everything we know about the universe at the moment is from studying photons" or light, said Olga Botner, an Uppsala University researcher and the official spokesperson for IceCube, in an interview with Science.
Light generally travels in straight lines, but can be blocked matter such as intergalactic dust, or altered in certain ways - for example, its colour may be affected by certain things.
On the other hand, neutrinos have little mass, no charge and interact very weakly with matter, meaning they can theoretically journey billions of light years to the Earth without slowing, stopping or touching anything.
"It really makes them an ideal messenger particle," Grant said.
That means the space neutrinos recently detected by IceCube are almost exactly as they were when they were produced and carry "pristine" information about the event that produced them.
They also likely point directly back to their source — also a likely source of cosmic rays — even if it is millions or billions of light years away.
Creating a skymap
The IceCube researchers have started creating a sky map of the directions that the high-energy neutrinos came from.
So far, while they cluster in certain areas, they don't yet pinpoint specific locations in the sky. Grant said that will happen as IceCube gathers more data.
"Then you turn to your favourite astronomer and you say what's in the sky? What's there that could produce these high energy events?" he said.
"That will give us the first definitive identification of what's causing the production of these particles."
That may also provide clues about the origin of the high-energy cosmic rays, which likely originate from the same events.
The reason why neutrinos from space have never been found before is that they are very hard to detect, partly because they rarely interact with anything.
Researchers needed a huge detector in order to increase the chance of interaction with such rare neutrinos.
One cubic kilometre detector
IceCube is a block of extremely clear ice one cubic kilometre in size and 1.5 to two kilometres below the surface, where it is very dark and high pressures keep the ice clear and bubble-free. The ice is embedded with 5,160 sensors that detect very faint amounts of light.
Very rarely, neutrinos will interact with the ice itself as they pass through the detector. When that happens, Grant said, they produce a charged particle such as an electron. Typically, light travels fastest in a vacuum and more slowly in ice. But a particle produced by a neutrino interacting with the ice travels faster than the typical speed of light in ice.
When it does that, a burst of blue light, known as Cherenkhov radiation, is produced. Grant described it as "almost the optical equivalent of sonic boom."
The higher energy the neutrino, the more ice it lights up, and a high-energy cosmic neutrino lights up "an enormous amount of the detector." In fact, a large detector is also needed to capture all of the energy from a single neutrino of this kind.
The IceCube collaboration first looked only for neutrinos above 1 petaelectronvolt and detected the two most energetic neutrinos in April 2012. They then went back and searched through their data and found 26 of slightly lower energies, but above 30 teraelectronvolts that were detected between May 2010 and May 2012. While some of these less high-energy neutrinos may have been produced by cosmic rays in the Earth's atmosphere, the calculations suggest that most of them likely came from space.
The data was analyzed in such a way as to exclude, as much as possible, neutrinos that didn't come from space and other types of particles that may have tripped off the detector.
Canada played a key role in the study by crunching data with a federally funded computer cluster at the University of Alberta called Westgrid.
"One hundred per cent of our reconstructions and simulations were run there, and without it, I don't think we would have been able to do all of the work that was required," said Nathan Whitehorn, a researcher at the University of Wisconsin-Madison who had presented the preliminary results at a conference in May.
The IceCube collaboration includes 250 physicists and engineers from Canada, the U.S., Germany, Sweden, Belgium, Switzerland, Japan, New Zealand, Australia, the U.K. and Korea. The detector cost $279 million US ($290 million) and was completed in December 2010, but had already started collecting data before completion.