Scientists detect neutrinos from outside the solar system

Scientists have detected cosmic neutrinos, elusive subatomic particles that stream through the Earth from outside the solar system, for the first time in 25 years, a feat that could usher in a new era of astronomy.

Using technology embedded in a cubic kilometer of ice buried deep beneath the surface of an Antarctic glacier, an international team of astrophysicists known as IceCube detected 28 neutrinos that are so energetic that whatever created them must have been extremely powerful.

The discovery means the technology could act as a telescope into the high-energy universe, possibly explaining what sent the neutrinos on their way and where they came from in the early universe.

"It's a 100-year-old mystery. We have theories but we don't really know," said Gregory Sullivan, a physics professor at the University of Maryland, College Park who led a team from the university working on the IceCube project. "It's looking through that window for the first time, so you don't know what you're going to see."

Astronomers previously observed the universe only through some portion of the electromagnetic spectrum. Every time they have looked at a new part of the spectrum beyond visible light, it has opened up a new way to view the universe.

Radio waves led to the discovery of the cosmic microwave background radiation left over from the birth of the cosmos. X-rays revealed black holes, neutron stars and other extremely powerful celestial objects. Both of these new tools gave birth to new telescopes and ultimately earned Nobel Prizes.

But those discoveries all depended on light. Now, after seven years of construction and two years in operation, IceCube is the first "telescope" to pick out interstellar neutrinos instead of photons — and it could shed light, so to speak, on black holes, dark matter and perhaps on yet-undiscovered phenomena in the universe.

"This is a landmark discovery — possibly a Nobel Prize in the making," said Alexander Kusenko, a UCLA astroparticle physicist who was not involved in the IceCube collaboration.

The initial discovery of cosmic neutrinos in 1987 earned their discoverers the Nobel, but even though they ought to ubiquitous, no one has seen them since — until IceCube.

Neutrinos are strange little particles, said Francis Halzen, lead scientist for the IceCube collaboration. The universe is filled with them — billions pass through your finger every second — but they're incredibly light, with less than a millionth the mass of an electron.

Neutrinos (along with muons, high-energy protons and other particles) are created in enormous blasts when something gigantic explodes. Scientists wonder whether dark matter — that stuff we can't see or feel or touch but makes up a large portion of the universe — might also give off neutrinos that we could detect.

There are important reasons for scientists to look for neutrinos. For one thing, they're neutral particles, which means they won't fall prey to the magnetic field lines crisscrossing the universe that cause light to bend and twist from its original direction. If a neutrino hits you, you know exactly where it came from.

Another reason is that a neutrino can't be stopped. It hardly interacts with matter, which means it won't get blocked by clouds, or planets, or entire galaxies. That means scientists can pick up neutrino signals from spots where light would never be able to reach us.

Here's the problem: This standoffishness is exactly what makes neutrinos so hard to detect. On top of that, they're created all the time. Cosmic rays hitting the atmosphere create a shower of particles, neutrinos and otherwise, that constantly bombard detectors. It's incredibly difficult to "hear" a neutrino over all that background noise.

Neutrino detectors, including one built under a mountain in Japan, take advantage of the Earth's bulk to block out as much of the background noise as they can. Most particles — protons, muons and even photons — don't make it through. But neutrinos can.

For this experiment, scientists decided to use a giant cube of pure and clear Antarctic ice. Into this underground cube they lowered 86 strings containing a total of 5,160 light detectors. When a neutrino hits the ice, it produces a flash of blue light that the detectors pick up.

The University of Maryland team led efforts to analyze the data from those detectors. They sifted through millions of irrelevant "events" to detect the impacts of the neutrinos, striking the surface with energy as powerful as a Major League Baseball pitch, Sullivan said.

After sifting through about 200,000 atmospheric neutrino hits, the scientists discovered 28 that they say could not have come from anywhere but outside of the solar system. These interstellar neutrinos seem to come from all over the sky, not from any particular location.

But 28 is a small sample, the scientists say. The hope, Halzen said, is to create a whole map of the universe, sketched out in neutrinos, and see what turns up.

The find is far more exciting than the discovery of the elusive Higgs boson, which took the physics community by storm last year, said John Learned, a neutrino physicist at the University of Hawaii.

"Finding the Higgs was really boring," Learned said. "It's a great triumph, but ... we didn't learn anything we didn't already know."

Baltimore Sun reporter Scott Dance and Tribune reporter Amina Khan contributed to this article.

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