Polar telescope sights first high-energy neutrinos

A novel telescope, buried deep in the Antarctic ice at the South Pole, has become the first instrument to detect and track high-energy neutrinos from space, setting the stage for a new field of astronomy that promises a view of some of the most distant, enigmatic and violent phenomena in the universe.

Writing in the March 22 edition of the British scientific journal Nature, an international collaboration of physicists and astronomers reports the first observation of high-energy neutrinos using the AMANDA Telescope, a large array of buried detectors designed to detect the fleeting signs of high-energy subatomic particles from the farthest reaches of space.
“We have proven the technique,” says Francis Halzen, a UW–Madison professor of physics and the lead author of the Nature paper. “We have a unique probe with a sensitivity well beyond other experiments, and the neutrinos we’ve seen are of a higher energy than has been seen before.”

Neutrinos are invisible, uncharged, nearly massless particles that can travel cosmological distances. Unlike the photons that make up visible light, or other kinds of radiation, neutrinos can pass unhindered through stars, vast magnetic fields and entire galaxies without skipping a beat.

To be able to detect high-energy neutrinos and follow their trails back to their points of origin promises unparalleled insight into such extraordinary phenomena as colliding black holes, gamma-ray bursters, the violent cores of distant galaxies and the wreckage of exploded stars.

Of all high-energy particles, only neutrinos can directly convey astronomical information from the edge of the universe — and from deep inside the most cataclysmic high-energy processes, notes Robert Morse, a UW–Madison professor of physics and the principal investigator for the AMANDA project.

Sunk more than one-and-a-half kilometers beneath the South Pole, the National Science Foundation-funded AMANDA Telescope is designed to look not up, but down, through the Earth to the sky in the Northern Hemisphere. Since neutrinos can and do skip through the Earth continuously, it is the logical direction to point the telescope in order to filter out other, confusing high-energy events. The Earth between the detector at the South Pole and the northern sky filters out everything but neutrinos.

The AMANDA telescope array consists of 677 optical modules, each the size of a bowling ball, arrayed on electrical cables set deep in the ice beneath the South Pole and arranged in a cylinder 500 meters in height and 120 meters in diameter.

The glass modules at the heart of AMANDA work like light bulbs in reverse, capturing the faint and fleeting streaks of light created when the occasional neutrino crashes head on into another particle such as a proton. The subatomic wreck creates a muon, another subatomic particle that, conveniently, traces an ephemeral trail of blue light through the ice identical to the path of the neutrino. In theory, that trail can be used to point back to the neutrino’s point of origin. The discovery of point sources of high-energy cosmic neutrinos is a long-standing quest of modern astrophysics.

Cosmic neutrinos are believed to be generated in the universe’s most violent events – exploding stars and active galactic nuclei, extremely violent and not-at-all understood phenomena at the heart of many galaxies.

The results presented in this week’s Nature are based on AMANDA observations of high-energy atmospheric neutrinos, neutrinos created when cosmic rays crash into the Earth’s atmosphere. While astrophysical in nature, they are not the cosmic neutrinos coveted by scientists. Instead, they simply prove that the AMANDA detector is a working neutrino telescope.

“This paper establishes the AMANDA experiment as a neutrino telescope,” according to Albrect Karle, a UW–Madison professor of physics and AMANDA scientist. “Now we can do astrophysics.”

However, while the new results from AMANDA represent a critical step toward establishing a new field of astronomy, a much bigger detector is required, the Nature paper’s authors write, to search the sky for the speculated sources of the cosmic neutrinos that constantly bombard the Earth. Toward that end, plans are being made to construct a much larger detector know as IceCube. To consist of 4,800 optical modules on 80 strings, the IceCube detector would effectively convert a cubic kilometer of Antarctic ice into the world’s largest scientific instrument.

Still, the success of AMANDA in detecting neutrinos at high energies effectively extends the reach of conventional neutrino physics beyond any existing experiment and is a promising step toward the 40-year-old dream of neutrino astronomy, says Morse, who has spent the last decade overseeing the building of AMANDA.

“This is our coming-out party,” he says. “Now we start the process of discovery.”

The paper published this week in Nature was the product of a collaboration between 119 scientists at various institutions from around the world.